Dissertations / Theses on the topic 'Precipitation variability'

Precipitation is one of the atmospheric variables that characterize the climate of a region. The South Island of New Zealand (SI of NZ) has an unusually large number of distinct regional climates and its climatic diversity includes the coldest, wettest, driest and windiest places in New Zealand. This thesis focuses on identifying precipitation trends and rainfall fluctuations for the SI of NZ. First, homogeneity of 184 precipitation series is assessed with the combination of three homogeneity tests (Standard Normal Homogeneity Test, Easterling & Peterson test, Vincent�s Multiple Linear Regression). More than 60% of tested time series are found to contain at least one inhomogeneity. About 50% of the inhomogeneities can be traced to information in the station history files with nearly 25% of all inhomogeneities caused by the relocation of the precipitation gauge. Five coherent precipitation regions are defined by the Principal Component Analysis. The objective of identifying the periods of water deficit and surplus in spatial and temporal domains is achieved by using Standardized Precipitation Index (SPI). The SPI series (for 3, 6, 12, 24 and 48 months time scales) are calculated for each region and used for analysis of dry and wet periods. Clear differences in the frequency, length and intensity of droughts and wet periods were found between individual regions. There is a positive (i.e. increase in wet periods) trend in SPI time series for the North, Westland and Southland regions during the 1921-2003 period at all times scales, and a negative trend for Canterbury during the same period. The results show longer wet periods than dry periods at all time scales. Extreme heavy precipitation, which causes floods, is the most common type of natural disaster accounting for about 40% of all natural disasters worldwide. A set of ten extreme indices is calculated for 51 stations throughout the South Island for the period 1951-2003. The west-east division is found to be the dominant feature of extreme precipitation trends for all extreme indices with more frequent and more intense extreme precipitation in the west/southwest and with a declining trend in the east. The significant decrease in extreme precipitation frequency was detected in Canterbury with 3 days less of precipitation above the long-term 95th percentile by 2003 as compared to 1951. The variability of precipitation, expressed by the SPI, is correlated with local New Zealand atmospheric circulation indices and large-scale teleconnections. The precipitation variability in the South Island is governed largely by the local circulation characteristics, mainly the strength and position of the westerly flow. The increase in precipitation in the West and SouthEast is associated with enhanced westerlies. The correlations between New Zealand�s circulation indices and regional SPI are seasonally robust. The SouthEast region exhibits a strong relationship with the Southern Oscillation Index on seasonal and annual time scales,and with Interdecadal Pacific Oscillation at the decadal scale. The predictability of seasonal precipitation one season ahead is very limited.

Pakistan, as an agriculture based economy, is vulnerable to various hydrometeorological hazards ranging from tropical cyclones, thunderstorms, tornadoes, drought, rain, hail, snow, lightning, fog, wind, temperature extremes, air pollution, and climatic change. However, three of the most pressing challenges in terms of water resource availability, that are different in nature, but are inter-linked to each other are discussed over here. We begin with the Karakoram Anomaly that is considered as one of the most mysterious and most speculated phenomena on Planet Earth. Though, it is confined to the glaciers in the eastern Hindukush, western Karakoram and northwestern Himalayan mountain ranges of Northern Pakistan that are not responding to global warming in the same manner as their counterparts elsewhere, because, their retreat rates are less than the global average, and some are either stable or growing. However, the Karakoram Anomaly has baffled scientific society for more than a decade since its earliest discovery in the year 2005. The reasons of the Karakoram anomaly were mainly associated to physiography of the area and role of climate was considered marginal till now, as climate is influencing glaciers differently all over the globe. Here, for the first time, we present a hydro-meteorological perspective based on five decades of synoptic weather observations collected by the meteorological network of Pakistan. Analysis of this unique data set indicates that increased regional scale humidity, cloud cover, and precipitation, along with decreased net radiation, near-surface wind speed, potential evapotranspiration and river flow, especially during the summer season, represent a substantial change in the energy, mass and momentum fluxes that are facilitating the establishment of the Karakoram Anomaly. In turn, it is influencing the availability of glacier melt in River Indus in summer season. Secondly, we developed a hydrometeorological data sets for Pakistan as they are extremely important for water related impact studies and future climate change scenarios. Presently, major sources of gridded temperature and precipitation data generation are in-situ observations, satellite retrieved information and outputs from numerical models. However, each has its own merits and demerits. Among them gridded observed data sets are considered superior if the gauge density is better. Unfortunately, precipitation gauge network of Pakistan is poorly presented in prior gridded products. Therefore, a daily in-situ observation based, 0.05º×0.05º gridded temperature and precipitation data set for Pakistan, for the period of 1960-2013 is developed. It is named as PAK-HYM-1.0, that is an abbreviation of Pakistan and Hydrometeorology, and 1.0 indicates that it is the first version. This data set is developed by utilizing data from 67 meteorological stations of Pakistan. This number of observation sites is 2 to 4 times higher than that used in prior similar products, and this product can be adopted as an operational information product that can be updated on daily basis. Finally, we focused on meteorological and hydrological droughts in Pakistan. We have reconstructed history of drought in Pakistan using in situ observations based high resolution gridded data through Standardized Precipitation Index (SPI) methodology on different time scales. Furthermore, we have explained the transition of meteorological drought to hydrological drought using river inflows data of large rivers of Pakistan, and explained the sensitivity of different rivers to rainfall and temperature of different seasons. On the basis of this analysis, we have proposed a solution of construction of water reservoirs to tap water resources from northern mountains as inflows from these mountains has potential to perform as a buffer against droughts in low-lying areas of Pakistan. In addition to that, we have demonstrated the potential of Palmer Drought Sensitivity Index (PDSI) as an operational tool for drought monitoring in Pakistan.

The Asian summer monsoon supplies around 3 billion people with much of their yearly supply of freshwater, necessary for human consumption as well as in agriculture and industry. In many regions, particularly along the Ganges River in India and in northern China, use of freshwater far exceeds natural recharge rates. Given the high population density of these regions, a substantial fraction of Asia's population is therefore critically sensitive to interannual changes in the supply of freshwater by the monsoon, as well as potential future change under 21st century warming. This dissertation focuses on understanding the atmospheric dynamics of the leading mode of July-August Asian Monsoon rainfall variability, which links two major subsystems: the South Asian and East Asian monsoons. (Abstract shortened by ProQuest.)

Thesis: Ph. D. in Hydrology, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, September, 2020
Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages 264-287).
Water is a critical factor limiting economic and social development in Morocco and the Mediterranean Basin. In addition to strong seasonality and high inter-annual variability, annual precipitation remains low (<500mm) across much of the region. Furthermore, the situation is not expected to improve under climate change as models project a sustained decline in precipitation in the Mediterranean, most pronounced during the winter season. Despite the significance of such projections, a comprehensive theory for Mediterranean winter climate change is still lacking. Here, we adopt a multi-faceted approach to investigate precipitation variability and change over Morocco and the Mediterranean, with a focus on resulting water availability. First, we link inter-annual variability of seasonal precipitation in Morocco to global sea-surface temperatures, and develop empirical forecast models that can predict up to 35% of this variability with a one-month lead time.
Turning our attention to regional climate change processes and impacts, we show how future winter precipitation trends in the Mediterranean directly result from projected circulation anomalies. The enhanced advection of dry air from the Sahara Desert caused by these anomalies is key in causing precipitation to decline over Morocco. In addition, a major contribution of this work is to propose a physical explanation for the circulation trends involving planetary-scale circulation shifts and reduced warming of the Mediterranean Sea compared to land. We develop high-resolution regional climate simulations over Morocco to assess future risks from drought and weather extremes relevant to agriculture. Our results point to robust declines of 25-45% in annual precipitation and confirm physical drivers identified at the regional scale.
Because snow is such an important component of the water cycle in this semi-arid region, we also investigate snowpack dynamics in the High Atlas and we quantify components of the snow water balance for the first time. Future trends in snowpack and associated runoff are also investigated: at best, snowpack volume will decline by at least 60%, which, combined with increased air dryness, will likely reduce mountain runoff by 60%. Our findings have important implications for climate change adaptation and water management in Morocco, particularly in agriculture, which uses 90% of all available water.
by Alexandre Tuel.
Ph. D. in Hydrology
Ph.D.inHydrology Massachusetts Institute of Technology, Department of Civil and Environmental Engineering

Climate variability and the carbon cycle (C-cycle) are tied together in complex feedback loops and due to these complexities there are still knowledge-gaps of this coupling. However, to make accurate predictions of future climate, profound understanding of the C-cycle and climate variability is essential. To gain more knowledge of climate variability, the study aims to identify recurring spatial patterns of the variability of precipitation anomalies over Scandinavia during spring and summer respectively between 1981 to 2014. These patterns will be related to the C-cycle through changes in summer vegetation greenness, measured as normalized difference vegetation index (NDVI). Finally, the correlation between the patterns of precipitation variability in summer and the teleconnection patterns over the North Atlantic will be investigated. The precipitation data was obtained from ERA5 from the European Centre for Medium-Range Weather Forecasts and the patterns of variability were found through empirical orthogonal function (EOF) analysis. The first three EOFs of the spring and the summer precipitation anomalies together explained 73.5 % and 65.5 % of the variance respectively. The patterns of precipitation variability bore apparent similarities when comparing the spring and summer patterns and the Scandes were identified to be important for the precipitation variability in Scandinavia during both seasons. Anomalous events of the spring EOFs indicated that spring precipitation variability had little impact on anomalies of summer NDVI. Contradictory, summer precipitation variability seemed to impact anomalies of summer NDVI in central- and northeastern Scandinavia, thus indicating that summer precipitation variability modulates some of the terrestrial C-cycle in these regions. Correlations were found between a large part of the summer precipitation variability and the Summer North Atlantic Oscillation and the East Atlantic pattern. Hence, there is a possibility these teleconnections have some impact, through the summer precipitation variability, on the terrestrial C-cycle.
Förändringar och variation i klimatet är sammankopplade med kolcykeln genom komplexa återkopplingsmekanismer. På grund av denna komplexitet är kunskapen om kopplingen mellan klimatvariation och kolcykeln fortfarande bristande, men för att möjliggöra precisa prognoser om framtida klimat är det viktigt att ha kunskap om denna koppling. För att få mer kunskap om klimatvariation syftar därför denna studie till att identifiera återkommande strukturer av nederbördsvariation över Skandinavien under vår respektive sommar från 1981 till 2014. Dessa relateras till förändringar i sommarväxtlighetens grönhet, uppmätt som skillnaden i normaliserat vegetationsindex (NDVI). Även korrelationen mellan sommarstrukturerna av nederbördsvariationen och storskaliga atmosfäriska svängningar, s.k. "teleconnections", över Nordatlanten undersöks. Nederbördsdatan erhölls från ERA5 analysdata från Europacentret för Medellånga Väderprognoser och strukturer av nederbördsvariationen identifierades genom empirisk ortogonal funktionsanalys (EOF) av nederbördsavvikelser. De tre första EOF av vår- respektive sommarnederbördsavvikelser förklarade tillsammans 73,5 % respektive 65,5 % av nederbördsvariationen. Strukturerna av nederbördsvariation under vår respektive sommar uppvisade tydliga likheter sinsemellan. Dessutom identifierades Skanderna vara av stor vikt för nederbördsvariationen i Skandinavien under båda årstider. Avvikande år av nederbördsvariation under våren indikerade att sagda nederbördsvariation haft liten påverkan på NDVI-avvikelser under sommaren. Emellertid verkade nederbördsvariationen under sommaren påverkat NDVI-avvikelser under sommaren i centrala och nordöstra Skandinavien. Detta indikerar att nederbördsvariationen under sommaren till viss del styr den terrestra kolcykeln i dessa regioner. För nederbördsvariationen under sommaren fanns korrelation mellan både Nordatlantiska sommaroscillationen och Östatlantiska svängningen. Det finns således en möjlighet att dessa "teleconnections" har en viss påverkan på den terrestra kolcykeln genom nederbördsvariationen under sommaren.

