Дисертації з теми "High-resolution Regional Climate Model"

Accurate regional and local scale information about seasonal climate variability and its impact on water availability is important in many practical applications like agriculture, water resource planning, long term decision making etc. Presently, the primary source for real-time seasonal climate forecast comes from the CPC within the NOAA-NCEP which uses its model forecast component (CFSv2) of North American Multi-Model Ensemble (NMME). But it has been observed that in comparison to the cool season, the level of skill in warm season seasonal forecasts of precipitation produced by the NMME is much lower (Kirtman et al. 2014) due to the poor climatological representation of warm season convective precipitation. To fully realise the potential in improving warm season seasonal forecasts using a dynamical modeling approach requires dynamical downscaling of NMME models to better improve their representation of convective precipitation at a convective-permitting (3km) grid. A decade-long CFSR (the reanalysis product of CFS) data is dynamically downscaled using WRF to demonstrate the value added of convective permitting modeling in the representation of mean and extreme warm season precipitation over the Southwest United States. The study shows evidence that the use of regional model adds value to the reanalyses in terms to better special and temporal representation which is also consistent with previous studies and appears to be an important initial step towards seasonal to subseasonal (S2S) forecasting. Empirical observations show that the structure and size of tropical cyclones (TCs) have dramatic impacts at landfall, including wind damage and storm surge. A better understanding of how the large-scale environment affects TC size and size change might be helpful in the predictions of the TC environment to infer how the TC size might change close to landfall. This study investigates the influence of environmental factors on TC size expansions using numerical simulations. Two periods of size change are investigated one in Hurricane Katrina (2005) as it moved through the Gulf of Mexico and one in Igor (2010) as it begins to undergo extratropical transition. Size changes are evaluated using the North Atlantic Hurricane Database second generation (HURDAT2) data set, which contains the maximum radial extent of the 64-, 50- and 34-kt wind in four quadrants. The average 34-kt wind radius (R34) is used as an indicator of the size of the TC. For the purposes of this study, the environment of a TC is investigated if the wind field either expanded or contracted in size at least 15 n mi radially in a 12-hour period. The regional model used is WRF-ARW. The results found from the simulation of Hurricane Katrina support previous results that increased surface fluxes and higher moisture availability is conducive to TC wind expansion and that as the moisture is depleted, the expansion of the wind field is no longer supported. In the case of Hurricane Igor, the influences of the midlatitude westerlies was evident in the increasing deep vertical wind shear, which is known to be detrimental to TC structure and intensity when strong enough.

This dissertation aims to better understand how various climate modeling approaches affect the fidelity of the North American Monsoon (NAM), as well as the sensitivity of the future state of the NAM under a global warming scenario. Here, we improved over current fully-coupled general circulation models (GCM), which struggle to fully resolve the controlling dynamics responsible for the development and maintenance of the NAM. To accomplish this, we dynamically downscaled a GCM with a regional climate model (RCM). The advantage here being a higher model resolution that improves the representation of processes on scales beyond that which GCMs can resolve. However, as all RCM applications are subject to the transference of biases inherent to the parent GCM, this study developed and evaluated a process to reduce these biases. Pertaining to both precipitation and the various controlling dynamics of the NAM, we found simulations driven by these bias-corrected forcing conditions performed moderately better across a 32-year historical climatology than simulations driven by the original GCM data. Current GCM consensus suggests future tropospheric warming associated with increased radiative forcing as greenhouse gas concentrations increase will suppress the NAM convective environment through greater atmospheric stability. This mechanism yields later onset dates and a generally drier season, but a slight increase to the intensity during July-August. After comparing downscaled simulations forced with original and corrected forcing conditions, we argue that the role of unresolved GCM surface features such as changes to the Gulf of California evaporation lead to a more convective environment. Even when downscaling the original GCM data with known biases, the inclusion of these surface features altered and in some cases reversed GCM trends throughout the southwest United States. This reversal towards a wetter NAM is further magnified in future bias-corrected simulations, which suggest (1) fewer average number of dry days by the end of the 21st century (2) onset occurring up to two to three weeks earlier than the historical average, and (3) more extreme daily precipitation values. However, consistent across each GCM and RCM model is the increase in inter-annual variability, suggesting greater susceptibility to drought conditions in the future.

The Himalaya plays a vital role in shaping the hydro-climate of South Asia and beyond, but their climate has not yet been monitored and modelled as well as some other regions. As the summer monsoon is the dominant climate system over South Asia, including the Himalaya, realistic simulation of the South Asian summer monsoon (SASM) should be a prerequisite for the satisfactory simulation of the Himalayan climate. The present research tests the assumption that higher resolution modelling will provide improved representation of the SASM, both regionally and over the Himalaya region. The first part of this research assesses the strength and stability of the temporal relationships between the monsoon rainfall indices (MRIs) and the large-scale monsoon circulation indices (MCIs), as a precursor to using such indices for model evaluation. The remainder of the thesis evaluates model performance in simulating various characteristics of SASM, mainly with regard to precipitation. In particular, the sensitivity of a regional climate model (RCM) simulation to domain size and added value of high resolution RCM simulation are evaluated. For this purpose, the Hadley Centre unified model - HadGEM is utilized in its regional and, in few instances, global configurations. The RCM simulations are performed at 0.44° and 0.11° horizontal resolutions and they are forced by the ERA interim dataset. Results show that i) the MRI-MCI relationship exhibits considerable low-frequency variability, ii) RCM simulation of SASM, particularly precipitation, shows sensitivity to domain size and simulation with a moderately sized domain that partially excludes bias prone equatorial Indian ocean outperform those with larger domains, iii) high resolution RCM simulation adds value in many aspects of SASM precipitation, including the seasonal mean, relative frequency distribution, extremes, and active and break monsoon composites, but the improvements are generally seen over the Indo-Gangetic plain rather than the Himalaya. The findings promote use of a high resolution RCM over a moderate sized domain (~ 25,000,000 sq. km) for the realistic simulation of SASM, but the study needs to be repeated with multiple realizations and different RCMs before arriving at a robust conclusion.

