Article

Some Aspects of Indian Monsoon Depressions and the Associated Rainfall

March 1973
Monthly Weather Review 101(3)

March 1973
101(3)

DOI:10.1175/1520-0493(1973)101<0271:SAOIMD>2.3.CO;2

Authors:

This study is based on all the monsoon depressions that moved westward across India between Calcutta, Allahabad, and Delhi on the right and Gopalpur, Nagpur, and Ahmadabad on the left during July and August for the period 1891–1960. Statistical distribution, of 24-hr motion and of the intensity of the depression, the relation between 24-hr motion and concurrent 24-hr rainfall, and the relation between the intensity of the depression and subsequent 24-hr rainfall are examined. In addition, the average rainfall per depression day and its standard deviation, the contribution of depression rainfall to the total rainfall, and the efficiency of the depression as a rain giver are computed. Mean patterns of 24-hr rainfall within 500 km of the center of the depression along longitudes 87°E, 80°E, and 75°E are obtained, and the main points of difference between them are discussed. In the quadrants to the right of the depression track, the rainfall field is flat. In the quadrants to the left, however, large...

Analysis of summer monsoon rainfall over Maharashtra and its relation with SSTs

Article

Full-text available

Dec 2020

This paper is based on the spatio-temporal analysis of daily, monthly and seasonal rainfall variability over Maharashtra state of the Indian region, for the period 1901–2017 during summer monsoon, viz., June through September. This analysis is carried out over Maharashtra’s four sub-divisions, where mean rainfall received for JJAS (1901–2017) is more over Konkan and Goa and east Vidarbha and less over Madhya Maharashtra. Coefficient of variation is considered to understand spatial variability of total rainfall received during JJAS by the state. The percentage of rainy days is found to be more for July and August months compared to June and September months as per the advancement of monsoonal flow noticeable through this study. Hence, contribution to JJAS rainfall more from July month proceeded with August month is considerable. September month contribution is the least; nevertheless its contribution is prominent from south Madhya Maharashtra. Rainfall variability is linked with El Niño and La Niña, by calculating composites of standardized rainfall anomalies and % rainy days during El Niño and La Niña years, which show that monthly and seasonal rainfall, are modulated by variations in SSTs of equatorial central Pacific Ocean. Thus this study aims to analyze the changing pattern of rainfall over Maharashtra and its sub-division as linking El-Niño and La-Niña events with rainfall received over small domain is the need of the hour as each region behaves differently during El-Niño or La-Niña.

Improving the subseasonal variability of the Indian summer monsoon in a climate model

Article

Full-text available

Jun 2023
INT J CLIMATOL

Many global climate models, including the Climate Forecast System version 2 (CFSv2), have a biased representation of subseasonal modes of variability of the Indian summer monsoon. For instance, they simulate a weaker summer mean monsoon low‐pressure systems (LPS) climatology, faster than observed northward propagation of monsoon intraseasonal oscillations (MISOs), and a systematic dry bias over Indian landmass. The Bay of Bengal (BoB), with its shallow mixed layers and unique thermal stratification, significantly modulates the convective activity in this region at subseasonal‐to‐seasonal timescales through modulation of sea surface temperature. The highly stratified upper ocean in the BoB is due to the enormous freshwater it receives from rains and rivers. A river routing model is coupled to the CFSv2 to account for the riverine freshwater and the improvements in modelling the upper‐ocean structure are analysed. Model simulations indicate that inclusion of temporally varying riverine freshwater improves the upper‐ocean state in the BoB and the observed mixed‐layer temperature gradients in the Bay are simulated reasonably after incorporating the time varying river runoff. This resulted in increased LPS lifetime and track density, and enhanced rainfall over central India. Better representation of the upper‐ocean stratification in the model leads to larger post‐convection shoaling of mixed layers at intraseasonal timescales, thereby forming thick barrier layers. Enhanced air–sea interactions restricted to the shallow mixed layer are associated with stronger vorticity, specific humidity and low‐level convergence to the north of the intraseasonal convection band. This enhanced low‐level moisture convergence north of the convection centre results in realistic northward propagation of MISO and aids LPS activity. It is demonstrated that better simulation of the upper‐ocean structure in coupled climate models can improve the representation of subseasonal modes of monsoon variability. These results bear important implications for operational forecasting.

Role of mean, variability and teleconnection of clouds behind Indian summer monsoon rainfall

Article

Full-text available

Apr 2023
INT J CLIMATOL

Skilful prediction of the seasonal Indian summer monsoon (ISM) rainfall (ISMR) at least one season in advance has great socio‐economic value. The ISM is a lifeline for about a sixth of the world's population. The ISMR prediction remained a challenging problem with the subcritical skills of the dynamical models due to a limited understanding of the interaction among clouds, convection and circulation. In this study, we have analysed the seasonal mean of high cloud fraction, ice mixing ratio and ice cloud fraction from satellite and reanalysis and demonstrated their importance for ISM. The variability of the mixing ratio of cloud ice in different time scales (3–7 days, 10–20 days and 30–60 days bands) is also examined from reanalysis during ISM. Here, we have shown the teleconnection of different cloud variables over the ISM region with global sea surface temperature. We found that they are tied with slowly varying forcing (e.g., El Niño and Southern Oscillation). Besides, the correlation of cloud ice with different indices (Niño, Pacific Decadal Oscillation, North Atlantic Oscillation and Extratropics) may enhance the potential predictability of ISMR. The representation of deep convective clouds, which involve the ice‐phase processes in a coupled climate model, strongly modulates ISMR variability in association with global predictors. The results from the two sensitivity simulations using coupled global climate model (CGCM) demonstrate the importance of the cloud ice on ISM rainfall predictability. Therefore, this study provides a scientific basis for improving the simulation of the seasonal ISMR by developing the physical processes of the cloud on a subseasonal time scale and motivating further research in this direction.