Microbial Induced Calcite Precipitation (MICP) is an attractive alternative for a variety geotechnical ground improvement practices commonly used today and has a variety of potential applications. This research focuses primarily on its use as a soil stabilization technique using the bacteria Sporosarcina Pasteurii and a single injection point percolation method adapted from previous research in granular soils. This method, and most published data, show an inherent variability in both physical and engineering properties due to the distribution of precipitated calcite within the specimen. The focus of this research is on the quantification of the variability in shear strength parameters induced by MICP treatment in sand. Also, on the initial development of a new treatment method which aims to reduce this inherent variability and offer a more feasible option for field applications. The MICP treated soil columns were sampled at constant intervals from the injection point and then subject to direct shear testing (DST) and calcite distribution analysis. This analysis reiterates previously documented reduction in cementation as distance from injection point increases. The reduction in cementation results in reduced shear strength parameter improvements. This research also concluded a minimum of two percent mass of calcite per total mass of treated soil for significant strength improvements.

È noto che le precipitazioni invernali europee sono ben stimate nei dati di rianalisi e nelle simulazioni dai modelli dato che sono altamente correlate con modi con variabilità su larga scala e a bassa frequenza come l'oscillazione del Nord Atlantico (NAO) e l'oscillazione artica (AO). Dal momento che NAO e AO sono principalmente modi di variabilità invernale, la capacità di stimare le precipitazioni è più limitata nelle altre stagioni, soprattutto in estate, in cui (quando) le precipitazioni sono principalmente dovute alla convezione a mesoscala. La prima parte dello studio utilizza dati osservativi, dati di rianalisi e l'output del modello WRF (Weather Research and Forecast) per studiare i cambiamenti recenti negli eventi estremi di precipitazione giornaliera in Europa. Si è stato riscontrato che nelle stagioni estive e di transizione, sono più le regioni che hanno registrato un aumento di eventi di precipitazioni estreme rispetto a quelle che hanno registrato una tendenza al ribasso. Questo è coerente con le tendenze del riscaldamento globale con per via della relazione di Clausius-Clapeyron. è stato studiato anche il valore aggiunto dell'utilizzo di un modello ad alta risoluzione che consenta la convezione per stimare gli estremi delle precipitazioni. I risultati mostrano che WRF riesca a correggere il fallimento della rianalisi ERA-Interim per I risultati mostrano che WRF riesce, al contrario di ERA-Interim, a catturare le tendenze positive delle precipitazioni estreme europee sia in estate che nelle stagioni di transizione indicate dai dati osservativi (EOBS) e dalla letteratura precedente. D'altra parte, sono di più le aree in Europa che hanno registrato tendenze negative nelle precipitazioni estreme rispetto alle aree con tendenze positive. Questo si tratta di una conseguenza del recente trend positivo)della NAO negli ultimi decenni, che ha causato eventi NAO positivi più frequenti, riducendo le precipitazioni estreme in più regioni in Europa. Dato che sia la NAO che l'AO, che sono altamente correlate, stanno cambiando, sono state svolte ulteriori indagini sulla natura delle due oscillazioni. I dati di rianalisi e le simulazioni del modello climatico, (sia per run climatiche che rappresentative di condizioni a temperatura più elevata), dei climi storici e caldi sono stati utilizzati per mostrare che la relazione tra le due oscillazioni cambia con il riscaldamento climatico. Le due modalità sono attualmente altamente correlate, poiché entrambe sono fortemente influenzate dalla propagazione verso il basso delle anomalie del vortice polare stratosferico nella troposfera. Tuttavia, se si considera uno scenario climatico molto caldo, il modello AO definito emisfericamente si sposta per riflettere la variabilità della rotta delle tempeste del Pacifico settentrionale, mentre il modello NAO definito a livello regionale rimane stabile. La stratosfera rimane un importante precursore della NAO e le anomalie di pressione superficiale eurasiatica e aleutiana precedono le anomalie stratosferiche. Simulazioni con modelli idealizzati di circolazione generale suggeriscono che queste modifiche sono legate al riscaldamento più forte del Pacifico rispetto al riscaldamento più lento dell'Oceano Atlantico, dovuto al rallentamento della Overturning Meridional Circulation (AMOC).
European wintertime precipitation is known to be skilfully estimated in reanalysis data and model simulations since it is highly correlated with large scale, low frequency modes of variability, namely the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). Since the NAO and AO are mainly a wintertime mode of variability, the skill of estimating precipitation becomes more limited in the other seasons, most importantly in the summer, in which precipitation is mainly a result of mesoscale convection. The first part of the study uses observational data, reanalysis data, and the output of Weather Research and Forecast (WRF) model to study the recent changes of extreme daily precipitation events over Europe. It is found that in summer and transition seasons, more regions recorded an increase of extreme precipitation events than regions that recorded a decreasing trend. This is consistent with the global warming trends with Clausius-Clapeyron relation. The added value of using a high resolution, convection-permitting model to estimate precipitation extremes is deduced. The results show that WRF succeeds to correct the failure of ERA-Interim reanalysis to capture the positive trends of European extreme precipitation in summer and transition seasons that are indicated by the observational data (EOBS) and previous literature. On the other hand, more regions in Europe recorded negative extreme precipitation trends than regions with positive trends. This is found to be a consequence of the recent positive trend of the NAO over the past decades, causing more frequent positive NAO events, reducing extreme precipitation outbreaks to more regions in Europe. As the NAO and the highly correlated AO are changing, further investigations to the nature of the two oscillations are carried out. Reanalysis data and climate model simulations of historical and warm climates are used to show that the relation between the two oscillations changes with climate warming. The two modes are currently highly correlated, as both are strongly influenced by the downward propagation of stratospheric polar vortex anomalies into the troposphere. However, when considering a very warm climate scenario, the hemispherically defined AO pattern shifts to reflect variability of the North Pacific storm track, while the regionally defined NAO pattern remains stable. The stratosphere remains an important precursor for NAO, and surface Eurasian and Aleutian pressure anomalies precede stratospheric anomalies. Idealized general circulation model simulations suggest that these modifications are linked to the stronger warming of the Pacific compared to the slower warming of the Atlantic Ocean, that is due to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC).

Palaeoclimate reconstruction is of paramount importance because the more we know about the patterns of past variability the better we understand the present climate. Precipitation is a key climate variable shaping the culture and economy of China. However, few studies are concerned with past changes in precipitation. The existing few are either reconstructed by one reconstruction approach or based on a single proxy, limiting reconstructions to a small region and a short period of time. The study presents the first high-resolution multi-proxy palaeoprecipitation reconstruction of China as a whole. Proxies used in the study are of annual-to-decadal resolution, mainly including historical written evidence, tree-ring chronologies and stable isotopic records. In accordance with the principle of “uniformity in the nature”, about 80 proxy series extend the precipitation records as far back in time as 2,000 years ago when there was a significant increase in natural proxy and documentary records of climate all over China. The top three popular reconstruction approaches are borrowed from multi-proxy temperature reconstructions to reconstruct the palaeoprecipitation. A pioneering attempt is taken to evaluate and compare these approaches in reconstructing precipitation over a long term in a large scale. The three common practices are respectively simple average, composite-plus-scale method, and covariance-based climate field reconstruction. All three reconstruction methods are modified catering to the unique characteristics of precipitation, especially in terms of its large spatial variance. Local reconstructions are carried out first before a national index in reconstructed. Though based on the same documentary and natural proxies, three reconstruction approaches adopt different rationales to decipher the precipitation variability, represented by different statistical models. Each model is assessed by conventional statistics and cross-compared. These models are found complementary to each other while the common shared among three models are considered the closest to the true precipitation condition. Though geographically biased to Eastern China, historical documents still capture the most inter-annual to multi-centennial variations in palaeoprecipitation, indicating the possible control of Asian Monsoon climate on China’s overall precipitation. Tree-ring widths preform worst due in part to the attribute of strong locality. Each reconstruction approach turns out to have its own strength and weakness. Three major falls at multi-centennial timescale are detected in most reconstructed precipitation series, which are at AD600-800, AD1000-1300 and AD1500-1700. Spectral analyses demonstrate a rhythmic pattern of around 400-500 years for all reconstructions. When the proxies are not extrapolated, an additional cycle of 700-800 years is significant in precipitation variability over the past 2,000 years. A cross-comparison with European reconstructions show a high agreement in precipitation variability for the past two millennia, leading to the suspect of dry Medieval Warm Period and wet Little Ice Age over the whole Eurasia.
published_or_final_version
Geography
Master
Master of Philosophy

Summer precipitation variability is investigated over the England and Wales region. The England and Wales Precipitation dataset (EWP) is an area average estimate derived from rain gauges, taking into account temporal stability and representativeness of the gang<' stations used. The observed daily precipitation distribution is well represented by a Weibull fit. Regional daily precipitation accumulations show large variability over the region, hilt no robust trends are found in mean or extremes over the last 80 years. Comparing the observations with the ERA-Interim short term forecast shows that the ECMWF reanalysis system represents the variability in EWP observations (correlation of 0.91). The shape parameter of the Weibull fit to ERA-Interim precipitation is indistiuguishable from the EWP estimate, but the ERA-Interim estimates are scaled down by 22%. Light precipitation events are overestimated by the ERA-Interim reanalysis. A novel Lagrangian approach is used to investigate monthly precipitation variability during summer for England and Wales. The ROTRAJ trajectory model calculates back trajectories to link precipitation to the origins of water and identifies the evolution of th(' moistun' content of trajectories along their path towards England and 'Vales. Precipitation estimates in the ROTRAJ trajectory model are calculated using specific humidity changes along trajectories in the last 6 hours prior to arrival. The ROTRA'} estimate has similar skill to the ECWMF short term forecast for moderate and extn'llle events (4mm/day), but tends to overestimate the observed light precipitation events (4mm/day) more strongly than the ECI\IWF forecast when compared to EWP.