TCs have caused death and great economic loss every year across the coastal area of the western North Pacific (WNP). Therefore it is important to improve the understanding of the climatology of TCs over this region and their modulation by natural climate variability and large-scale circulation systems. It is also important to improve our ability to predict possible changes in TC activity over the WNP under climate change conditions. The most appropriate approach to study this is to use numerical models. However, high model resolution is required to resolve the complex physical structure of TCs so that realistic climatologies of TCs can be produced. In this thesis, regional climate model (RCM) simulations, from the Unified Model of the UK Met Office (MetUM) with resolutions of 25 and 12km and two different dynamical cores, are used to project the future change in TC activity over Vietnam, Philippines and the South China Sea (SCS). These simulations are driven by data from the ERA-Interim reanalysis for the period 1990-2005 and the HadGEM2-ES global climate model covering the historical period (1961-2005) and a future period (2069-2099) under two IPCC greenhouse gas emission scenarios (RCP4.5 and RCP8.5) to investigate the impact of anthropogenic warming on TCs in the study region. An objective algorithm is used to identify and track the simulated TCs. First, the ability of the RCM to simulate TCs and their associated large-scale environments, for a current climate period of 1990-2005, with different model resolutions is evaluated. For the period of 1961-2005, the downscaled HadGEM2-ES simulations are also used to evaluate the model's ability to reproduce the modulation of TC activity associated with El Nino/Southern Oscillation (ENSO). Both the 25 km and 12 km models can reasonably simulate the TC activity over the SCS compared with the observed TCs. The associated large-scale environments are found to be simulated correctly compared with the ERA-Interim reanalysis. The observed weakened TC activity during El Nino events is also captured by the downscaled HadGEM2-ES. Compared with the 25 km model, the 12 km model has a better ability to simulate the large-scale environments and generally improves the simulation of the spatial distribution and structure of TCs. Improved simulated TC-ENSO response (in terms of TC frequency, track density, intensity, structure and associated large-scale environments) is also found in the 12 km model. However, the 12 km model does not produce stronger 10-m maximum wind speeds of TCs compared with the 25km model. For the projections towards the end of the 21st century, the downscaled HadGEM2-ES at both 25 km and 12 km resolution present an insignificant decrease in TC frequency with rates of -0.001 per year and -0.007 per year for RCP4.5 and RCP8.5 respectively. The Had2-25km projection shows a more than 3% increase of the intense (10 m wind speed >35 mS-1) TCs over the South China Sea, while weak TCs (wind speed >25 ms-1) decrease by 10% under RCP8.5. Also, both the RCMs simulate a seasonal shift of TC activity in a warming climate, with an increase in TCs during winter related to the more favourable large-scale conditions and a decrease in TCs is projected in summer.

Les épisodes méditerranéens qui concernent chaque automne le nord-ouest de la Méditerranée sont des événements de précipitation extrême à fort impact. L'étude de leur évolution aux échelles climatiques revêt donc une grande importance et constitue encore un défi pour la communauté de modélisation du climat. Depuis quelques années, il est possible d’utiliser des modèles régionaux de climat à résolution kilométrique (Convection-Permitting Regional Climate Models ou CP-RCMs, 1-3 km) dont la convection profonde est simulée de manière explicite. Ces modèles permettent de se rapprocher des échelles spatio-temporelles en jeu et ouvrent de nouvelles perspectives en termes d'analyse. Cette thèse a pour objectif de déterminer la réponse des épisodes méditerranéens du nord-ouest de la Méditerranée au changement climatique d’origine humaine, en s'appuyant sur cette nouvelle génération de modèles de climat associée à une approche orientée objet. Le suivi des systèmes fortement précipitants est appliqué à la fois aux jeux de données d'observation, aux simulations réalisées avec le CP-RCM CNRM-AROME et aux simulations du premier ensemble de CP-RCMs disponible dans le cadre du programme international CORDEX FPS Convection sur un domaine commun couvrant le nord-ouest de la Méditerranée. La première partie de cette thèse est consacrée à l’évaluation des performances des CP-RCMs par comparaison à des données d’observation de référence à haute résolution. La valeur ajoutée des CP-RCMs par rapport aux modèles régionaux à résolution plus grossière (12-15 km) est démontrée pour les extrêmes de précipitation, en particulier au pas de temps horaire. L’approche objet permet également de montrer que, malgré quelques biais résiduels, les CP-RCMs sont capables de représenter correctement les principales propriétés des systèmes fortement précipitants, que ce soit en termes de nombre et de positionnement sur l'ensemble du domaine, ou en termes de durée, d'intensité, de surface, de volume, de vitesse et de sévérité sur le pourtour méditerranéen français où les observations permettent d’évaluer finement ces propriétés. La bonne performance de ces modèles permet de renforcer la confiance en leurs projections futures. La deuxième partie s’intéresse à l’évolution future des épisodes méditerranéens avec l’approche objet appliquée aux simulations milieu et fin de siècle de l’ensemble de CP-RCMs en mode scénario pour étudier les changements des propriétés des systèmes fortement précipitants dans un climat plus chaud. En fin de siècle et selon un scénario de fortes émissions, certains changements se retrouvent dans la plupart des simulations et peuvent être qualifiés de robustes. Ainsi, une augmentation de la fréquence des systèmes fortement précipitants de l’automne dans une grande partie du domaine, en particulier du centre de l'Italie au nord des Balkans, accompagne un doublement des zones touchées par ces événements. Sur la région méditerranéenne française, les modèles s’accordent sur une augmentation d'intensité, de surface et de volume des systèmes précipitants. Toutefois, même avec cette nouvelle génération de modèles, d'importantes incertitudes persistent, notamment pour les changements de fréquence dans le sud-est de la France, probablement dus à des différences dans les conditions synoptiques imposées par les modèles forceurs des CP-RCMs. De même, l’ensemble projette une large gamme de changements possibles dans les propriétés des systèmes, en particulier pour les plus intenses et y compris lorsqu’on normalise par le réchauffement régional correspondant. Si les CP-RCMs sont les outils de modélisation adaptés pour l'étude des extrêmes de précipitation, les efforts doivent être poursuivis pour produire des ensembles plus larges, mieux construits et probablement complétés par des méthodes d'apprentissage machine, afin d’apporter des informations climatiques utiles aux échelles pertinentes pour les politiques d’adaptation
The Mediterranean Heavy Precipitation Events (HPEs) that affect the northwestern Mediterranean every fall are high-impact weather events. The study of their evolution on climate scales is therefore of great importance and remains a challenge for the climate modelling community. For some years now, it has been possible to use kilometre-scale regional climate models (Convection- Permitting Regional Climate Models or CP-RCMs, 1-3 km) in which deep convection is explicitly simulated. These models make it possible to get closer to the spatio-temporal scales involved and open up new perspectives in terms of analysis. The aim of this thesis is to determine the response of northwestern Mediterranean HPEs to human-induced climate change, using this new generation of climate models combined with an object-oriented approach. The tracking of heavy precipitation systems is applied to observational datasets, to simulations carried out with the CNRM-AROME CP-RCM and to simulations of the first ensemble of CP-RCMs available as part of the international CORDEX FPS Convection programme over a common domain covering the north-western Mediterranean. The first part of this thesis is devoted to evaluating the performance of CP-RCMs in comparison with high-resolution reference observation data. The added value of CP-RCM compared with regional models with coarser resolution (12-15 km) is demonstrated for precipitation extremes, particularly at hourly time steps. The object-oriented approach also shows that, despite a few residual biases, CP-RCMs are capable of correctly representing the principal properties of heavy precipitation systems, both in terms of number and position over the entire domain, and in terms of duration, intensity, surface area, volume, speed and severity over the French Mediterranean, where observations enable these properties to be assessed in detail. The good performance of these models lends greater confidence to their future projections. The second part focuses on the future evolution of Mediterranean HPEs using the object-oriented approach applied to mid and end-of-century simulations of the CP-RCMs ensemble in scenario mode to study changes in the properties of heavy precipitation systems in a warmer climate. At the end of the century, and according to a scenario of high emissions, certain changes are found in most of the simulations and can be described as robust. For example, an increase in the frequency of fall heavy precipitation systems over a large part of the domain, particularly from central Italy to the northern Balkans, is accompanied by a doubling of the areas affected by these events. Over the French Mediterranean region, the models agree on an increase in the intensity, surface area and volume of precipitating systems. However, even with this new generation of models, significant uncertainties remain, particularly for changes in frequency over southeastern France, probably due to differences in the synoptic conditions imposed by the CP-RCMs driving models. Similarly, the ensemble projects a wide range of possible changes in the properties of systems, particularly for the most intense ones and even when standardised by the corresponding regional warming. While CP-RCMs are the appropriate modelling tools for studying precipitation extremes, efforts must be continued to produce larger, better constructed ensembles, probably supplemented by machine learning methods, in order to provide useful climate information at scales relevant to adaptation policies

