Academic literature on the topic 'Western Ghats orography'

ABSTRACT. The effect of change in orientation and surface roughness of terrain on the daytime up slope flow is investigated using a 2-dimensional mesoscale model. A realistic orography profile of western ghat is chosen for the purpose. Twelve hour of integrations are performed starting from sunrise. The numerical simulation have shown that the intensity of up slope flows remained practically unaffected by change of orientation of terrain. However, increase in roughness length decreases the intensity of developed flows. For comparison purposes, the results of previous investigators are verified with a change in slope angles.

Rainfall over the coastal regions of western India [Western Ghats (WG)] and Myanmar [Arakan Yoma (AY)], two regions experiencing the heaviest rainfall during the Asian summer monsoon, is examined using a Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) dataset spanning 16 years. Rainfall maxima are identified on the upslope of the WG and the coastline of AY, in contrast to the offshore locations observed in previous studies. Continuous rain with slight nocturnal and afternoon–evening maxima occurs over the upslope of the WG, while an afternoon peak over the upslope and a morning peak just off the coast are found in AY, resulting in different locations of the rainfall maxima for the WG (upslope) and AY (coastline). Large rainfall amounts with small diurnal amplitudes are observed over the WG and AY under strong environmental flow perpendicular to the coastal mountains, and vice versa. Composite analysis of the boreal summer intraseasonal oscillation (BSISO) shows that the rain anomaly over the WG slopes lags behind the northward-propagating major rainband. The cyclonic systems associated with the BSISO introduces a southwest wind anomaly behind the major rainband, enhancing the orographic rainfall over the WG, and resulting in the phase lag. This lag is not observed in the AY region where more closed cyclonic circulations occur. Diurnal variations in rainfall over the WG regions are smallest during the strongest BSISO rainfall anomaly phase.

Abstract The city of Mumbai, India, frequently receives extreme rainfall (>204.5 mm day−1) during the summer monsoonal period (June–September), causing flash floods and other hazards. An assessment of the meteorological conditions that lead to these rain events is carried out for 15 previous cases from 1980 to 2020. The moisture source for such rain events over Mumbai is generally an offshore trough, a midtropospheric cyclone, or a Bay of Bengal depression. The analysis shows that almost all of the extreme rain events are associated with at least two of these conditions co-occurring. The presence of a narrow zone of high sea surface temperature approximately along the latitude of Mumbai over the Arabian Sea can favor mesoscale convergence and is observed at least 3 days before the event. Anomalous wind remotely supplying copious moisture from the Bay of Bengal adds to the intensity of the rain event. The presence of midtropospheric circulation and offshore trough, along with the orographic lifting of the moisture, give a unique meteorological setup to bring about highly localized catastrophic extreme rainfall events over Mumbai. The approach adopted in this study can be utilized for other such locales to develop location-specific guidance that can aid the local forecasting and emergency response communities. Further, it also provides promise for using data-driven/machine learning–based pattern analysis for developing warning triggers. Significance Statement We have identified the meteorological conditions that lead to extreme heavy rains over Mumbai, India. They are that 1) at least two of these rain-bearing systems, offshore trough, midtropospheric circulation, and Bay of Bengal depression moving north-northwestward are concurrently present, 2) an anomalous high SST gradient is present along the same latitude as Mumbai, and 3) the Western Ghats orography favors the rainfall extreme to be highly localized over Mumbai.