The thesis concerns different arguments of research, but strictly related between them. All the activities of research were carried out simultaneously during the period of study. The aim is to provide a more accurate rainfall estimation in mountainous areas, improving the capability to detect intense events leading to flash floods Another object is to investigate the role of rainfall spatial variability in flash flood triggering. Flash floods are caused by heavy precipitation over small area (usually not bigger than 500 km2), and have a timescale that normally does not exceed 12 hours. What makes flash flood very dangerous is their sudden nature. They cause great damages to the human activities. In order to reduce their effects and to get a real-time system monitoring, it is necessary to have an instrument that allows to have a spatial resolution of the data, that the conventional weather stations network not guarantees. For these reasons in recent decades had a strong diffusion the meteorological radar; this tool allows to locate the precipitation and to calculate the intensity of rain in real- time, with a resolution that no other instruments might reach. The meteorological forecasters use this instrument to provide short-time forecasts, named nowcasting (1-3 hours). For such short interval time the mathematical models are useless, and the only instruments that support the forecasters are the data from satellite and the rain maps from the radar. Radar data are unfortunately affected by several sources of error; between them the main important are the occlusion, the ground clutter, the attenuation, the vertical variation of the reflectivity. By using double-polarimetric radar operating at low wavelenght (X-band radar) is possible to reduce the effects of these errors, acquiring more accurate data than the ones provided by the traditional tool. A part of this work of thesis aims to assess if the eventual introduction of this new technology (double polarimetric radar operating at low wavelengths) in the Alpine area might ensure more accurate rain estimation. This instrument were tested during the summer of 2007 in Folgaria, and it was used in different conditions to monitorate the events over several hydrological basins. On the same area are available the data collected by two C-band radar, one managed by the Provincia Autonoma of Trento, and the other one by the Veneto Region. The aim of the study is to evaluate the advantages/disadvantages of these different technologies, and finally to set if by using this X-band polarimetric radar, would be possible to get a better description of the rainfall patterns, and to improve the predictability of flood events. The meteorological precipitations detected in real-time works as input for the hydrological models, that operate a simplification of the real processes to obtain a discharge hydrograph for different closing sections. The accuracy of the input data allows to improve the flood predictability and to reduce the catastrophic impacts that these event have on human activities. Within the present work the efforts have been focused in understanding the dynamics underlying the generation of a flood hydrograph; systematic studies were carried out to describe the accuracy of rainfall volumes at basin scale and the effect of spatial variability within the basin. The analysis focused on the flash flood event that affected the Weisseritz Region (Saxony) in the summer of 2002. Some statistics were applied that allow to put in direct relation the rainfall distribution, the shape of the basin, and their influence on the hydrograms at the closing section. Particularly, in a place with the characteristics of the pre-Alpine area, orography strongly influences the rainfall distribution. The events are mostly concentrated in the basins that are affected by a great increase of the precipitation with the altitude. The meteorological radar often describes only partially the precipitation in the mountain regions, because the beam, along its propagation path, can be occluded by orography. It often happens then, that the weather stations are located in the valleys, where the majority of the population lives, and where it is easier for the person assigned to control their working. For this reason, particularly in the last year of the studies, the efforts have been mainly focused in detecting some methodologies that might allow to estimate correctly the lapse rate of the precipitation (even in places where there are no rainfall measurements i.e. high mountains), and the total volumes of rain. The methodology uses the data collected by the tradition weather network, and allows to calculate rainfall maps for the whole territory The remainder of this dissertation is organised as follows: Chapter 1 → Short description of the main topics selected. Bibliography on the different arguments. Methodology applied to study flash flood events; what has been dome to mitigate the effects of these catastrophic events, and how to improve in understanding their dynamics. Chapter 2 → The meteorological radar. Data acquiring and processing. Most common errors. The double-polarimetric radar. The measure campaign of Folgaria: short description of the basin object of the analysis and events selected. Procedures and algorithms to correct the data. Different algorithms used and comparison. Analysis of the results and some statistics calculated by merging radar data and conventional raingauge network. Significant advantages incoming from this new technology. Chapter 3 → The Hydrological model KLEM. Characteristic of the model and simplifications. An hydrological event in the Weisseritz: short description of the characteristics of the basin and of the flash flood event of the summer of 2002. The application of the hydrological model. The role of rainfall spatial variability in flash flood triggering .Statistics to describe the relationship between the variability catchment scale and the rainfall distribution. Comments and conclusions Chapter 4 → Alto Adige: brief description of the territory and development of a methodology of spatialisation from the rainfall data. Underestimation of snowfall. Calculation of the rain maps. Verification of the results, comparing the estimated rainfall and the flow rates for different closing sections throughout the Province. The introduction of some additional weather stations of Austria, and its effect in achieving a more accurate estimate of the precipitation lapse rate. Possible future developments
La presente attività di ricerca abbraccia tematiche diverse tra loro, ma allo stesso tempo fortemente interconnesse. Esse sono state sviluppate simultaneamente nel corso dell’ attività di ricerca. L’ obiettivo finale è fornire una stima più accurata dei campi di pioggia in un territorio ad orografia complessa come quello pre-alpino, per poter migliorare la capacità previsionale di eventi di piena improvvisi. Si vuole inoltre valutare l’influenza che il movimento dei campi di pioggia ha nel generare l’idrogramma di piena finale, e metterlo in relazione al tipo di bacino e alla sua morfologia. Gli eventi di piena improvvisi (flash flood) si generano quando intense precipitazioni si riversano in un breve lasso temporale su un territorio circoscritto; essi. non superano la durata di 6-12 ore e coinvolgono bacini non superiori ai 500 km2. Data la loro improvvisa formazione essi producono notevoli disagi e danni anche ingenti. Per poter mitigare i loro effetti, e per poter arrivare a monitorare in ogni istante quello che succede sul territorio, è importante avere uno strumento che consenta di ottenere in tempo reale stime di pioggia con una risoluzione spaziale che la tradizionale rete pluviometrica non riesce a garantire. Per questo motivo nel corso degli ultimi decenni ha avuto una forte diffusione il radar meteorologico; questo strumento permette di localizzare e misurare le precipitazioni in tempo reale, consentendo di calcolare mappe di pioggia con una risoluzione spazio-temporale che nessun altro tipo di strumento consente di ottenere. I meteorologi utilizzano il radar meteorologico per formulare previsioni a brevissimo termine (1-3 ore), comunemente definite nowcasting. Per intervalli temporali così limitati difatti i modelli matematici, generalmente utilizzati per formulare le previsioni per i giorni successivi, sono insufficienti, e gli unici strumenti che possano supportare i previsori sono i rilevamenti dati dal satellite e dal radar. Le misure rilevate dai radar sono però soggette a diverse forme di errore che possono alterare la qualità delle misure. Tra questi vale la pena citare l’occlusione, il ground clutter, l’attenuazione e la variazione del profilo di riflettività con la quota. L’impiego di radar a doppia polarizzazione operanti nel campo delle basse lunghezze d’onda (radar in banda X) consente di ridurre l’effetto di questi errori, rispetto al tradizionale radar meteorologico. Parte del lavoro di tesi è volto a valutare se l’introduzione di questi strumenti innovativi (radar a doppia polarizzazione) possa garantire nell’area pre-alpina misure dei campi di pioggia più accurate. A tale scopo ci si avvale delle misure rilevate nel corso dell’estate del 2007 da un radar a doppia polarizzazione installato sull’altipiano di Folgaria (Trento). Su tale area operano, accanto a una fitta rete di pluviometri disseminati alle varie quote, due radar doppler in banda C appartenenti al sistema nazionale (uno gestito dalle Province di Trento e Bolzano, l’altro di proprietà della Regione Veneto). Si vuole pertanto valutare la qualità delle misure associate alle diverse tecnologie, e in pratica testare se un' eventuale introduzione di questi radar (doppio polarimetrici) garantirebbe in territorio montano una migliore descrizione dei fenomeni di precipitazione e una maggiore tutela nei confronti di fenomeni alluvionali improvvisi. Le misure di pioggia vengono rilevate in tempo reale e vengono fornite ai diversi modelli idrologici, che le utilizzano per formulare una previsione dell’idrogramma di piena finale, relativo alle diverse sezioni di chiusura di interesse. Si capisce quindi come la possibilità di disporre di misure quanto più precise e rappresentative permetta di dare come input ai modelli valori più rappresentativi e di conseguire una buona capacità previsionale, migliorando la capacità predittiva di questi eventi catastrofici. All’ interno del percorso di ricerca parte degli sforzi sono stati volti a migliorare le conoscenze di come lavorano questi modelli idrologici, e a comprendere se il movimento dei campi di pioggia durante l’evento abbia una reale influenza sull’andamento dell’idrogramma. Si vuole quindi capire se per un dato bacino è sufficiente avere una stima dei volumi complessivi di precipitazione caduti, oppure è importante conoscere la distribuzione e il movimento dei campi di pioggia per ottenere simulazioni più accurate. All’interno del presente lavoro si è cercato di capire quali dinamiche siano sottese alla generazione di un idrogramma di piena, e si è cercato di sviluppare una metodologia che consenta di valutare per ogni singolo bacino quale influenza abbia la dinamica dell’evento nel generare la piena finale. Si è a tale scopo analizzato un evento specifico (evento di piena del 2002 che ha interessato il bacino del Weisseritz in Sassonia), e sono stati sviluppati alcuni statistici che consentono di metter in relazione la distribuzione dei campi di pioggia e l’effetto che essi producono nella generazione dell’idrogramma dio piena finale. In particolare in un territorio come quello pre-alpino l’orografia influenza fortemente la distribuzione delle precipitazioni. Gli eventi più significativi si concentrano nei bacini in cui la particolare orografia determina un forte incremento delle precipitazioni con la quota, Il radar meteorologico non sempre riesce a descrivere correttamente l’andamento delle precipitazioni nei territori montani, in particolare in quei bacini in cui il fascio risulta occluso. Capita poi sovente che le stazioni meteorologiche siano concentrate nel fondovalle ove risiede la maggior parte della popolazione e dove è più agevole il controllo dal personale qualificato. Per questo motivo, in particolare nell’ultimo anno dell’attività di ricerca, gli sforzi sono stati maggiormente indirizzati a sviluppare altre tecniche che consentano di stimare correttamente i volumi complessivi di pioggia, anche nei punti in cui non sono disponibili misure pluviometriche, e quindi in particolare in alta montagna. Lo studio si propone di partire dai dati di precipitazioni rilevati dalla tradizionale rete pluviometrica . e di sviluppare una metodologia di spazializzazione, che consenta di ottenere delle mappe di pioggia continue e rappresentative per tutto il territorio. La struttura dell’elaborato di tesi si presenta così articolata: Capitolo 1 → Fornisce una breve inquadratura dell’argomento prescelto. Viene fornita una rapida illustrazione dello stato dell’arte, delle metodologie che vengono applicate per studiare eventi di piena improvvisi, di cosa viene fatto per mitigarne gli effetti, e degli studi che vengono condotti per migliorare la comprensione di questo tipo di eventi. Capitolo 2 → Il radar meteorologico. Principi di funzionamento. Principali forme di errore. Il radar doppio polarimetrico. L’esperimento di Folgaria: breve descrizione del bacino monitorato e eventi selezionati. Procedure di correzione applicate ai dati. Confronto tra i diversi algoritmi di calcolo. Analisi dei risultati ottenuti e calcolo di indici statistici, ottenuti confrontando tra loro i dati radar e i dati pluviometrici. Significativi vantaggi che un eventuale introduzione di questo tipo di strumenti consentirebbe di ottenere. Capitolo 3 → Il modello idrologico KLEM. Schema di funzionamento e semplificazioni adottate. Il caso del Weisseritz: breve descrizione del bacino e dell’evento di piena dell’estate del 2002. Applicazione del modello idrologico. Influenza spaziale delle precipitazioni nella generazione dell’idrogramma di piena .Sviluppo di alcuni statistici che permettono di mettere in relazione tra loro l’andamento delle precipitazioni all’interno del bacino, e la morfologia stessa del bacino. Commenti e conclusioni. Capitolo 4 → Alto Adige: breve descrizione del territorio e sviluppo di una metodologia di spazializzazione delle precipitazioni, partendo dai dati pluviometrici. Sottostima delle precipitazioni nevose. Calcolo delle mappe di pioggia. Verifica dei risultati ottenuti, confrontando tra loro le precipitazioni stimate, e le portate misurate, in prossimità delle diverse sezioni di chiusura disseminate nel territorio provinciale. Effetto dell’ introduzione di alcune stazioni meteorologiche austriache, per conseguire una stima più precisa dell’incremento di precipitazione con la quota. Possibili sviluppi futuri.