The field of urban hydrology is in need of high temporal resolution data series in order to effectively model and analyse existing and future trends in extreme precipitation. When high resolution data sets are, for any number of reasons, not available for a given location, the technique of disaggregation using a random cascade model can be applied. Previous studies have demonstrated the relevance of random cascades in the context of rainfall data disaggregation with temporal resolutions usually down to 1 hour. In this study, an attempt at disaggregation to a resolution of 1 minute was made. Using newly disaggregated rainfall data for different regions in Sweden, the possibility of clustering rain events into separate regional hyetographs was investigated. The random cascade model was calibrated using existing municipal rainfall data with a temporal resolution of 1 minute, in order to disaggregate continuous 15 minutes data series provided by the Swedish Meteorological and Hydrological Institute (SMHI). The disaggregation process was then performed in multiple stochastic realisations, in order to correct the uncertainties inherent to the random cascade model. The disaggregation results were assessed by comparing them with calibration data: two main rainfall parameters, EV and ED, were analysed by determining their behaviours and distribution. The possibility of transfering calibration parameters from one station to another was also assessed in a similar manner, again by studying EV & ED for different scenarios. Finally, hyetographs were clustered, compared and contrasted, in order to ascertain previously theorized differences between regions. This research showed the feasibility of applying a random cascade model to very high temporal resolutions in Sweden, while replicating rainfall characteristics from the calibration data quite well. The analysis of the spatial transferability of calibration parameters yielded inconclusive results, as rainfall characteristics were preserved in some cases but failed in others. Lastly, distinct regional differences in hyetographs were noted, but no clear conclusions could be drawn owing to the delimitations of this study.
Inom småskalig hydrologisk modellering finns det idag ett behov av dataserier med hög tidsupplösning för att effektivt kunna modellera och analysera både aktuella och kommande trender hos extrema regnhändelser. När högupplösta dataserier är otillgängliga vid en önskad mätplats kan disaggregering med hjälp av en slumpmässig kaskadmodell tillämpas. Tidigare forskning har visat att kaskadmodeller är användbara för disaggregering av regndata med en tidsupplösning av 1 timme. I denna studie disaggregerades dataserier med syftet att uppnå en tidsupplösningav av 1 minut. För att kunna analysera eventuella skillnader mellan regioner klustrades även hyetografer med de framtagna dataserierna. Den slumpmässiga kaskadmodellen kalibrerades med befintlig kommunal data med en tidsupplösning på 1 minut, för att sedan kunna disaggregera 15 minuters data från SMHIs databaser. Disaggregeringen genomfördes i ett antal olika stokastiska realisationer för att kunna ta hänsyn till, och korrigera, de inneboende osäkerheterna i den slumpmässiga kaskadmodellen. Disaggregeringsresultaten bedömdes genom en jämförelse med kalibreringsdata: två regnegenskaper, regnvaraktighet (ED) och regnvolym (EV), analyserades för att kunna bestämma derasfördelningar och beteenden. Kalibreringsparametrarnas överförbarhet analyserades också med hjälp av ED & EV för olika scenarier. Slutligen klustrades hyetografer för att fastställa potentiella skillnader mellan regioner. Studien påvisade möjligheten att använda en slumpmässig kaskadmodell till höga tidsupplösningar i Sverige. Modellen lyckades återskapa regnegenskaper från kalibreringsdata vid disaggregeringen. Möjligheten att överföra kalibreringsparametrar från en station till en annan visade sig dock inte vara helt övertygande: regnegenskaper återskapades endast i vissa fall, men inte i samtliga. Slutligen konstaterades regionala skillnader i hyetografer, men tydliga slutsatser kunde inte dras på grund av underliggande begränsningar med studien.

Includes bibliographical references
The main aim of this thesis is to investigate the potential benefits of increasing resolution in regional climate models in the simulation of climate variability and change over East Africa. This study is based on two high resolution regional climate simulations with a horizontal resolution of 50km and 10km, respectively. These represent present day climate and a projection of future climate change over East Africa. The regional climate model (RCM) used here is HIRHAM5, which is driven by the global circulation model (ECHAM5). Downscaled ECHAM5 output is used to drive the 50km HIRHAM5 simulation for the period 1950-2100, and output from this simulation is used to drive the 10km simulation for three time slices: 1980-1999, representative for present-day climate and two time slices for near future (2046-2065) and far future (2080- 2099), respectively. HIRHAM5 is evaluated with respect to the observed mean climatologies of rainfall, surface temperature and surface winds over East Africa, and representations of the observed annual cycles and inter-annual variability of rainfall and surface temperature. This study utilizes reanalysis and observational datasets: a hindcast of HIRHAM5 forced with ERA Interim, as well as two observation datasets for temperature and rainfall. Since reanalyses aim to make "best use" of all available observations by making a physically consistent representation continuous in time and space, and since there is a paucity of observations over many parts of Africa, the ERAI reanalysis is also used as a best estimate for model evaluation. Additionally, for evaluation of the bimodal nature of East Africa's rainfall, especially over Tanzania, three stations run by the Tanzania Meteorological Agency were used. The model data used in th is evaluation ranges from 1980 to 2006 iv HIRHAM5 demonstrates reasonable skill in the reproduction of observed patterns of mean climatology of rainfall, surface temperature and winds over East Africa. Moreover, the patterns of annual cycles of rainfall and surface temperature in the bimodal nature of East Africa are well represented. Furthermore, the model showed reasonable skill in the representation of the inter- annual variability and ENSO signals as suggested by the observation. Despite these strengths, HIRHAM5 shows some shortcomings. One weakness of the model is the simulation of the magnitude of a given variable over a specific region. For example, HIRHAM5 driven by ERAI underestimates rainfall and overestimates surface temperature over the entire domain of East Africa. The higher resolution HIRHAM5 (10km resolution) overestimates rainfall over high ground. The model bias could be due in part to the inadequacy of the observation networks in East Africa, represented in this thesis by the CRU and FEWS datasets. However, these two datasets draw on some different sources and neither do they have the same resolution. FEWS is a high resolution data (0.1 o ) gridded satellite-derived precipitation estimate covering the entire African continent while CRU datasets is a relatively low resolution (0.5 o ) dataset based on rain gauge monthly precipitation only; in addition , near surface temperature is also available. As no reliable wind observations exist, wind data was taken from the ERA-Interim reanalysis. The different observational datasets do not agree particularly well, which impedes evaluating the quality of the HIRHAM5 simulations, in particular the high resolution one. So while the higher resolution HIRHAM5 appears to be generally reliable, caution must be exercised in formulating conclusions from the results, especially over high ground and remote areas without adequate observation data. Under these constraints, the results suggest HIRHAM5 may be useful for assessing climate variability and change over East Africa. A weakness of the analysis presented here is that only one combination of GCM and RCM could be investigated in depth due to computer and time constraints. Therefore the results presented here, if used in application for climate change adaptation, should be considered in conjunction with a broader suite of data, such from the CORDEX programme. This has potential to increase the reliability of information about climate variability and change at a regional to local level necessary for impact assessment.