Abstract The structure of the mean precipitation of the south Asian monsoon is spatially complex. Embedded in a broad precipitation maximum extending eastward from 70°E to the northwest tropical Pacific Ocean are strong local maxima to the west of the Western Ghats mountain range of India, in Cambodia extending into the eastern China Sea, and over the eastern tropical Indian Ocean and the Bay of Bengal (BoB), where the strongest large-scale global maximum in precipitation is located. In general, the maximum precipitation occurs over the oceans and not over the land regions. Distinct temporal variability also exists with time scales ranging from days to decades. Neither the spatial nor temporal variability of the monsoon can be explained simply as the response to the cross-equatorial pressure gradient force between the continental regions of Asia and the oceans of the Southern Hemisphere, as suggested in classical descriptions of the monsoon. Monthly (1979–2005) and daily (1997–present) rainfall estimates from the Global Precipitation Climatology Project (GPCP), 3-hourly (1998–present) rainfall estimates from the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) estimates of sea surface temperature (SST), reanalysis products, and satellite-determined outgoing longwave radiation (OLR) data were used as the basis of a detailed diagnostic study to explore the physical basis of the spatial and temporal nature of monsoon precipitation. Propagation characteristics of the monsoon intraseasonal oscillations (MISOs) and biweekly signals from the South China Sea, coupled with local and regional effects of orography and land–atmosphere feedbacks are found to modulate and determine the locations of the mean precipitation patterns. Long-term variability is found to be associated with remote climate forcing from phenomena such as El Niño–Southern Oscillation (ENSO), but with an impact that changes interdecadally, producing incoherent responses of regional rainfall. A proportion of the interannual modulation of monsoon rainfall is found to be the direct result of the cumulative effect of rainfall variability on intraseasonal (25–80 day) time scales over the Indian Ocean. MISOs are shown to be the main modulator of weather events and encompass most synoptic activity. Composite analysis shows that the cyclonic system associated with the northward propagation of a MISO event from the equatorial Indian Ocean tends to drive moist air toward the Burma mountain range and, in so doing, enhances rainfall considerably in the northeast corner of the bay, explaining much of the observed summer maximum oriented parallel to the mountains. Similar interplay occurs to the west of the Ghats. While orography does not seem to play a defining role in MISO evolution in any part of the basin, it directly influences the cumulative MISO-associated rainfall, thus defining the observed mean seasonal pattern. This is an important conclusion since it suggests that in order for the climate models to reproduce the observed seasonal monsoon rainfall structure, MISO activity needs to be well simulated and sharp mountain ranges well represented.

Abstract. Proxy records suggest that the Northern Hemisphere during the mid-Holocene (MH), to be assumed herein to correspond to 6000 years ago, was generally warmer than today during summer and colder in the winter due to the enhanced seasonal contrast in the amount of solar radiation reaching the top of the atmosphere. The complex orography of both South and Southeast Asia (SA and SEA), which includes the Himalayas and the Tibetan Plateau (TP) in the north and the Western Ghats mountains along the west coast of India in the south, renders the regional climate complex and the simulation of the intensity and spatial variability of the MH summer monsoon technically challenging. In order to more accurately capture important regional features of the monsoon system in these regions, we have completed a series of regional climate simulations using a coupled modeling system to dynamically downscale MH global simulations. This regional coupled modeling system consists of the University of Toronto version of the Community Climate System Model version 4 (UofT-CCSM4), the Weather Research and Forecasting (WRF) regional climate model, and the 3D Coastal and Regional Ocean Community model (CROCO). In the global model, we have taken care to incorporate Green Sahara (GS) boundary conditions in order to compare with standard MH simulations and to capture interactions between the GS and the monsoon circulations in India and SEA. Comparison of simulated and reconstructed climates suggest that the dynamically downscaled simulations produce significantly more realistic anomalies in the Asian monsoon than the global climate model, although they both continue to underestimate the inferred changes in precipitation based upon reconstructions using climate proxy information. Monsoon precipitation over SA and SEA is also greatly influenced by the inclusion of a GS, with a large increase particularly being predicted over northern SA and SEA, and a lengthening of the monsoon season. Data–model comparisons with downscaled simulations outperform those with the coarser global model, highlighting the crucial role of downscaling in paleo data–model comparison.

Abstract Summer precipitation over the Western Ghats and its adjacent Arabian Sea is an important component of the Indian monsoon. To advance understanding of the physical processes controlling this regional precipitation, a series of high-resolution convection-permitting simulations were conducted using the Weather Research and Forecasting (WRF) Model. Convection simulated in the WRF Model agrees with TRMM and MODIS satellite estimates. Sensitivity simulations are conducted, by altering topography, latent heating, and sea surface temperature (SST), to quantify the effects of different physical forcing factors. It is helpful to put India’s west coast rainfall systems into three categories with different causes and characteristics. 1) Offshore rainfall is controlled by incoming convective available potential energy (CAPE), the entrainment of midtropospheric dry layer in the monsoon westerlies, and the latent heat flux and SST of the Arabian Sea. It is not triggered by the Western Ghats. When offshore convection is present, it reduces both CAPE and the downwind coastal rainfall. Strong (weak) offshore rainfall is associated with high (low) SSTs in the Arabian Sea, suggested by both observations and sensitivity simulations. 2) Coastal convective rainfall is forced by the coastline roughness, diurnal heating, and the Western Ghats topography. This localized convective rainfall ends abruptly beyond the Western Ghats, producing a rain shadow to the east of the mountains. This deep convection with mixed phase microphysics is the biggest overall rain producer. 3) Orographic stratiform warm rain and drizzle dominate the local precipitation on the crest of the Western Ghats.