Spatial variability of precipitation is examined over the state of Kentucky and surrounding areas. The study focuses on the analysis of monthly precipitation totals from the period of 1961-2000. The purpose of the study is to develop a set of indices to represent the spatial variability of the study area for a given month. Various exploratory data analysis methods such as variography, kriging, and cluster analysis were used. The study attempts to quantify the second order (local) effects of the spatial variation of precipitation as a means to provide insight into the prediction of precipitation randomness. This task can be a difficult one due to the distinction between first and second order effects being somewhat arbitrary. The study proposes that a qualitative map of mean monthly precipitation can be classified through the use of a quantitative measure. This approach allowed for the unique classification of numerous months of precipitation through the use of a standard methodology. The researcher found that trying to capture the spatial variation of precipitation with two indices is an arduous task. Months were classified based on percentiles of the variogram cloud. The data were condensed into distance bins for analysis and used for calculation of the indices. A Short Range Index (SRI) and Long Range Index (LRI) were calculated for each month. The indices were then analyzed through the use of cluster analysis. The Partioning Around Medoids (PAM) method was used for the analysis providing an average silhouette value of .32. The study found that the methods applied did not efficiently capture the spatial variability of precipitation across the study area. However, this study has provided insight into the methodologies that can be applied to investigate spatial patterns of precipitation.

This PhD thesis investigates the relative importance of oceanic and atmospheric influences on extremes, long-term trends, and seasonal to interannual variability of precipitation for different regions in the Southern Hemisphere in observations, reanalysis data, and output from general circulation models (GCM). Examination of interannual rainfall extremes over southwest Western Australia (SWWA) reveals a characteristic dipole pattern of Indian Ocean sea surface temperature anomalies (SSTA). This coincides with a large-scale reorganization of the wind field over the tropical/subtropical Indian Ocean changing SSTA, via anomalous Ekman transport in the tropical Indian Ocean and via anomalous air-sea heat fluxes in the subtropics, and altering moisture advection onto SWWA. The potential impact of these Indian Ocean SSTA in driving modulations of mid-latitude precipitation across southern and western regions of Australia is assessed in atmospheric GCM simulations. The SSTA give rise to changes in the thermal properties of the atmosphere, meridional thickness gradient, subtropical jet, thermal wind, and baroclinicity over southern regions of Australia, thus modulating precipitation. In addition, links between anomalous wet conditions over East Africa and these characteristic Indian Ocean SSTA are explored during the "short rain" season in October-November. Interannual extremes m New Zealand rainfall and their modulation by modes of Southern Hemisphere climate variability, namely the Southern Annular Mode (SAM) and El Nino-Southern Oscillation (ENSO), are investigated. Late twentieth Century trends in New Zealand precipitation are examined for the period 19792006 to quantify the relative impact of recent changes in the large-scale atmospheric circulation related to the SAM and ENSO. Increasingly drier conditions over much of New Zealand can be partially explained by the SAM and ENSO. Cool season rainfall variability in southeastern Australia is investigated via a classification and characterization of the predominant types of synoptic systems occurring in the region, focusing on frontal and cutoff low systems. Two definitions of the autumn break developed for northwestern Victoria are employed to produce a synoptic climatology of the break phenomenon. Trends in characteristics of the autumn break indicate that the most recent drought in southeastern Australia is comparable in severity with the two major droughts in the twentieth Century.

Thesis (doctoral) - Delft University of Technology, Delft, 2002.
"Dissertation submitted in fulfillment of the requirements of the Board for Doctorates of Delft University of Technology and of the Academic Board of the International Institute for Infrastructural, Hydraulic and Environmental Engineering for the Degree of Doctor to be defended in public on Monday, 29 April 2002 at 13:30 hours in Delft, The Netherlands." Includes bibliographical references (p. [191]-199).

As population and urbanization increase across the United States, the effects of natural hazards may well increase, as extreme events would increasingly affect concentrated populations and the infrastructure upon which they rely. Extreme precipitation is one natural hazard that could stress concentrated populations, and climate change research is engaging heavy precipitation frequency and its impacts. This research focuses on the less-studied phenomenon of an extreme precipitation reversal - defined as an unusually wet (dry) period that is preceded by an unusually dry (wet) period. The magnitude is expressed as the difference in the percentiles of the consecutive periods analyzed. This concept has been documented only once before in a study that analyzed extreme precipitation reversals for a region within the southwestern United States. That study found that large differences in precipitation from consecutive winters, a hydrologically critical season for the region, occurred more frequently than what would be expected from random chance, and that extreme precipitation reversals have increased significantly since 1960. This research expands upon the previous work by extending the analysis to the entire continental United States and by including multiple temporal resolutions. Climate division data were used to determine seasonal and annual precipitation for each of nine climate regions of the continental United States from 1895-2014. Precipitation values were then ranked and given percentiles for seasonal and annual data. The season-to-season analysis was performed in two ways. The first examined consecutive seasons (e.g., winter–spring, spring–summer) while the second analyzed the seasonal data from consecutive years (e.g., spring 2014–spring 2015). The annual data represented precipitation for the period October 1–September 30, or the 'water year' used by water resource managers. Following the approach of the previous study, a secondary objective of the research was to examine large-scale climate teleconnections for historical relationships with the occurrence of precipitation reversals. The El Nino-Southern Oscillation was chosen for analysis due to its well-known relationships with precipitation patterns across the United States. Results indicate regional expressions of a propensity for extreme precipitation reversals and relationships with teleconnections that may afford stakeholders guidance for proactive management. Precipitation reversal (PR) and extreme precipitation reversal (EPR) values were significantly larger for the second half of the study period for the western United States for the winter-to-winter, spring-to-spring, and year-to-year analyses. The fall-to-fall analysis also revealed changes in PR/EPR values for several regions, including the northwest, the Northern Rockies and Plains, and the Ohio Valley. Relationships between the winter-to-winter PR time series and an index representing the El Nino-Southern Oscillation (ENSO) phenomenon were examined. The winter-to-winter PR time series of the Northern Rockies and Plains region and the South exhibited significant relationships with the time series of Niño 3.4 values. El Niño (La Niña) coincided with more wet-to-dry (dry-to-wet) PR/EPR values for the Northern Rockies and Plains, while El Niño (La Niña) coincided with more dry-to-wet (wet-to-dry) PR/EPR values for the South.
Master of Science

It has been demonstrated in past studies that drainage area (Ada) and mean annual precipitation depth (Pmean) are significant independent variables for predicting bankfull channel geometry. Bankfull width (Wbf), a particularly useful measure of bankfull channel geometry, is used in hydrology, fluvial geomorphology and stream ecology applications. Since precipitation depth does not generally have a normal distribution over a basin it is reasonable to question whether or not Pmean is the best measure of precipitation across a basin. That is, perhaps the median precipitation depth (P50) or the mean annual precipitation depth at the basin outlet (Poutlet) is more representative of precipitation in a basin. The objective of the study is to see if Pmean, P50, or Poutlet is a significantly better measure of precipitation in a watershed. Along with Ada, each of these precipitation measures was used to develop relationships for predicting Wbf for basins located across the continental U.S. Using several goodness-of-fit statistics i.e., coefficient of determination(R2), Standard error of estimates(SEE) and Akaike Information Criterion(∆AIC), the relationships were compared to determine which one had the greatest predictive strength. Estimates of Pmean, P50, and Poutlet were derived from a precipitation model developed by PRISM. Results from this study tentatively indicate that all three measures of precipitation depth yield models for predicting Wbf that have comparable goodness-of-fit statistics.