In this work high-resolution numerical simulation (Large-Eddie Simulation, LES) has been used to study the characteristic factors causing and influencing the development of moist convective clouds. Through this work a 1D cloud-model was derived from first principles to represent the vertical profile of individual convective clouds. A microphysics framework was implemented to ensure identical behaviour in LES and cloud-model integration where the microphysical processes represented are numerically integrated using a novel adaptive step microphysics integration which uses the physical speed at which a process takes place to adjust the integration step size (in space and time). This work also introduces a simple representation of cloud-droplet formation which allows for super-saturation to exist in-cloud and through this provide more physical representation of the in-cloud state. Together with high-resolution simulation of isolated individual and interacting multiple clouds in environmental conditions leading to shallow convection, the 1D cloud-model was used to infer that the principal influence on moist convective clouds is the entrainment of air from a cloud’s immediate environment which is significantly different from the environmental mean state. This suggests that convection parameterisations must represent the influence of moist convective downdrafts to properly predict the vertical structure of convective clouds so as to correctly predict the cloud-top height and vertical transport. Finally it was found that cloud-base radius is not in itself adequate as a means of classification for defining cloud-types as clouds with the same cloud-base radius showed large variation (≈ 600m) in cloud-top height. Based on simulations of individual convective clouds it was found that 3D simulations are necessary to capture the full dynamic behaviour of convective clouds (2D axisymmetric simulations have too little entrainment) and that agreement with the 1D cloud-model could only be found when entrainment was diagnosed from simulation instead of being parameterised by the traditional Morton-Turner model and only for 2D axisymmetric simulations, suggesting that the 1D cloud-model will require further extension or the diagnosis of entrainment improved.

The Indian Ocean plays an important role in modulating climate over the surrounding region and around the globe, particularly through its major mode of climate variability, the Indian Ocean dipole (IOD). In this thesis, the representation of the Indian Ocean mean state and the IOD in the HiGEM high resolution coupled climate model has been investigated. To understand air-sea coupled processes during the IOD, it is first necessary to evaluate the representation of the Indian Ocean mean state in HiGEM. Although HiGEM has a very good representation of the tropical Indian Ocean, there are some regional biases in the model. Firstly biases in the Arabian Sea. HiGEM has biases in various parameters over the northern Arabian Sea during boreal winter. A major result of this thesis is that a winter cold SST bias in the Arabian Sea in HiGEM is shown to be common among most of the CMIP3 models, relating to cold dry air advection from north-west India! Pakistan during the winter monsoon. This also feeds enhanced convergence over the western equatorial Indian Ocean, potentially interacting with biases along the equator and the IOD. Secondly biases in mean equatorial winds, reversing the tilt of the equatorial thermocline during SON and DJF, which may lead to biases in the evolution of the IOD. To test the impact of the equatorial mean state biases in HiGEM on the IOD, an ensemble experiment correcting ocean surface wind stress over the equatorial Indian Ocean has been performed. When applied the wind stress correction to the ocean wind stress, it reduced the biases in the Cold SST and thermocline depth in the east equatorial Indian Ocean (EElO). This suggests that the seasonal evolution of the IOD is sensitive to the model mean state. As the anthropogenic warming can spin down the tropical circulations, the impact of climate change on the IOD in HiGEM is also investigated. Analysis showed an earlier initiation and termination of the roD in HiGEM in a warm climate compared to the HiGEM control run. This shift in initiation and termination in the seasonal evolution of the IOD is consistent with the change in the Indian Ocean mean state in a wann climate. This study shows the importance of understanding the Indian Ocean processes and their representation in models for regional climate studies.

Thesis (Ph. D.in Physical Oceanography)--Naval Postgraduate School, September 2003.
Dissertation supervisor: Mary L. Batteen. Includes bibliographical references (p. 109-123). Also available online.

A Regional Climate Model (RCM) is a comprehensive tool to simulate high- resolution climatic factors. A RCM is driven by low resolution data obtained from a Global Climate Model (GCM). In the one-way nested method, is the GCM data fed into the RCM as a Lateral Boundary Condition (LBC) in certain updates in time, the boundary data interval resolution. The necessary information in between these updates is obtained by using linear interpolation techniques. The ability to reproduce high-resolution RCM output with low-resolution GCM data depends on the accuracy of this LBC. This thesis investigates whether third order interpolation methods lead to a more accurate approximation than the linear method. This is investigated in combination with lowering the boundary data interval resolution. The conclusion is that a third order interpolation method does not lead to a more accurate approximation for a high boundary data interval resolution. But when the resolution is lowered, the linear interpolation method looses its accuracy earlier than the third order method. This results in a boundary data interval resolution of 1.5 hours for the linear method compared to 7.5 hours for the third order method. Implementing a lower boundary data interval resolution in combination with the third order method lead to significant gain in computational time and storage for the RCMs.
En Regional Climate Modell (RCM) är ett avancerat verktyg för att simulera klimatet med hög upplösning. En RCM drivs av data från en Global Climate Modell (GCM), ofta med låg upplösning. I den så kallade envägsnästade metoden sparas GCM-data med vissa tidsintervall för att användas som ett lateralt randvillkor (LBC) till RCM. Den nödvändiga informationen mellan sparade data erhålls med användning av linjära interpoleringstekniker. Möjligheten att reproducera ett hög upplöst GCM-resultat med en RCM med liknande upplösning genom att använda envägsnästning med låg upplösning beror på exaktheten hos denna LBC. Denna avhandling undersöker huruvida tredje ordningens interpoleringsmetoder leder till en mer exakt inställning än den linjära metoden. Detta undersöks i kombination med minskningen av intervallupplösningen av randdata. Slutsatsen är att en tredje ordningens interpolationsmetod inte leder till en mer exakt approximation för en hög dataintervallupplösning. Men när upplösningen sänks, förlorar den linjära interpolationsmetoden sin noggrannhet, tidigare än den tredje ordningens metod. Detta resulterar i en begränsad dataintervallupplösning på respektive 1, 5 timmar respektive 7,5 timmar. Tredje orderinterpolering är således en signifikant vinst för RCM.