Many mountainous regions in the tropics witnessed extreme orographic rainfall episodes in the recent past. The portions of the Western Ghats that fall on the Kerala state also experienced extreme climatic conditions in floods and landslides in 2018 and 2019. More than a thousand small and large landslides occurred during that period in the State's Western Ghats regions. The landslide at Kavalapara in the Malappuram district in 2019 is at the top in the state regarding the causalities, financial loss, and spatial spread. This study is based on a comprehensive field investigation at the Kavalappara landslide site and we developed a detailed landslide susceptibility map with the local community's involvement. The massive landslide covers 0.34 Sq.km (34 hectares) triggered by the unprecedented monsoon rainfall coupled with unsustainable agricultural practices. The area's risk zones have been identified and spatially mapped with the help of a detailed field investigation using Geographic Information System (GIS) and remote sensing technology. The output of the study can be used for the policymakers and planners working in landslide-prone areas.

ABSTRACT. The low/depression over northwest (NW) Bay of Bengal is the largest contributor to seasonal monsoon rainfall over all stations in Orissa and Orissa as a whole. The Low Pressure Systems (LPS) and cyclonic circulation (cycir) extending upto 500 hPa level over NW Bay of Bengal alone contribute about 22% to the seasonal monsoon rainfall through about 12 days. The monsoon trough without any significant embedded systems over Orissa and adjoining regions contributes about 28% to seasonal rainfall through about 55 days. All types of LPS including low, depression and cyclonic storm yield maximum rainfall in their left forward (southwest) sectors. The maximum rainfall belt lies more southward due to a depression compared to that due to a low. The spatial distribution of rainfall due to cycir is less systematic. The interaction due to Eastern Ghat plays a significant role in spatial distribution of rainfall over western and eastern sides of the Eastern Ghat due to monsoon lows and depressions over Orissa and adjoining Bay and land regions. The orographic interaction due to Eastern Ghat with the cycirs over Orissa and adjoining Bay and land regions is significantly less leading to no significant difference in spatial distribution of rainfall over eastern and western sides of the Eastern Ghat.