Asian climate varies on various spatial and temporal scales and has a wide spectrum of climatic characteristics. Climate variability, especially decadal to inter-annual scale rainfall variability across Asia has gained considerable attention of climatologists over the last century due to the fact that rainfall variability is known to have caused considerable damage to southern Asian nations. Until recent, much of the existing literature on southern Asian climate focused on India and it is only recently that studies have focused on countries other than India. Although the Maldives is a nation within southern Asia (lying in the Indian Ocean southwest of India), literature on precipitation patterns over the Maldives and its connection to the Asian monsoon is lacking. This thesis examines the variability of precipitation over the Maldives in relation to the Asian monsoon, since proper knowledge of the spatial and temporal variations of precipitation is essential for managing the water resources and agricultural sector of the Maldives. Yearly and monthly rainfall across the Maldives indicates that the rainfall varies temporally and spatially. Despite spatial variability of mean annual rainfall (January-December total) showing rainfall increasing from north to south, it was found that on average the northern and southern parts of the Maldives have received less rainfall during the monsoon season (May-November). This suggests that the mean annual rainfall maximum for the Maldives occurs between central and southern parts of the Maldives during the monsoon season. The Maldives monsoon rainfall is characterised by inter-decadal and inter-annual periodicities with a frequency of 12.9 and 2.5-4 years, and intra-seasonal periodicities (10-20 days and 30-60 day) in daily time series of monsoon rainfall for different regions of Asia. The fact that no objective criteria previously existed to identify monsoon onset and withdrawal dates in the Maldives, the criteria developed here for defining the monsoon season objectively for this region indicates that on average the rainy season or monsoon commences between 4 May and 13 May (mean onset dates based on outward longwave radiation (OLR) index and rain and wind criteria, respectively) and terminates in late November (21 and 23 November: mean withdrawal dates based on rain and wind, and OLR index criteria, respectively) for the Maldives. The mean length of the rainy season (LRS) based on the OLR index is 204 days, the mean LRS based on rain and wind is 11 days shorter (193 days). Results also demonstrate that the earliest monsoon onset for the Asian region occurs in the south of the Maldives in April. Correlation coefficient maps generated between Maldives monsoon rainfall and meteorological parameters suggest that the most significant parameters that influence the interannual variability of the Maldives monsoon rainfall (MMR) are mean sea level pressure, surface air temperature, OLR, sea surface temperature (SST), and the zonal wind and relative humidity at various levels. Temporal consistency checks carried out for these parameters with the MMR led to the elimination of some of these predictors (which have less influence in the variance of MMR). The predictors which explained a significant amount of variance in the MMR were retained, including surface relative humidity during April (SRHAPR), 850 hPa level relative humidity during May (850RHMAY) and 500 hPa relative humidity for May (500RHMAY). These parameters were then used to formulate a regression model (using backward regression) for the prediction of Maldives monsoon rainfall. The predictors included in the model account for a significant part of the variance (76.6%, with a correlation coefficient, CC = 0.9) in MMR, indicating the usefulness of the model for medium-range prediction of MMR before the core monsoon season commences. Global scale processes such as the El Niño-Southern Oscillation (ENSO) phenomenon influence the weather and climate around the globe, with ENSO considered to be one of the strongest natural phenomena influencing the climate of Asia on inter-annual time-scales. The association between the Maldives monsoon rainfall and ENSO events demonstrates that deficient/excess monsoon rainfall over the Maldives and India region is linked to the strong/moderate El Niño and La Niña events, respectively. During strong/moderate El Niño events, about 71.4% of the time the Maldives/India region experiences deficiencies in monsoon rainfall, while the Maldives/India region experiences excessive monsoon rainfall about 75% of the time during strong/moderate La Niña events. One of the regional scale processes that influence the climate of Asia is Eurasian snow cover. No previous studies have directly examined possible relationships between Eurasian snow and Maldives monsoon rainfall. The possible relationship between Eurasian snow cover (ESC) and the Maldives monsoon rainfall, explored in this research for the first time, appears to be only very weak. The results also demonstrate that the inverse relationship between the ESC and the Indian monsoon has weakened over recent decades. The correlation coefficient (-0.34) between Indian monsoon rainfall and ESC obtained for the 1973-94 period dropped to -0.18 for the 1979-2007 period. The inter-annual variability of the Indian and Australian monsoon rainfall experiences a remarkable biennial oscillation, which has been referred to as the tropospheric biennial oscillation (TBO). It is believed that the land and ocean surface conditions in March-May (MAM) over the Indo-Pacific region play an important role in monsoon transitions. The Maldives monsoon rainfall transition from relatively strong/weak to relatively weak/strong in consecutive years demonstrates a TBO connection (via a biennial tendency in Maldives monsoon rainfall). In relation to the Maldives monsoon rainfall, TBO strong years occur about 47.1% of the time, while weak TBO years occur about 52.9% of the time. Only some of the El Niño and La Niña onset years correspond to strong TBO years, with El Niño onset years (1982, 1987 and 2002) corresponding to weak TBO years, while La Niña onset years (1988 and 2000) corresponding to strong TBO years. Variability (spatial and temporal) in Maldives precipitation associated with global and regional scale processes results in flood and drought events that have downstream impacts, such as on water resources and the agricultural sector of the Maldives. Excess (wet) or deficient rainfall years identified for the period 1992-2008 indicate that the central region is most vulnerable to flooding (5 years with excess rainfall: 27.8% of the time), while the southern region is least vulnerable to both flooding (2 years with excess rainfall: 11.1% of the time) and drought (2 years with deficit rainfall: 11.1% of the time). The northern and central regions show an equal number of years with deficit rainfall (3 years: 16.7% of the time), indicating that they are equally prone to drought events. Furthermore, field survey results demonstrate that about 23, 31 and 37% households (respondents) from the northern, central and southern regions experienced flood events. About 79, 58 and 77% of the farmers from the northern, central and southern areas also experienced floods on their farms. On the other hand, field survey results also suggest that the 49-63% of the households in outer islands of the Maldives and 48-62% of farmers experience shortage of rainwater.

Throughout the Holocene, bison have always been more abundant east of the Rocky Mountains with considerably fewer bison found west of the Rocky Mountains. It is likely that drought frequency and snowfall characteristics have influenced the pattern of historical bison occurrence across the northwest United States. Using monthly average snow and precipitation data from the past several decades, average April snow water equivalent (SWE) and summertime drought frequency were analyzed at sites across the northwest United States. A climatic stress index (CSI) was developed by combining average SWE and drought frequency for sites, as these are the climate factors that will most likely affect bison success. The results of the CSI revealed that locations west of the Rockies experience heavier snowfall and a greater frequency of droughts, thus presenting a “double whammy” of climate conditions that bison would have to endure. The locations of highest combined snow and drought frequencies coincide with locations of low bison occurrence.

As precipitation is a very important parameter of climate and hydrology, exploring spatial and temporal distribution and variation of this variable can give an idea about climate conditions and water resources in the future. Therefore accurate mapping of the temporal and spatial distributions of precipitation is important for many applications in hydrology, climatology, agronomy, ecology and other environmental sciences. In this thesis, spatiotemporal distributions and variations of total annual, seasonal and monthly precipitation of the Eastern Mediterranean (EM) are analysed. The Eastern Mediterranean is one of the most prominent hot spot of climate change in the world further; extreme climatic events such as drought are expected to become more frequent and intense in this region. Main data source is instrumental data of monthly and daily precipitation at 103 and 70 meteorological stations, respectively. Spatial coherence analysis, coefficient of variation (CV), rainfall seasonality index (SI), incomplete gamma distribution and precipitation concentration index (PCI) are applied to evaluate the seasonality and variability of annual, seasonal and monthly precipitation amounts and their distribution. Daily precipitation concentration index (CI) is used as an important index for specifying daily rainfall characteristics. Additionally, rainfall entropy is also calculated for monthly and daily data for finding the most suitable probability distribution under the available information. High to moderate irregularity and rainfall concentration are the two very characteristic features of rainfall in the EM. Highest values of daily CI are detected in the southern parts of the EM. Distribution of annual precipitation CI trends indicate a statistically significant increase in the northern and northwestern regions of the EM. Four meteorological drought indices (DIs) are calculated at monthly time scale, the Standardized Precipitation Index (SPI), the Modified China Z Index (MCZI), the Statistical Z -scores and the Rainfall Decile based Drought Index (DI) while the Effective Drought Index (EDI) is calculated at daily time scale. All selected DIs with multiple time steps are applied to compute the severity for five time steps of 3, 6, 9, 12 and 24-month, and compared with each other and EDI. The most significant factor affecting the spatial accuracy of drought indices is seasonality. Study of DIs shows that the DIs are highly correlated at same time steps and can alternatively be used and the DIs computed for 6 and 9-month time step are best correlated with each other. SPI and MCZI are more consistent in detecting droughts for different time steps. EDI is found to be best correlated with other DIs when considering all time steps. The investigation shows that the use of an appropriate time step is as important as the type of DI used to identify drought severities. Principal Component Analysis (PCA) is employed and reveals the main modes and spatiotemporal variability of seasonal and annual precipitation and droughts over the EM. The preliminary analysis indicated tendencies towards a drier climate due to a statistically significant decrease in annual precipitation over the EM. The analysis of indicators of extreme events reveals a much more complex transformation of the climatic pattern with strong regional and seasonal variation. The results demonstrated that an extreme wet spells in the EM will shorten in all seasons, except autumn. Precipitation extremes are projected to become more pronounced in the northern parts of the EM than in southern ones and tend to be more significant during autumn. Extreme and heavy precipitation events showed a statistically significant decrease in whole parts of the EM and in the southern ones, respectively with a significant decreasing in total precipitation amount. A significant increase in daily intense precipitation events in the northern parts of the EM. In addition, climate extreme indices recommended by the joint World Meteorological Organization (CCL/CLIVAR/JCOMM) Expert Team on Climate Change Detection and Indices (ETCCDI) are also calculated for daily precipitation data. Finally, Mann- Kendall test, cumulative sum chart (CUSUM), regime shift index (RSI) calculated by a sequential algorithm are applied for detecting the shifts in the means of seasonal and annual precipitations. Mann-Kendall test and the linear slopes of trends are calculated using Sen’s slope estimator to determine the trend magnitude for SPI, CI, extreme precipitation indices, dry and drought periods. The highest decreasing of rainfall amounts is found in spring and winter and these seasons are the overall driving factor of trends in annual precipitation. Overall results of the trend analysis on the reconstructed shows that over last 52 years the drought events are more sever and frequent after 1990s over the EM which reflects negative effects on socio- economic sectors as well as water resources in this region. The findings of this study could be used or extended in further studies in the future to gain insights regarding the precipitation variability, drought patterns and extreme events over the domain of the EM.
La precipitación es una variable muy importante del clima y la hidrología. La exploración de su distribución espacial y temporal y su variación puede dar una idea acerca de las condiciones del clima y los recursos hídricos en un futuro. Por lo tanto, el mapeo amento? Preciso de la distribución temporal y espacial resulta importante para muchas aplicaciones ya sea en hidrología, climatología, agronomía, ecología, y otras ciencias ambientales. En esta tesis, se analizan las distribuciones espacio-temporales y variaciones de la precipitación total anual, estacional y mensual en el Mediterráneo Oriental (MO). El Mediterráneo Oriental es uno de los puntos calientes más prominentes al cambio climático por lo que espera que los eventos climáticos extremos, como las sequías, sean más frecuentes e intensas en esta región. La principal fuente de datos es instrumental, de estaciones meteorológicas. Con un total de 103 a escala mensual y 70 a escala diaria. El análisis espacial de coherencia, el coeficiente de variación (CV), el índice de estacionalidad y precipitaciones (SI), la distribución gamma incompleta y el índice de concentración de la precipitación (PCI), se aplican para evaluar la estacionalidad y la variabilidad de las cantidades de precipitación anual, estacional y mensual y su distribución. El índice de concentración de la precipitación diaria (CI) se utiliza como índice imprescindible para la especificación de las características diarias de lluvia. Además, la entropía de las precipitaciones también se calcula para los datos mensuales y diarios con el fin de encontrar la distribución de probabilidad más adecuada de la información disponible. Una irregularidad moderada y la concentración de las precipitaciones son los dos rasgos más característicos de las precipitaciones en la EM. Los valores más altos de CI diario se detectan en las partes meridionales de la EM. La distribución de las tendencias anuales de (CI precipitación) indican un aumento estadísticamente significativo en las regiones del norte y noroeste de la EM. Cuatro índices de sequía meteorológica (DIs) se calculan a escala de tiempo mensual, el Índice de Precipitación Estandarizada (SPI), la modificación de China Índice Z (MCZI), los -scores Z estadísticos y el Índice de Sequía basada precipitaciones débil (DI), mientras que la sequía Efectiva Índice (EDI) se calcula en la escala de tiempo diario. Todos los DIs seleccionados con múltiples pasos de tiempo se aplican para calcular la gravedad durante cinco pasos de tiempo de 3, 6, 9, 12 y 24 meses, y se compararon entre sí y con EDI. El factor más importante que afecta a la precisión espacial de índices de sequía es la estacionalidad. El estudio de DIs muestra que los DI están altamente correlacionados en mismos pasos de tiempo y de forma alternativa se pueden utilizar, y los DI calculados para los pasos de tiempo de 6 y 9 meses son los más correlacionados entre sí. SPI y MCZI son más consistentes en la detección de las sequías para los diferentes pasos de tiempo. El EDI tiene la mejor correlación con otros DIS en diferentes pasos de tiempo. La investigación muestra que el uso de un paso de tiempo apropiado es tan importante como el tipo de DIs utilizado para identificar severidades de sequía. Los análisis de los Componentes Principales (PCA) se emplean y revelan los principales modos de la variabilidad espacio-temporal de la precipitación estacional, anual y también los episodios de las sequías del MO. El análisis preliminar indica tendencias hacia un clima más seco debido a una disminución estadísticamente significativa en la precipitación anual durante el MO. El análisis de los indicadores de los fenómenos extremos revela una transformación mucho más compleja de la pauta climática con fuerte variación regional y estacional. Los resultados demostraron que episodios de lluvia extrema en el EM se acortarán en todas las estaciones, excepto otoño. Las precipitaciones extremas se proyectan a ser más pronunciadas en la parte norte de la MO que en las del sur, y tienden a ser más significativas en el otoño. Eventos extremos de precipitación mostraron una disminución estadísticamente significativa en partes enteras de la EM, y en las del sur, respectivamente, con una significativa disminución en la cantidad total de precipitación y un aumento significativo de las precipitaciones intensas diarias en la parte norte de la EM. Además, los índices de extremos climáticos recomendados por la Organización Meteorológica Mundial (CCL / CLIVAR / CMOMM), con un equipo de expertos sobre detección del cambio climático e índices (ETCCDI), también se calculan para los datos de precipitación diaria. Por último, la prueba de Mann-Kendall, carta suma acumulativa (CUSUM), el índice de cambio de régimen (RSI), calculado mediante un algoritmo secuencial fueron aplicadas para detectar los cambios en el medio de las precipitaciones estacionales y anuales. La prueba de Mann-Kendall y las pistas lineales de las tendencias se calculan utilizando la pendiente estimadora de Sen para determinar la magnitud tendencia de SPI, CI, los índices de precipitación extrema, los períodos secos y sequías. La mayor disminución de las cantidades de lluvia se encuentra en la primavera e invierno. Estas estaciones son el factor de conducción general de las tendencias de la precipitación anual. Y los resultados generales del análisis de las tendencias en los programas reconstruidos indican que durante los últimos 52 años las sequías son más frecuentes después el año 1990 sobre el MO, los cuales reflejan los efectos negativos sobre los sectores socioeconómicos, así como los recursos hídricos en esta región. Los hallazgos de este estudio podrían utilizarse o ampliarse en otros estudios en el futuro para obtener información con respecto a la variabilidad de la precipitación, patrones de sequía y fenómenos extremos sobre el dominio del MO.