The investigation of climate variability in different timescales such as daily, monthly, seasonal and inter-annual has utmost importance for managing the socio-economic processes on regional to global scale. Indeed, the variation in the climate has a crucial impact on agriculture, water, health, tourism, economy and transportation. Therefore the development of climate forecasting tools is necessary which helps to manage these sectors more efficiently. However, there are limitations on producing accurate climate forecast for more than two weeks in advance due to the chaotic nature of the climate system, especially for the region like the Mediterranean, which is characterized by high interannual variability. Due to its importance and challenging nature, a collective effort is being done to improve the skill of models and climate forecasting in the Mediterranean. The contribution of this thesis is a overall effort, which consists of developing a high resolution model application in the Mediterranean, which can provide reliable estimate of Sea Surface Temperature (SST) and Mixed Layer Depth (MLD). This approach is based on the fact that the atmospheric predictability in seasonal to interannual time scale is significantly dependent on slowly varying lower boundary conditions (e.g. Charney and Shukla 1981) i.e. Mediterranean SSTs. The spatial resolution of model is increased for taking into account the mesoscale processes in the Mediterranean. Since the first internal Rossby radius of deformation in the Mediterranean sea is of the order of 10-15 kms, the spatial resolution of an eddy resolving model should have at least a resolution one half of the Rossby radius. Based on this assumption, the spatial resolution is explored to the order of -5 km (1/16°). The regional ocean modeling system (ROMS) adopted from the Rutgers University is used in the current study. The objective is to validate certain fields (such as SST and MLD) obtained from model simulations and study air-sea interactions. The validation is done by performing two experiments namely, climatological and interannual simulations. The model simulated results are validated with observations as well as intercompared to evaluate the skill of model. The monthly mean SST climatology is obtained from ten years of model run forced with climatological air-sea fluxes is well captured by the model configuration and follows the annual cycle. Model simulated summer SST climatology shows biases of the order of 0.8-1.0 °C with observation (MedAtlas) and 1.0-1.2 °C with other datasets (intercomparison). The vertical structure of temperature climatology is found to be well simulated by model in which upper layer shows a difference of 1.0 °C and it further decreased at intermediate layers. The simulated sea surface height and surface currents is validated with Aviso altimetry data. On the large scales the surface currents generated by model captures general structures of surface circulation. The monthly mean mixed layer depth (MLD) climatology computed from model is validated with observed monthly MLD climatology and found that the winter MLD is overestimated by model. In second experiment, model is forced with six hourly air-sea interaction fluxes from ERA-Interim and interannual simulations are obtained for the period 1998-2007. The monthly mean SST climatology obtained from above interannual simulation follows climatological annual cycle with cold biases in summer season. The weak SSTs (bias of the order of 1.0 to 1.5 °C) are observed in the summer for the period 2002-2007 in the model simulations. The monthly mean SST anomalies are well simulated by model except for the year 2006. The time evolution of monthly mean SST anomalies area averaged over different sub-basins are exhibits interannual variability. The comparison with satellite derived SSTs reveals that our model is able to capture both, the seasonal and inter-annual variability, although it still has a bias of the order of 1 to 1.2 °C. The model is able to reproduce the temperature at subsurface layer having the signatures of existence of intermediate water masses. The monthly mean mixed layer climatology derived from interannual simulations is quite well reproduced by model. In the Gulf of Lions, MLD values are reached upto 1500 meter deep in winter whereas it shows 50 meter in summer season. The time evolution of monthly mean mixed layer climatology derived from the model is able to reproduce annual variability. The interannual variability of monthly mean mixed layer depth is simulated quite well by model for the year 2004-2007. The timeseries of climatological, monthly and daily mixed layer depth which is area averaged over various sub-basins follows seasonal cycle. The high resolution regional model application developed in the current study is thus able to reproduce certain fields. The surface currents and eddy kinetic energy in the model shows small scale structures and strong variability. The model is also capable to generate mesoscale eddies in the western Mediterranean although model overestimated surface fields.
La investigación de la variabilidad climática en diferentes escalas de tiempo, como diario, mensual, estacional e interanual tiene suma importancia para la gestión de los procesos socio-económica en la región a escala global. De hecho, la variación en el clima tiene un impacto crucial en la agricultura, el agua, la salud, el turismo, la economía y el transporte. Por lo tanto el desarrollo de herramientas de predicción del clima es necesario que ayuda a gestionar estos sectores de manera más eficiente. Sin embargo, existen limitaciones en la producción de pronóstico climático precisa durante más de dos semanas de antelación debido a la naturaleza caótica del sistema climático, especialmente para la región como el Mediterráneo, que se caracteriza por una alta variabilidad interanual. Debido a su importancia y naturaleza desafiante, se está haciendo un esfuerzo colectivo para mejorar la habilidad de los modelos y la predicción del clima en el Mediterráneo. La contribución de esta tesis es un esfuerzo global, que consiste en el desarrollo de una aplicación de modelo de alta resolución en el Mediterráneo, que puede proporcionar una estimación fiable de la temperatura superficial del mar (TSM) y la profundidad de la capa mixta (MLD). Este enfoque se basa en el hecho de que la previsibilidad atmosférica en estacional a interanual escala de tiempo es significativamente dependiente de variación lenta condiciones límite inferior (por ejemplo, Charney y Shukla 1.981), es decir TSM mediterráneos. La resolución espacial de modelo se incrementa por tomar en cuenta los procesos de mesoescala en el Mediterráneo. Desde la primera radio interno de deformación de Rossby en el mar Mediterráneo es del orden de 10-15 kms, la resolución espacial de un remolino resolución de modelo debe tener al menos una resolución de la mitad del radio de Rossby. Basándose en esta suposición, la resolución espacial se explora a la orden de — 5 km (1/16°). El sistema de modelado regional de los océanos (ROMS), aprobada por la Universidad de Rutgers se utiliza en el estudio actual. El objetivo es validar ciertos campos (como SST y MLD) obtenidos a partir de simulaciones de modelos y estudiar las interacciones aire-mar. La validación se realiza mediante la realización de simulaciones de dos experimentos saber, climatológicos e interanuales. Los resultados del modelo simulado se validan con las observaciones, así como intercomparados para evaluar la habilidad del modelo. La media mensual climatología SST se obtiene a partir de diez años de ejecución del modelo forzada con climatológicas flujos aire-mar es bien capturados por la configuración del modelo y sigue el ciclo anual. Simulado Modelo SST climatología verano muestra sesgos del orden de 0,8 a 1,0 °C con observación (MEDATLAS) y 1,0-1,2 °C con otros conjuntos de datos (de intercomparación). La estructura vertical de la climatología de temperatura se encuentra para ser bien simulado por modelo en el que la capa superior muestra una diferencia de 1,0 °C y disminuyó aún más en las capas intermedias. Las corrientes simulados altura y la superficie de la superficie del mar se valida con los datos de altimetría aviso. En las grandes escalas de las corrientes superficiales generadas por el modelo capta las estructuras generales de la circulación superficial. La climatología media mensual profundidad de la capa mixta (MLD) calculada a partir del modelo se valida con la observada climatología mensual MLD y encontró que el invierno MLD se sobreestima el modelo. En segundo experimento, el modelo se ve obligado a seis por hora aire-mar flujos de interacción a partir de simulaciones ERA-Interim e interanuales se obtienen para el período 1998-2007. La climatología TSM mensual media obtenida desde arriba simulación interanual sigue el ciclo anual climatológica con sesgos fríos en la temporada de verano. Los TSM débiles (sesgo del orden de 1,0 a 1,5 °C) se observan en el verano para el período 2002-2007 en las simulaciones del modelo. Las anomalías medias mensuales de la TSM son bien simuladas por el modelo, excepto para el año 2006. La evolución en el tiempo de media área de anomalías de TSM mensual promedio durante diferentes subcuencas son exposiciones variabilidad interanual. La comparación con TSM satélite derivada revela que nuestro modelo es capaz de capturar tanto, la variabilidad estacional e interanual, a pesar de que todavía tiene un sesgo del orden de 1 a 1,2 °C. El modelo es capaz de reproducir la temperatura a la capa subsuperficial tener las firmas de la existencia de masas de agua intermedias. La media climatología capa de mezcla mensual derivado de simulaciones interanuales está bastante bien reproducido por modelo. En el Golfo de León, los valores de MLD se alcanzan hasta 1.500 metros de profundidad en invierno mientras que muestra 50 metros en temporada de verano. La evolución en el tiempo de la media climatología capa de mezcla mensual derivada del modelo es capaz de reproducir la variabilidad anual. La variabilidad interanual de media profundidad de la capa mixta mensual se simula bastante bien por el modelo para el año 2004-2007. Las series de tiempo de profundidad de la capa mixta climatológica, mensual y diaria, que es el área de media sobre varias subcuencas del siguiente ciclo estacional. La aplicación modelo regional de alta resolución desarrollado en el presente estudio es, pues, capaz de reproducir ciertos campos. Las corrientes superficiales y la energía cinética de Foucault en el modelo muestra las estructuras de pequeña escala y fuerte variabilidad. El modelo también es capaz de generar remolinos de mesoescala en el Mediterráneo occidental, aunque el modelo sobreestima campos superficiales.