The Western Ghats (WG) orography that runs along the western coast of peninsular India, is a long and narrow mountain chain with an average height of about 1200 meters. The orientation of this orography is approximately perpendicular to the mean low–level winds over the eastern Arabian Sea during boreal summer season (June–September; JJAS). In JJAS, while western sides of this orography receive high intensity of precipitation, a region to the lee side of the mountain, termed as the Bay of Bengal Cold Pool (BoB–CP), receive very less precipitation. In this thesis, we have investigated the role of WG orography in existence of the BoB–CP. In addition, we have shown the influence of WG on climate around the globe as well as the intraseasonal variability over the Indian region and the equatorial Indian Ocean. In the first part of the thesis, we have documented the climatology of BoB–CP and how the region is peculiar compared to other parts of south Asia. In boreal summer (JJAS), most of the Indian land and its surroundings experience rainrates exceeding 6 mm/day with considerable spatial variability. Over southern Bay of Bengal (BoB) along the east coast of the Indian peninsula (BoB–CP), the rain intensity is significantly lower (<2 mm/day ) than its surroundings. This low rainfall occurs despite the fact that the sea surface temperature in this region is well above the threshold for convection and the mean vorticity of the boundary layer is cyclonic with a magnitude comparable to that over the central Indian monsoon trough where the rainrate is about 10 mm/day. It is also noteworthy that the seasonal cycle of convection over the BoB–CP shows a primary peak in November and a secondary peak in May. This is in contrast to the peak in June–July over most of the oceanic locations surrounding the BoB–CP. We use an Atmospheric General Circulation Model (AGCM) to understand this paradox. Decade long simulations of the AGCM were carried out with varying (from 0 to 2 times the present) heights of the WG. We find that the lee waves generated by the strong westerlies in the lower troposphere in the presence of the WG mountains cause descent over the BoB–CP. Thus, an increase in the height of the WG strengthens the lee waves and reduces rainfall over the BoB–CP. More interestingly in the absence of the WG mountains, the BoB–CP shows a rainfall maxima in the boreal summer similar to that over its surrounding oceans. The redistribution of rainfall with the increase in height also resulted in the increase in Indian summer monsoon rainfall (ISMR) by almost 15%. The WG also impacts the climate over the middle and high latitude regions by modifying the upper tropospheric circulation. In the second part of the thesis, we have investigated the role of convection over northern BoB in controlling the rainfall over BoB–CP. Even after the removal of WG, the BoB–CP shows low level divergence, which leads us to speculate the role of acceleration/deceleration of meridional winds by convection over northern BoB. Intraseasonal variations (ISVs) over BoB–CP also depicts the existence of the see–saw between precipitation over head Bay of Bengal and southern Peninsular India, including BoB–CP. Based on these findings, we performed decade long simulations with varying Sea Surface Temperature (SST) gradients over northern BoB. The SST gradient– experiments reveal that convection over north BoB further reduces rainfall over BoB–CP by intensifying the upper level lee–waves, causing down–draft and accelerating the low level winds causing divergence near the surface. A combined effect of WG and SST gradients shows that even though the SST gradients influence convection over BoB–CP, the effect is overshadowed by the absence of WG indicating that the WG has dominant control on the convection over BoB–CP than the other. In the third part of our study, we analyzed the implications of the perturbations in WG orography on ISVs over India as well as over the equatorial Indian Ocean region. The increase in height of WG leads to the intraseasonal oscillations (ISO) to strengthen over the equatorial region. With the absence of WG, the northward propagations have become stronger compared to the mean state. These variations in ISVs also altered the ISVs over the equatorial Indian Ocean. Madden Julian Oscillation (MJO) is the most important component of ISVs over the equatorial belt, which we have investigated in this study. The model captures the MJO signal reasonably well with slight underestimation in its strength and meridional extent. With the increase in WG height, there is a change in circulation pattern around WG region, increasing the meridional as well as the westerly component of wind over the equatorial region. This provides more moisture as well as an increase in boundary layer convergence, eventually leading to the increase in convective activity associated with MJO over the region. This also suggests that it is essential to represent the orographic features near the equatorial region in order to simulate the MJO reasonably well in a model. In the last part of the thesis, we document the observed changes in the variability of rainfall and outgoing longwave radiation (OLR) associated with the MJO during 1998 to 2015, when reliable satellite derived daily rainfall and OLR are available. Observations show recent weakening of variance of convective activity with MJO across the equatorial Indian Ocean (EQIO) and Maritime Continent (MC) during boreal summer as well as winter seasons. However, during boreal winter MJO variance increased significantly over northern Australia and north–eastern Pacific. Using rain gauge based observations we further show that the decreasing trend of 30–60 day intraseasonal mode over MC is significant for an extended range of period (1958–2007). During northern summer, the MJO variability in the POST (2007–2015) period display remarkable reduction in convection for all the wavenumbers compared to PRE (1998–2006) period. During northern winter, along with reduction of intensity, the maximum variance of MJO related activity is shifted from lower to higher wavenumber in recent years. Thus, during the POST period, the convection associated with an MJO is broken down into smaller scales, reducing the variability in rainfall along the longitudes. The multivariate MJO index (The Wheeler–Hendon Index) exhibits a shift from a higher probability of stronger events to weaker events over EQIO, MC and Western Pacific Ocean in the recent years both during summer and winter seasons. There is a southward shift in the location of maximum variance from northern latitudes towards the southern latitudes either weakening the northern branch of maximum variance or reducing (increasing) the asymmetry along those longitudes during summer (winter). The relationship between OLR and rainfall has also modified from PRE to POST between the Indian Ocean and Maritime Continents possibly due to increase in cloud top height and recent sea surface warming over the Indian Ocean. These variations in MJO strength can have a huge impact on the local and remote climate systems across the globe and can modulate the extreme events across the globe. This study highlights the importance of WG orography in modulating the convection over BoB–CP, redistribution of rainfall over the subcontinent and the climate over the globe. These mountains can impact the ISVs over India as well as the equatorial Indian Ocean. The results of this study underline the importance of narrow mountains like the WG in the tropics in altering the global climate and possibly calls for a better representation of such mountains in climate models. It also provides insights into the recent weakening of MJO and its possible influence on the climate as well as the extreme events across the globe.