Monthly and annual changes in precipitation in South China during 1990-2011 are examined in this study. As located in the sub-tropical climate zone, the annual rainfall in South China is concentrated in the summer. During the same period of time, an average of 9 tropical cyclones approaching South China each year which also contribute ample of rainfall to South China during the tropical cyclone season (June-September). Since the climatic system is interconnected, so this study attempts to examine the combined effect on inter annual rainfall variations in South China due to El Nino Southern Oscillation (ENSO), tropical cyclones affecting South China and the Asian Monsoon. It is generally agreed that during the El Nino onset years, South China would be more wetter, but with less than average tropical cyclones affecting South China. As such, effects on precipitation in South China caused by these two factors offsetting each other and data indicate there is no consistent trend of either more or less rainfall recorded for the El Nino onset years during 1990-2011. On the other hand, the first half of the year right after the El Nino years are in general more wetter than normal. This can be due to the lagging effect of El Nino events normally happened in the winter time, plus the fact that the strength of winter monsoon during the El Nino years is weak, resulting in more rainfall in the following year. Furthermore, annual changes in precipitation in South China directly affect the surface runoff of the Pearl River and a strong positive correlation between the two variables exists for the period 2000-2011. The +/- 25% annual variations and the seasonal variations of rainfall in South China post a challenge to the water resource management in the Pearl River Delta Region. Therefore dams and reservoirs built along the Pearl River are necessary to store surplus rainwater in order to supply sufficient fresh water to the Pearl River Delta Region during the dry seasons or years. On the other hand, further studies and monitoring programs are recommended to continue assessing and evaluating impacts of dam constructions to the environment and the ecosystem of the Pearl River Delta Region.
published_or_final_version
Applied Geosciences
Master
Master of Science

Resumo: O objetivo deste trabalho foi analisar as variabilidades espacial, sazonal e interanual da precipitação pluviométrica nas regiões Sudeste e Sul do Brasil. Foram utilizadas 164 séries mensais e anuais de chuvas para o período de 1972 a 2002 e dados de médias mensais de radiação de ondas longas emergentes (ROLE), para o período de junho de 1974 a dezembro de 2002, com pontos de grade com resolução espacial de 2,5º x 2,5º, entre as latitudes 20ºN e 35ºS e longitudes entre 0º; 90ºW. A partir das análises de agrupamento, de componentes principais e geoestatística foram identificadas 13 regiões homogêneas de regime de precipitação. A precipitação anual variou de 956 mm nos vales do São Francisco- Jequitinhonha a 1830 mm na Bacia do Iguaçu-Chapecó. As três primeiras componentes principais (CP) da matriz de médias mensais de precipitação explicam conjuntamente, 61,49% da variância. A primeira CP representa o campo médio de precipitação, a segunda CP representa o período de verão e a terceira CP apresenta dois pontos extremos: o mês de novembro e o mês de fevereiro. Através dos dados de ROLE analisou-se a influência da Zona de Convergência do Atlântico Sul (ZCAS) na precipitação. As três primeiras componentes principais da matriz de médias mensais de ROLE explicam conjuntamente 96,27% da variância. A primeira CP representa a convecção média. A segunda CP representa o período de primavera e a terceira CP representa o período de verão. As correlações, significativas ao nível de 5%, obtidas entre os autovetores das matrizes de precipitação pluviométrica e de ROLE, corroboram a influência da ZCAS no regime de precipitação das regiões Sudeste e Sul do Brasil.
Abstract: The spatial, seasonal and interannual variability of rainfall in Southeastern and Southern Brazil were analyzed. For that, 164 series of rainfall for the period of 1972 to 2002 and data about outgoing longwave radiation (OLR) for the period of 1974 to 2002 were utilized. From the clusters analysis and components analysis (PCA), 13 homogeneous regions under precipitation regime were identified. The annual rainfall varied from 956 mm in São Francisco-Jequitinhonha valleys to 1830 mm in Iguaçu-Chapecó basin. The three first matrix principal components (PC) of monthly average rainfall (average of 30 years) explain together 61.49% of the variance. The first PC represents the mean rainfall field, the second PC represents the extended rainy period (September to May) and the third PC presents two extreme points: November indicates the end of spring and the beginning of summer; and February indicates the end of summer and the beginning of autumn. The OLR influence on the rainfall variability in the Southeast and in the South was also evaluated. Through the PCA, the behavior of the South Atlantic Convergence Zone (SACZ) was studied together with its influence on the rainfall variability. The three first matrix main components of OLR monthly average (average of 30 years) explain together 96.27% of the variance. The first main component (CP) is associated to the period of autumn. The second PC represents the winter/summer period, and the third PC represents summertime. The correlations, significative to the level of 5%, obtained between the rainfall and OLR eigenvectors, attest their influence on the rainfall regime in Southeastern and Southern Brazil.
Orientador: Luzia Aparecida Trinca
Coorientador: Jonas Teixeira Nery
Banca: Angelo Cataneo
Banca: João Francisco Escobedo
Banca: Shigetoshi Sugahara
Banca: Paulo Henrique Caramori
Doutor

Numerous studies have analyzed isotopic variation of meteoric and dripwater in karst environments for paleoclimate reconstructions or aquifer recharge capacity. What is poorly understood is how the isotopic signal of δ18O and δ2H is transferred through the hydrologic cycle based upon storm type, frequency, intensity, and teleconnection activity in the tropical karst areas. At Harrison’s Cave, Barbados, a Hobo Onset event data logger was attached to a tipping bucket rain gauge to count the tips and record the total rainfall every 10 minutes. In the cave a Hobo data logger was used to record relative humidity and temperature at 10-minute intervals. Rainwater, dripwater, and stream water samples were collected at a weekly resolution and refrigerated before sample analysis. The study period was from July, 2012 to October, 2013, with data from the data loggers only until June, 2013 due to inability to reach the study site. The samples were analyzed using the Picarro Cavity Ring Down Spectroscopy Unit-Water L1102-I through laboratories at the University of Kentucky and the University of Utah. The samples were reported in per mil and calibrated. The teleconnection (NAO, AMO, and ENSO) and other atmospheric data were obtained from the Climate Prediction Center or the NOAA Earth System Research Laboratory-Physical Sciences Division. The weekly isotope signatures were linearly regressed against total rainfall for Harrison’s Cave and surface temperature with no statistically significant correlation, indicating the amount effect was not present at a weekly resolution. The amountweighted precipitation δ18O values were calculated on a monthly basis and compared to TRMM monthly rainfall and island-wide monthly rainfall, and a statistically significant negative correlation was found between both datasets. This confirmed that the amount effect dominates the island’s rainfall isotopic signature at a monthly resolution, and that specific atmospheric influences represented in weekly rainfall were less influential on a weekly basis. It is hypothesized that the variation in weekly rainfall is due to quick initiating, rain-out, and dissipation of convective storm systems over the island. In terms of evaporative influences, the samples do not deviate much from the Global Meteoric Water Line (GMWL), indicating minimal evaporation, which is typical for tropical locations. When the d-excess parameters were calculated, there were distinct variations with minimal evaporation occurring in the 2013 calendar year. This is attributed to coastal storm formation in the tropics.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 59-60).
This study investigates recent changes in the variability and seasonality of temperature and precipitation in the Northern Hemisphere. The mean and variance of daily temperature and precipitation anomalies are calculated for each year over a 35-year period and compared to a base period. For temperature in the Northern Hemisphere, a noticeable warming trend amplified in the higher latitudes was observed, as well as a significant decrease in variability in the mid and high latitudes. For precipitation in the Northern Hemisphere, a drying trend and decreasing trend in variability were observed in the mid latitudes during summer. The seasonal cycles of both temperature and precipitation were also analyzed. The trends in temperature seasonal amplitude and phase were studied and revealed some influence of Arctic sea ice loss that changes the seasonality of local temperature, and Arctic amplification that potentially influences temperature seasonality in the mid and high latitude land regions. To determine whether the changes in temperature seasonality may affect temperature variance, analyses were performed by removing the phase trends from the temperature data using two methods. The phase trend-removed temperatures were found to have no prominent trends in variance. This suggests that changes in the temperature variance may be related to changes in temperature seasonality. To study what affects precipitation variability, the coefficient of variation (ratio of standard deviation to mean), which determines the shape of the mixed gamma probability distribution function (PDF) of precipitation, was studied. It was found that the mean and variance of precipitation have a fixed ratio over time, suggesting that the shape of the precipitation PDF has not changed. Therefore changes in the precipitation variance in the midlatitudes could be simply explained by the change in the mean precipitation in the same region.
by Katrina L. Hui.
S.B.