The North American monsoon (NAM) is the principal driver of summer severe weather in the Southwest U.S. With sufficient atmospheric instability and moisture, monsoon convection initiates during daytime in the mountains and later may organize, principally into mesoscale convective systems (MCSs). Most monsoon-related severe weather occurs in association with organized convection, including microbursts, dust storms, flash flooding and lightning. The overarching theme of this dissertation research is to investigate simulation of monsoon severe weather due to organized convection within the use of regional atmospheric modeling. A commonly used cumulus parameterization scheme has been modified to better account for dynamic pressure effects, resulting in an improved representation of a simulated MCS during the North American monsoon experiment and the climatology of warm season precipitation in a long-term regional climate model simulation. The effect of urbanization on organized convection occurring in Phoenix is evaluated in model sensitivity experiments using an urban canopy model (UCM) and urban land cover compared to pre-settlement natural desert land cover. The presence of vegetation and irrigation makes Phoenix a "heat sink" in comparison to its surrounding desert, and as a result the modeled precipitation in response to urbanization decreases within the Phoenix urban area and increase on its periphery. Finally, analysis of how monsoon severe weather is changing in association with observed global climate change is considered within the context of a series of retrospectively simulated severe weather events during the period 1948-2010 in a numerical weather prediction paradigm. The individual severe weather events are identified by favorable thermodynamic conditions of instability and atmospheric moisture (precipitable water). Changes in precipitation extremes are evaluated with extreme value statistics. During the last several decades, there has been intensification of organized convective precipitation, but these events occur with less frequency. A more favorable thermodynamic environment for monsoon thunderstorms is the driver of these changes, which is consistent with the broader notion that anthropogenic climate change is presently intensifying weather extremes worldwide.

La zone France-Méditerranée est fréquemment exposée à de fortes précipitations en automne dont l'accumulation quotidienne peut parfois dépasser 300 millimètres. Quelques études montrent une tendance à l'augmentation de la fréquence et de l'intensité de ces événements (par exemple Vautard et al., 2015 ; Ribes et al., 2019). Cependant, une attribution formelle des événements extrêmes qui lie ces changements au changement climatique induit par l'homme pour cette région n'a jamais été faite. Ce sujet de thèse vise à quantifier le rôle du changement climatique induit par l'homme dans l'altération des propriétés statistiques des précipitations convectives extrêmes survenant sur la région France-Méditerranée, en se concentrant sur la chaîne de montagnes des Cévennes et en utilisant pour la première fois une approche de modèle à haute résolution incluant un modèle permettant la convection. J'analyse d'abord l'ensemble EURO-CORDEX, qui comprend différentes combinaisons de modèles climatiques globaux et de modèles climatiques régionaux. Ensuite, j'ai effectué une série de simulations numériques avec le modèle WRF à une résolution permettant la convection. J'ai également comparé les simulations avec les observations et les ré-analyses à haute résolution. Les résultats montrent que les modèles régionaux peuvent reproduire des événements extrêmes de pluie convective avec une meilleure concordance avec les observations en augmentant leur résolution horizontale, en particulier à une résolution permettant la convection (environ 3 km). En utilisant ces simulations, je montre que le changement climatique induit par l'homme rend les précipitations quotidiennes et trihoraires sur 100 ans au moins deux fois plus probables dans le climat actuel. Les résultats suggèrent également la nécessité d'utiliser une approche multi-modèle pour réduire les incertitudes dans ce type d'étude d'impact
The France-Mediterranean area is frequently exposed to heavy precipitation events in the autumn whose daily accumulation can sometimes exceed 300 millimeters. There are a few studies showing the increasing trend in the frequency and intensity of these events (e.g. Vautard et al., 2015; Ribes et al., 2019). However, a formal extreme event attribution that links those changes to human-induced climate change for this area has never been done. This PhD subject aims at quantifying the role of human-induced climate change in altering the statistical properties of extreme convective precipitation event occurring over the France-Mediterranean focusing on the Cevennes mountain range and using a high-resolution model approach including convection-permitting model for the first time. I first analyze the EURO-CORDEX ensemble, which includes different combinations of global climate models and regional climate models. Then I conducted a set of numerical simulations with the WRF model at a convection-permitting resolution. I also compared the simulations with observations and high-resolution re-analyses. The results show that regional models can reproduce extreme convective rainfall events with better agreement with observations by increasing their horizontal resolution, especially to convection-permitting resolution (approx. 3 km). By using these simulations, I show that human-induced climate change consistently makes the 100-year 3-hourly and daily precipitation event at least 2 times more likely under current climate. The results also suggest the need of using multi-model approach to reduce the uncertainties in this type of impact study

La mer des Salomon est une mer semi-fermée située dans le Pacifique subtropical. Elle connecte les masses d'eau des subtropiques à l'équateur via les courants de bord ouest de faibles latitudes (LLWBCs) et pourrait de ce fait moduler à l'échelle décennale le climat du Pacifique tropical. Très peu d'observations sont disponibles pour l'étude de cette région. Un des objectifs principaux de cette étude est la mise en place d'un modèle réaliste d'océan à haute résolution (1/36°) de la mer des Salomon permettant la résolution d'une large gamme d'échelles, particulièrement la mésoéchelle et marginalement la sous-mésoéchelle. La circulation générale est étudiée ainsi que la variabilité à mésoéchelle et à sous-mésoéchelle. La représentation de la circulation simulée par le modèle 1/36° est non seulement validée par les observations disponibles mais aussi améliorée par rapport à celle simulée par les modèles antérieurs. La variabilité mésoéchelle simulée dans le modèle à haute résolution est fortement augmentée par rapport à celle issue des modèles antérieurs et est en bon accord avec les observations. Des études spectrales en nombre d'onde de la température de surface, de la dénivellation de la surface libre et de l'énergie cinétique ont été réalisées dans la mer des Salomon et suggèrent que les pentes spectrales obtenues sont proches de la théorie classique de la quasi-géostrophie de surface (SQG).