Drought variability and change is assessed in this study across the Iberian Peninsula along the 20th century and the first decade of the 21st century using state of the art drought indices: the Sc-PDSI, the SPI and the SPEI. The analysis conducted in this study confirms that drought conditions are worsening for most of the Iberian Peninsula across time except in the western and in the north-western area, where a wetting trend is identified. Furthermore, the severity and duration of droughts show a remarkable increase in the south-western area, while drought magnitude is increasing significantly in the Mediterranean region strongly induced by global warming especially during the last decades. Severity, duration and magnitude of wet events do not experience significantly variations. Our results are in agreement with other studies which project this evolution to the whole 21st century. For this reason, and as a link of our study to societal needs, the application of effective water management strategies will be crucial to minimize the impact of the hydrological droughts over the Iberian Peninsula into the near future.
La següent tesi doctoral proporciona un anàlisi exhaustiu sobre la variabilitat i les tendències observades en l’evolució de les sequeres a la Península Ibèrica al llarg del s.XX i la primera dècada del s.XXI. El Sc-PDSI, el SPI i el SPEI són els índexs de sequera calculats a escala mensual usant una xarxa de sèries climàtiques d’alta qualitat per avaluar l’evolució espacio-temporal de les sequeres hidrològiques i els events humits a escala anual i estacional, tenint en compte la severitat, la magnitud i la durada d’aquests events extrems. A partir dels resultats, una clara tendència a la sequera ha estat detectada a gran part de la Península Ibèrica al llarg del s.XX a excepció de la regió oest i nord-oest, on ha estat identificada una tendència a l’increment de les condicions humides. Llavors, la disponibilitat d’aigua podria estar empitjorant en aquesta àrea provocant impactes ambientals i socio-econòmics realment difícils de solucionar. L’aplicació d’estratègies efectives de gestió de l’aigua, seran crucials per minimitzar l’impacte de les sequeres a la Península Ibèrica en un futur pròxim.

Thesis (Ph. D.)--University of Oregon, 2002.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 170-178). Also available for download via the World Wide Web; free to University of Oregon users.

Atmospheric reanalyses have been used in many studies to investigate the variabilities and trends of precipitation because of their global coverage and long record; however, their results must be properly analyzed and their uncertainties must be understood. In this study, precipitation estimates from five global reanalyses [ERA-Interim; MERRA, version 2 (MERRA2); JRA-55; CFSR; and 20CR, version 2c (20CRv2c)] and one regional reanalysis (NARR) are compared against the CPC Unified Gauge-Based Analysis (CPCUGA) and GPCP over the contiguous United States (CONUS) during the period 1980-2013. Reanalyses capture the variability of the precipitation distribution over the CONUS as observed in CPCUGA and GPCP, but large regional and seasonal differences exist. Compared with CPCUGA, global reanalyses generally overestimate the precipitation over the western part of the country throughout the year and over the northeastern CONUS during the fall and winter seasons. These issues may be associated with the difficulties models have in accurately simulating precipitation over complex terrain and during snowfall events. Furthermore, systematic errors found in five global reanalyses suggest that their physical processes in modeling precipitation need to be improved. Even though negative biases exist in NARR, its spatial variability is similar to both CPCUGA and GPCP; this is anticipated because it assimilates observed precipitation, unlike the global reanalyses. Based on CPCUGA, there is an average decreasing trend of -1.38mm yr(-1) over the CONUS, which varies depending on the region with only the north-central to northeastern parts of the country having positive trends. Although all reanalyses exhibit similar interannual variation as observed in CPCUGA, their estimated precipitation trends, both linear and spatial trends, are distinct from CPCUGA.

The central United States is a region of the country plagued by frequent catastrophic flooding (e.g., flood events of 1993, 2008, 2011, 2013, and 2014). In the twentieth and twenty-first centuries, flooding has taken a devastating societal and economic toll on the central United States, contributing to dozens of fatalities and causing billions of dollars in damage. Moreover, previous studies have shown that flood damage has been increasing over the past century across this region, and seems to foreshadow a future increase in flood activity. Despite these large repercussions, the use of historical records to ascertain the changes over time in flooding has thus far proved inconclusive. It is therefore of paramount importance to examine whether the characters (i.e., magnitude and frequency) of recent flooding are different from the long-term averages over the central United States. The results of this thesis are based on long-term discharge records at 774 U.S. Geological Survey sites and show limited evidence suggesting increasing or decreasing trends in the magnitude of flood peaks over the study region. In contrast, there is much stronger evidence of increasing frequency of flood events. While the detection of changes in flood characteristics is essential, it is also of critical importance to start exploring what caused these changes. Therefore, in addition to the aforementioned investigation on the stream flow records, precipitation records were used to inspect whether possible changes in flood characteristics can be linked to the changes in heavy precipitation characteristics. The results indicate that there is a stronger signal of change in the frequency rather than in the magnitude of heavy precipitation events, similar to what found for the discharge records. Given that heavy precipitation is responsible for the observed changes in flooding, further analyses were performed to examine the climatic driving forces that are responsible for the observed changes in the frequency of precipitation, and consequently flooding at the seasonal scale; particular emphasis was paid to the role played by the Atlantic and Pacific Oceans. The results of this dissertation indicate that changes in the climate system play a significant role in explaining the variations in the frequency of heavy precipitation and flooding over the central United States at both the seasonal and sub-seasonal scales. The Pacific North American (PNA) teleconnection pattern was found to play a particularly prominent role. Therefore, these results suggest that recent observed changes in the frequency of flood events over the central United States can be largely attributed to changes in the frequency of heavy precipitation events, which were in turn driven by changes in the climate system.

In addition to causing an increase in mean temperatures, climate change is also altering precipitation regimes across the globe. General circulation models project both latitude-dependent changes in precipitation mean and increases in precipitation variability. These changes in water availability will impact terrestrial primary productivity, the fixation of carbon dioxide into organic matter by plants. In my thesis, I addressed the following three questions: 1.) What will be the relative effect of changes in the mean and standard deviation of annual precipitation on mean annual primary production? 2.) Which ecosystems will be the most sensitive to changes in precipitation? 3.) Will increases in production variability be disproportionately greater than increases in precipitation variability? I gathered 58 time series of annual precipitation and aboveground net primary production (ANPP) from long-term ecological study sites across the globe. I quantified the sensitivity of ANPP at each site to changes in precipitation mean and variance. My results indicated that mean ANPP is about 40 times more sensitive to changes in precipitation mean than to changes in precipitation variance. I showed that semi-arid ecosystems such as shortgrass steppe in Colorado or typical steppe in Inner Mongolia may be the most sensitive to changes in precipitation mean. At these sites and several others, a 1% change in mean precipitation may result in a change in ANPP that is greater than 1%. To address how increases in interannual precipitation variability will impact the variability of ANPP, I perturbed the variability of observed precipitation time series and evaluated the impact of this perturbation on predicted ANPP variability. I found that different assumptions about the precipitation-ANPP relationship had different implications for how increases in precipitation variability will impact ANPP variability. Increases in ANPP variability were always directly proportional to increases in precipitation variability when ANPP was modeled as a simple linear or a lagged function of precipitation. However, when ANPP was modeled as a nonlinear, saturating function of precipitation, increases in ANPP variability were disproportionately low compared to increases in precipitation variability during wet years but disproportionately high during dry years. My thesis addresses an existing research gap regarding the long-term impact of increases in interannual precipitation variability on key ecosystem functioning. I showed that increases in precipitation variability will have negligible impacts on ANPP mean and have disproportionately large impacts on ANPP variability only when ANPP is a concave down, nonlinear function of precipitation. My work also demonstrates the importance of the precipitation-ANPP relationship in determining the magnitude of impacts to ANPP caused by changes in precipitation. Finally, my thesis highlights the potential for considerable changes in ANPP variability due to increases in precipitation variability.

The availability of high-resolution datasets describing the spatio-temporal evolution of precipitation is becoming increasingly important in order to analyse the long-term variability and trends of the climatic signal and possible impacts over specific areas of interest. These datasets are therefore crucial not only for research purposes, but also for decision-makers in a wide range of fields, such as agriculture, energy production, hydrology, natural risk monitoring and resource management. The reconstruction of high-resolution climate descriptions requires both accurate in-situ observations and suitable interpolation schemes to project the station data onto regular grids. The study focuses on the development and improvement of interpolation methods for monthly precipitation data and on the reconstruction and analysis of gridded datasets from dense and high-quality rain-gauge records covering specific study domains. The first goal of the work was the reconstruction of the high-resolution monthly precipitation climatologies over Italy for the 1961-1990 period. The observation database was set up thanks to a relevant effort of data rescue and collection from a great number of national and international sources and led to more than 4500 quality-checked monthly records available for the climatological reconstruction. Considering the heterogeneous Italian orography and the influence of surface features on precipitation distribution, a local weighted linear regression (LWLR) of precipitation versus elevation was applied to interpolate the station monthly normals onto a 30-arc second resolution Digital Elevation Model (DEM). Leave-one-out (LOO) validation and inter-comparison proved that the approaches modelling the local precipitation-orography relationship provide more accurate results in respect with methods considering larger spatial scales or not including the topography at all. The computed 30-arc second resolution monthly precipitation climatologies for Italy were on-line released and they represent an updated and highly detailed description of precipitation normal over the whole national territory in digital form. In order to evaluate the temporal evolution of precipitation over some of the most vulnerable Italian regions, gridded datasets of long-term precipitation series were produced by means of the anomaly-based approach. The secular precipitation series were reconstructed for the upper part of Adda river basin (Central Italian Alps), with an additional focus over the Forni glacier, and for Sardinia, as case-study for the Mediterranean area. New records were collected, especially thanks to the integration of the recent automatic station records with those of the previous mechanical networks and to digitisation activities of the most ancient data from hardcopy archives. All the series underwent statistical procedures aiming at avoiding inhomogeneities due to non-climatic signals. The gridded dataset allowed to get the secular areal precipitation records for the study regions and to evaluate their trends. As regards the Adda basin, the reconstructed 1800-2016 series showed statistically significant negative trends for annual and autumn precipitation. The comparison with the 1845-2016 annual basin runoff record, which is one of the longest runoff series available in Italy, allowed both to depict a strong decrease in annual runoff driven by the increase of evapotranspiration and to evaluate the possible contribution of gauge undercatch, especially in mountainous sites, to the underestimation of basin precipitation. The trend analysis over the 1922-2011 areal monthly precipitation record computed for Sardinia highlighted statistically significant decreases of -2.3% and -4.1% decade-1 in annual and winter values, respectively, and a positive but not statistically significant trend in summer precipitation. These outcomes agreed with other literature studies focusing on precipitation variability over Mediterranean area. The final part of the work focused on the development of interpolation schemes to improve the accuracy of gridded precipitation fields for domains unevenly covered by station networks. Norway represents a very interesting case-study, where the severe climatic conditions and the complex orography limit the management of in-situ observations over the most remote regions leading to an unbalanced station distribution between North and South and between low and high elevation. At this aim, a new method to compute the Norwegian monthly precipitation climatologies (1981-2010) at 1 km resolution was implemented and the gridded dataset was on-line released for both research and operative purposes. In this scheme, named HCLIM+RK, the HCLIM-AROME climate numerical model fields, which are not based on rain-gauge data and describe the precipitation gradients also over unsampled areas, are combined with available in-situ observations by the kriging interpolation of station residuals. In HCLIM+RK the high-resolution reliable precipitation patterns provided by the numerical model are retained and the station data are used to correct the biases affecting the numerical fields. The LOO reconstruction errors of Norwegian station normal showed that the combined approach almost removes the biases affecting the original HCLIM-AROME dataset and it provides much lower errors than conventional interpolation procedures based on stations only.

A tree-ring width chronology from Scots pine (Pinus sylvestris L.) was constructed from a xeric site in Stockholm to investigate the relationships between climate and tree growth and to reconstruct past moisture variability. The measure of moisture conditions employed here is a self-calibrating Palmer Drought Severity Index (PDSI). The index is derived from temperature, precipitation, and available water capacity of the soil, and assesses the intensity and duration of drought. It is widely used in tree-ring based climate reconstructions, a method which has never before been tested in the Nordic countries.

The comparison of the Stockholm tree-ring chronology with monthly temperature and precipitation data from a nearby meteorological station shows that tree growth is reduced by high summer temperatures, whereas high precipitation at the beginning of the growing season favours growth. The comparison with a PDSI calculated from this meteorological data shows that negative PDSI values are associated with narrow rings. Although tree growth in the humid climate of central Sweden is generally not limited by precipitation, the trees sampled for this study prove to be sensitive to changes in water supply. Their rings thus provide a record of past moisture variability and enable the reconstruction of precipitation and drought. The transfer function models for the reconstructions are calibrated using linear regression. A detailed verification of the results using the more than 200-year long meteorological record from Stockholm affirms the good model performance. May–June precipitation sums and the July PDSI could be reconstructed back to 1625.

The Palmer Drought Severity Index is found to be a useful tool in a tree-ring based reconstruction of past moisture variability, approximating the fraction of rainfall which is actually available to the tree, by including soil moisture storage, runoff, and the influence of temperature on evapotranspiration. It cannot completely account for the combined temperature and precipitation forcing of tree growth, and the use of the index does not improve the reconstruction compared to using precipitation alone. However, a reconstruction of both precipitation and the PDSI is possible when selecting an adequate sample site.

Abstract In northern regions such as Finland, snowpack represents a major water resource for aquatic and terrestrial ecosystems and different human uses, and snowmelt usually controls peak annual river discharge. To understand potential impacts of climate change, this thesis explored the effects of changes in surface air temperature (SAT) and precipitation patterns on snowpack accumulation and snowmelt processes in Finland. Two initial studies provided national and spatial pictures of long-term variability and changes in observed SAT and precipitation in Finland and their relationships with the atmospheric circulation patterns (ACPs) of the Northern Hemisphere (NH). On national scale, mean annual SAT and annual precipitation increased significantly in Finland during the past century. Seasonal analysis revealed increases in spring (MAM) and summer (JJA) SAT and in winter (DJF) and summer precipitation. SAT and precipitation were most significantly correlated with the Arctic Oscillation (AO), East Atlantic/West Russia (EA/WR), East Atlantic (EA) and Scandinavia (SCA) patterns. However, there were spatial differences within Finland as regards climate trends and relationships with ACPs. A study examining the dependence of snow resources on winter climate in Finland revealed a decline in snow water equivalent (SWE) and shortening of continuous snow cover duration (CSCD) in Finland during 1909-2008. This was mainly due to reductions in snowfall owing to winter warming and decreased precipitation in southern areas, but only to decreases in winter precipitation at higher latitudes. In southern Finland SWE and CSCD were associated with ACP affecting winter SAT (AO), and in central and northern parts with ACPs influencing winter precipitation (EA, EA/WR and AO). Analysis of snowmelt hydrology in Finland indicated that snowmelt flux has a much greater influence than rainfall on simulated peak snowpack outflow in Finland, which has decreased and shifted towards earlier in the year
Tiivistelmä Pohjoisilla alueilla kuten Suomessa lumi edustaa merkittävää vesivarantoa mikä vaikuttaa vesi- ja maaekosysteemeihin sekä yhdyskuntien vesienkäyttöön. Lisäksi lumen sulanta aiheuttaa suurimmat vuosittaiset tulvahuiput. Tämä väitöstutkimus keskittyy ilmanlämpötilan ja sadannan muutoksien aiheuttamiin vaikutuksiin lumen kertymisessä ja lumen sulannan prosesseissa Suomessa, jotta ilmaston muutoksen aiheuttamia mahdollisia muutoksia voidaan paremmin ymmärtää. Väitöskirjan kaksi tutkimusta antavat kansallisen ja spatiaalisesti kattavan kuvan pitkäaikaisvaihtelusta ja muutoksista ilmanlämpötilassa ja sadannassa Suomessa ja niiden yhteyden ilmastojärjestelmiin (ACP) pohjoisella pallonpuoliskolla. Kansallisella tasolla, vuosittainen keski-ilmanlämpötila ja vuosisadanta lisääntyivät merkittävästä Suomessa viimeisen vuosisadan aikana. Kausittainen analyysi paljasti kevät ja kesä ilmanlämpötilojen sekä talvi ja kesä-sadantojen lisääntyneen. Ilmanlämpötila ja sadanta korreloi merkittävimmin Arktisen (AO), Itä Atlantti/Länsi Venäjä (EA/WR), Itä-Atlantti (EA) ja Skandinaavisen (SCA) oskillaatioiden kanssa. Eri puolilla Suomea tuloksissa oli spatiaalista eroa liittyen ilmasto trendeihin ja yhteyksiin ilmakehän kiertoliikkeisiin. Tässä tutkimuksessa selvitettiin lisäksi lumivarojen riippuvuutta talviajan ilmastosta Suomessa. Vuosien 1909-2008 aineiston perusteella lumenvesiarvossa (SWE) havaittiin laskua sekä pysyvän lumipeitteen kestossa (CSCD) lyhenemistä. Etelässä tämä todettiin pääosin olevan seurausta talviajan lumisateiden vähentymisestä, jota vuorostaan selitti ilman lämpeneminen ja kokonaissademäärän pienentyminen. Pohjoisilla leveysasteilla ainoastaan talviaikaisen kokonaissademäärän pienentyminen selitti vähentyneitä lumivaroja. Tutkimuksessa Etelä-Suomen aineiston osalta tulokset SWE ja CSCD:ssa voitiin yhdistää muutoksiin talviseen SAT:hen (AO) vaikuttavissa ilmastojärjestelmissä (ACP). Keski- ja pohjoisosissa Suomea ilmastojärjestelmät (EA, EA/WR ja AO) vaikuttivat ainoastaan talviaikaiseen sadantaan. Aineiston perusteella voitiin myös todeta, että Suomessa kevätaikaisesta lumensulannasta johtuvaan valuntahuippuun vaikuttaa enemmän välitön lumen sulanta kuin sulannan aikana esiintynyt vesisade. Lumen sulannan huippuarvo havaittiin myös laskeneen sekä sen ajoittuminen aikaistuneen

The Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO) strongly influence atmospheric circulation over Western North America and therefore play a major role in modulating surface climate. This study represents an extension of Stahl et al. (2006a) in which a PCA - based approach was used to develop a synoptic climatology of mean sea level pressure patterns for the period 1948-2003 and in which relations between winter climate and major climate indices (PDO and ENSO) were explored. For both ENSO and PDO teleconnections, the synoptic analysis demonstrated that surface regional climate anomalies were linked to changes in both (a) the frequency of surface circulation types and (b) within-type variations. As the influence of major climate indices on the magnitude of within-type variability is not well understood, and is crucially important to the success of downscaling efforts, this study explores the nature of such variability in major winter precipitation producing types. Descriptive precipitation statistics are calculated for each synoptic type and compared between teleconnection phases for 56 years of data and for several key stations across British Columbia. Preliminary results suggest that teleconnections are responsible for a significant component of within-type precipitation variability and that this arises as a result of systematic differences in meteorology within types as expressed by such variables as vorticity, vertical velocity, precipitable water and geopotential heights. Based on these findings, creating of synoptic sub-types based upon teleconnection indices is proposed as a method for improving downscaling from GCMs by accounting for within-type variability associated with teleconnections. The downscaling methodology presented in this study assumes that GCM output, consisting of daily weather patterns and long-range teleconnection indices, can be used to classify the daily patterns into synoptic sub-types based upon the teleconnection indices, given mean precipitation amounts established for each type based on the historic record, downscaling scenarios can be calculated for any station in BC. In order to validate this approach, the statistical model was tested on the historic record using a bootstrapping approach. When repeated many times, this provides a statistically robust means of model assessment. The performance of a "basic" model, which employs the thirteen synoptic types of Stahl et al. (2006a), is assessed. The modelling methodology performs generally well for most of BC, but varies by region. The best model performance is found along the coast where precipitation amounts are the greatest, reflecting the consistent precipitation climatology of the region. The performance of three additional models, which utilise PDO, ENSO and PDO / ENSO subtypes, is contrasted with the basic model. It is discovered that the model using PDO / ENSO sub-types affords the greatest improvement over the basic model, with improvements varying in different regions of British Columbia. This study suggests that accounting for teleconnection indices can improve downscaling efforts; however, further research and development is recommended before an operational tool can be produced.
Arts, Faculty of
Geography, Department of
Graduate