Weather and climate models applied with sufficiently fine mesh grids to enable a large part of atmospheric deep convection to be explicitly resolved have shown a significantly improved representation of local, short-duration and intense precipitation events compared to coarser scale models. In this thesis, two studies are presented aimed at exploring the dependence of horizontal resolution and of parameterization of convection on the simulation of precipitation. The first examined the ability of HARMONIE Climate (HCLIM) regional climate model to reproduce the recent climate in Europe with two different horizontal resolutions, 15 and 6.25 km. The latter is part of the ”grey-zone” resolution interval corresponding to approximately 3-10 km. Particular focus has been given to rainfall and its spatial and temporal variability and other characteristics, for example intensity distributions. The model configuration with the higher resolution is much better at simulating days of large accumulated precipitation amounts, most evident when the comparison is made against high-resolution observations. Otherwise, the two simulations show similar skill, including the representation of the spatial structure of individual rainfall areas of primarily convective origin. The results suggest a ”scale-awareness” in HCLIM, which supports a central feature of the model’s description of deep convection as it is designed to operate independently of the horizontal resolution. In the second study, summer season precipitation over the Alps region, as simulated by HCLIM at different resolutions, is investigated. Similar model configurations as in the previous study were used, but in addition a simulation at the ”convection-permitting” 2 km resolution has been made over Central Europe. The latter considerably increases the realism compared to the former regarding the distribution and intensities of precipitation, as well as other important characteristics including the duration of rain spells, particularly on sub-daily time scales and for extreme events. The simulations with cumulus parameterization active underestimate short-duration heavy rainfall, and rainspells with low peak intensities are too persistent. Furthermore, even though the 6.25 km simulation generally reduces the biases seen in the 15 km run, definitive conclusions of the benefit of ”grey-zone” resolution is difficult to establish in context of the increased requirement of computer resources for the higher-resolution simulation.

La modélisation du climat à haute résolution est nécessaire aux études d'impact du climat et, de nos jours, les modèles de circulation générale (MCG) n'ont pas encore une résolution suffisante pour satisfaire ces besoins. Les modèles régionaux du climat (MRC) ont été développés dans le but de fournir des détails sur le climat à fine échelle sur des régions spécifiques de la Terre. Les MRC ont démontré leur capacité à produire de la variabilité spatiale à petite échelle qui manque dans les simulations de MCG ; pour cette raison, les MRC sont de plus en plus utilisés dans les études sur le climat actuel et futur. Malgré ce succès, les avantages découlant de la production d'une variabilité climatique de fine échelle - autres que l'effet visuel saisissant des animations réalistes - ont rarement été clairement identifiés. Les tentatives pour quantifier ces avantages, généralement désignés comme étant la valeur ajoutée (VA) des MRC, ont été relativement rares et ont prouvé que la question de la VA est très complexe. Compte tenu de cette complexité, ce projet se concentre sur un aspect particulier de cette question : l'étude des conditions préalables que doivent satisfaire certaines statistiques climatiques pour permettre aux MRC d'ajouter de la valeur aux données utilisées comme pilote. Ces conditions sont basées sur l'idée que la VA des MRC ne peut survenir que si les statistiques climatiques d'intérêt contiennent de l'information à fine échelle qui n'est pas négligeable. Des données observées et simulées par des MRC peuvent ensuite être utilisées pour quantifier l'influence relative des fines échelles dans les statistiques climatiques, comme un proxy, pour estimer la valeur ajoutée potentielle (VAP) des MRC. Deux méthodes différentes ont été utilisées pour étudier la VAP sur l'Amérique du Nord, respectivement pour la température de surface et la précipitation. Les deux méthodes comprennent 3 étapes : l'utilisation d'une technique de décomposition pour séparer les variables atmosphériques en plusieurs échelles temporelles et spatiales, le calcul de statistiques climatiques et la définition d'une quantité pour estimer la VAP. Pour la température, nous constatons que la VAP se dégage presque exclusivement dans des régions caractérisées par des forçages de surface importants, soit la présence de topographie de fine échelle ou de contrastes terre-mer. Par ailleurs, certains des processus qui produisent la variabilité de petite échelle semblent être liés à des mécanismes relativement simples tels que la réponse linéaire aux différentes propriétés physiques de la surface et la variation générale de la température avec l'altitude dans l'atmosphère. Le potentiel des MRC à ajouter de la valeur dans les projections futures de la température moyenne est brièvement étudié. L'analyse montre que la variabilité de fine échelle du signal du changement climatique est généralement très faible par rapport à celle de grande échelle, ce qui suggère que peu de VA est attendue pour cette statistique climatique. Pour les précipitations, les résultats montrent que la VAP est fortement liée à des instabilités hydrodynamiques de fine échelle. La VAP est beaucoup plus élevée sur de courtes échelles temporelles (par exemple, pour des données sur 3 heures) et pour la saison chaude en raison de la proportion plus élevée de précipitations produites par de petits systèmes météorologiques et systèmes convectifs. Dans les régions à topographie complexe, le forçage orographique induit une composante supplémentaire de VAP, peu importe la saison ou l'échelle temporelle considérée. Les résultats montrent aussi que les MRC ont tendance à reproduire relativement bien la VAP par rapport aux observations bien qu'ils montrent une légère surestimation de la VAP en saison chaude et dans des régions montagneuses. Les résultats démontrent l'utilité du cadre utilisé pour étudier la VAP dans le climat actuel et dans des projections futures. Il est souligné que l'étude approfondie de la VA des MRC devrait aider à comprendre comment utiliser le mieux les divers produits climatiques disponibles en appui aux études en impact et adaptation face au climat changeant. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : modèles régionaux du climat, valeur ajouté potentielle, Amérique du Nord, température, précipitation.

The study aims to generate a simulated, ultra high-resolution climatology over the southwestern Cape of South Africa, and in particular the Stellenbosch wine producing region, by the dynamical downscaling of observed synoptic-scale circulation. A variable-resolution global model, the conformal-cubic atmospheric model (CCAM), and a multiple-nudging strategy are applied in order to reach this goal. CCAM is employed in stretched-grid mode as a regional climate model (RCM) to simulate climate for the period 1976-2005 at four different spatial resolutions. Nudging from coarse-resoltion (2.5° in latitude and longitude), the model was first applied at a 60 km resolution over southern Africa in order to obtain a simulation of the synoptic-scale circulation over the region. Two higher resolution simulations, at 8 km and 1 km resolution, were obtained consecutively over the western and southwestern Cape, nudging from the 60 km and 8 km simulations, respectively. Finally, a 200 m simulation was performed over the Stellenbosch region. Because of the high computational requirements of high-resolution runs, each progressively higher resolution simulation is performed over a progressively smaller area of interest over which the spatial resolution is high. The simulations verify well against observed datasets, and generally capture the important climatic features over the area of interest. The 60 km CCAM simulation gives a good representation of the synoptic scale weather over southern Africa, with realistic seasonal circulation patterns and rainfall percentages as well as intra-annual rainfall totals over various regions. The mesoscale climate over the Western Cape of South Africa is captured by the 8 km simulation, especially with respect to seasonal variations in temperature and rainfall percentages - although the actual rainfall over the southwestern tip of the Western Cape is severely underestimated. The ultra high-resolution simulated diurnal cycle of temperature, relative humidity and screen level wind speed compared well against observations for the month of February. The CCAM climate simulations might not be accurate enough for some of the very sensitive studies of the wine industry, but it can have great value for the demarcation of areas which are climatically suited for viticulture and some more general viticultural studies. Ultra high-resolution climate parameter maps are presented for 1976-2005.
Dissertation (MSc)--University of Pretoria, 2009.
Geography, Geoinformatics and Meteorology
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Palaeohydrological response to internal and external climate forcing need to be understood in the context of current climate change and modelling future climate scenarios. One area that is particularly lacking in the global framework of Holocene climate reconstructions is from the southern hemisphere, and particularly from mainland Australia. It is unclear how the prominent drivers of present day climate, such as the El Niño Southern Oscillation in the pacific region have acted on longer centennial-millennial timescales. We explore these changes using a multi-proxy geochemical analysis of lacustrine organic matter from West Basin Lake, south-eastern Australia. The record is constrained by an age-depth model using newly acquired ¹⁴C radiocarbon dates, an important feature in a study encompassing 10795 BP to Present. Our analysis reveals that the hydrological balance of West Basin was high, with a generally wetter climate between 10795-7000 BP before increasingly arid conditions established from 5000 BP- Present. Continuous and cross wavelet transformation shows a common millennial periodicity linking aridity in south-eastern Australia with increased precipitation in western South America. Aridity also appears linked to periods of increased total solar irradiance in the late Holocene suggesting that the intensification of El Niño Southern Oscillation at millennial scales may be driven by solar forcing.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2016

The Indonesian Throughflow (ITF) is the only low latitude connection of the global circulation and is an essential pathway for mass, heat and salt exchange between the Pacific and Indian Oceans. The ITF is a boundary current constrained by topography and is characterised by two source pathways, a western and an eastern. At the exit to the Indian Ocean, observations show the ITF partitions amongst the three major outflow straits. The westernmost, Lombok Strait, has the lowest transport even though it is expected to carry most of the flow given that the ITF is a boundary current and this strait is a direct continuation of the western pathway. Heat and saltwater transports are different in each outflow strait and thus exchanged properties depend on the partitioning, consequently affecting the contribution to the Indian Ocean. In this study, we explore the ITF circulation and local dynamics that control the ITF partitioning. To explore what controls ITF partitioning in the context of western boundary current theories, we simulate a steady ITF in a high-resolution (4-km) regional model. The forcing consists of time-averaged velocity, temperature and salinity fields from the global model Ocean Forecasting Australia Model v3 (OFAM3). We investigate what sets the amount of western pathway water that exits via Lombok Strait in the regional model. Our reference simulation confirms the two ITF pathways and gives a mean ITF of 14.1 Sv and an outflow partitioning of 0.2:0.4:0.4 (Lombok:Ombai:Timor), consistent with observations. Focusing on the western pathway, comprising 70% of the total ITF, we investigate the routes this water follows to the Indian Ocean. Here we consider partitioning as the ratio between transport in Lombok and Makassar Straits. Relative to only Makassar Strait transport, Lombok Strait still has the lowest transport portion of the three outflows (27%). Idealised perturbation experiments help us to investigate boundary current dynamics; combinations of slip/non-slip boundary conditions and linear/non-linear advection in the momentum equations illustrate the effects of current width (CW) on partitioning. Our key finding from this analysis is that the CW in the Makassar Strait controls the Lombok Strait transport; a narrower boundary current can fit more flow through a narrow strait. To understand what sets the CW, we perform a vorticity budget. The reference simulation reveals that the leading order term that balances change in planetary vorticity is advection of vorticity. The vorticity term diagnostics for the perturbation experiments suggest non-linearity is the main term controlling the ITF current width. We test how CW influences partitioning in more realistic conditions and evaluate how low-frequency variability affects partitioning, analysing 18-yr of a global fully-realistic model OFAM3. Consistent with our perturbation experiments, we find CW in Makassar Strait controls transport in Lombok Strait. Further, we find that, on the inter-annual scale, the Makassar Strait CW is approximately constant. This suggests that Makassar Strait may be saturated and similar dynamics could also take place upstream in the ITF western pathway. Specifically, we find that the width of Makassar Strait constrains the CW, independent of variations in CW at the upstream inflow ITF at Mindanao boundary current. As a consequence, during years when Mindanao current is wide at the Indonesian Seas entrance, the flow does not entirely fit in Makassar Strait. This flow that did not fit in Makassar Strait joins the eastern pathway. The increase of inflow in the eastern pathway produces a change in transport at Timor Passage, providing a link between variability at Timor Passage and that of the Mindanao. Our results suggest that, given the ITF complexity, the simple concept of partitioning cannot be easily used as a proxy for ITF transports and predictions cannot be made based on single strait measurements. The ITF western pathway provides a more direct connection to the Indian Ocean compared to the eastern pathway. The changes eastern pathway waters undergo while circulating in the Indonesian Seas are crucial for understanding heat and tracers exchange between the two oceans. Finally, understanding what controls the ITF circulation and its variability is critical to better predicting how the ITF responds to climate change.

Dust-climate interaction over the Middle East and North Africa (MENA) has long been studied, as it is the "dustiest" region on earth. However, the quantitative and qualitative understanding of the role of dust direct radiative effect on MENA climate is still rudimentary. The present dissertation investigates dust direct radiative effect on MENA climate during summer with a special emphasis on the sensitivity of climate response to dust shortwave absorption, which is one of the most uncertain components of dust direct radiative effect. Simulations are conducted with and without dust radiative effect, to differentiate the effect of dust on climate. To elucidate the sensitivity of climate response to dust shortwave absorption, simulations with dust assume three different cases of dust shortwave absorption, representing dust as a very efficient, standard and inefficient shortwave absorber. The non-uniformly distributed dust perturb circulations at various scales. Therefore, the present study takes advantage of the high spatial resolution capabilities of an Atmospheric General Circulation Model (AGCM), High Resolution Atmospheric Model (HiRAM), which incorporates global and regional circulations. AMIP-style global high-resolution simulations are conducted at a spatial resolution of 25 km. A significant response in the strength and position of the local Hadley circulation is predicted in response to meridionally asymmetric distribution of dust and the corresponding radiative effects. Significant responses are also found in regional circulation features such as African Easterly Jet and West African Monsoon circulation. Consistent with these dynamic responses at various scales, the tropical rainbelt across MENA strengthens and shifts northward. Similarly, the temperature under rainbelt cools and that over subtropical deserts warms. Inter-comparison of various dust shortwave absorption cases shows that the response of the MENA tropical rainbelt is extremely sensitive to the strength of shortwave absorption. Further analyses reveal that the sensitivity of the rainbelt stems from the sensitivity of the multi-scale circulations that define the rainbelt. Importantly, the summer precipitation over the semi-arid strip south of Sahara, including Sahel, increases in response to dust radiative effect. The maximum response and sensitivity are predicted over this region. The sensitivity of the responses over Sahel, especially that of precipitation, is comparable to the mean state. Locally, the precipitation increase reaches up to 50% of the mean, while dust is assumed to be a very efficient absorber. As the region is characterized by the "Sahel drought", the predicted precipitation sensitivity to the dust loading over this region has a wide-range of socioeconomic implications. The present study, therefore, suggests the importance of reducing uncertainty in dust shortwave absorption for a better simulation and interpretation of the MENA climate in general, and of Sahel in particular.