Trends and Variabilities of Different Rainfall Intensities Over the West Coast and Central India and Their Association with Global SSTs

Article

Full-text available

Nov 2022
PURE APPL GEOPHYS

In this study, characteristics of summer monsoon rainfall (SMR) are investigated for two regions, West Coast (WC) and Central India (CI), in the Indian subcontinent during 1901–2020. We classified rain events into different intensity bins, viz. dry, low, moderate, high, very high and extreme. SMR in CI exhibits a significant decreasing trend of −5.6 mm season⁻¹ decade⁻¹. Similarly, a significant increasing trend in low and decreasing trend in very high intensity bins are noticed. In WC, the extreme intensity bin shows a significant increasing trend. From the coherence analysis of intensity bins between WC and CI, it is found that the correlation of rain events increased during 1991–2020, especially in extreme intensity bins. Analysis of the relationship of SMR with global sea surface temperature (SST) was carried out. The relationship of WC-rainfall with the Arabian Sea (AS) and the Bay of Bengal (BoB) SST shows significant changes from 1950. The correlation was previously positive but became negative after 1950. CI-rainfall shows a significant positive correlation with the southwest Pacific Ocean (SWPO) SST before 1960, and thereafter it became negative. Extreme intensity bin correlation with the Niño 3 index has changed from 1960 in both WC and CI regions. When the SST variations in various ocean domains are examined, it is discovered that the SST is increasing significantly in the AS, BoB, north central Pacific Ocean (NCPO), Niño 3, SWPO, and north Atlantic Ocean (NAO). In general, recent changes in rainfall and rainfall-SST relationship clearly indicate the climate shift.

Storm types in Bangladesh: duration, intensity and area of intra‐daily wet events

Article

Full-text available

Sep 2022
INT J CLIMATOL

We explore the characteristics of 96,190 wet events (WEs) defined as consecutive 3‐hourly rainfall ≥ 1 mm/3 hr from a network of 34 stations across Bangladesh. Nearly 60% (5%) of WEs last ≤ 3 (≥ 15) hr. The WEs are dynamically clustered into four “canonical” storm types (STs), mostly discretized by their duration, but also their mean and maximal intensity. While durations, total amounts and wet contiguous areas of WEs are positively related, their mean intensity is nearly independent of them. Approximately 60% of WEs are associated with ST#1, characterized by short and small WEs and very low rainfall amounts (usually <10 mm), ~30% of WEs are associated with either (ST#2) short/small WEs but with intense rainfall, probably mostly related to scattered thunderstorms or (ST#3) longer/larger WEs but with less intense rainfall. The last ST (ST#4) is rare (~6%), related to very long durations and large wet areas and includes the wettest WEs. It is especially frequent over southeastern Bangladesh. ST#2–ST#4 contribute almost equally to the local‐scale total amount of rainfall (27–29% each in mean) while ST#1, despite its individual low rainfall amount, still includes ~15% of it. ST#2 (ST#4) is related to the highest probability of occurrence of 3‐hourly (daily) extremes. ST#4 occurrence is the most impacted by synoptic Indian lows/depressions as well as the main modes of intra‐seasonal variation, while ST#1 and ST#2 are also significantly impacted by intra‐seasonal modes but in reverse manner than ST#4.

Diagnostic of the massive flood event and flood hazard mapping in Tons River basin

Article

Full-text available

May 2022
THEOR APPL CLIMATOL

This study diagnoses the Satna flood event in the Tons River basin. The occurrence of this intense flood is attributed to the rainfall associated with the movement of the monsoon depression during the peak monsoon season. The study uses Weather Research Forecast (WRF) model to examine the origin, movement, and dissipation of the monsoon depression over India. The study also incorporates remote sensing techniques and field campaigns to better understand the impact on the areas. The model captures the origin of the monsoon depression and the highest precipitating days fairly well. However, it underestimates the rainfall magnitude with respect to different observations due to the limited ability of the model to capture the rainfall maxima spatially. Moreover, the conditional instability of the second kind (CISK) mechanism seems to drive the monsoon depression. A positive feedback mechanism is observed along the track of the depression between rainfall and convective activity leading to excess rainfall over the Tons basin. The remote-sensing-based analysis using Landsat 8 products shows that around 1309 km 2 area of the Tons basin was inundated during the event. Based on the computed Flood Hazard Index values, the entire basin has been divided into the low, medium, high, and very high flood hazard zones with several affected hamlets 19, 178, 155, and 91, respectively. The flood hazard values are important for the planners to adopt appropriate adaptation and mitigation to minimize the impact of future flooding in the basin.

Examining the variability of cloud hydrometeors and its importance on the Indian summer monsoon rainfall predictability

Preprint

Full-text available

Jan 2021

Skilful prediction of the seasonal Indian summer monsoon (ISM) rainfall (ISMR) at least one season in advance has great socio-economic value. It represents a lifeline for about a sixth of the world's population. The ISMR prediction remained a challenging problem with the sub-critical skills of the dynamical models attributable to limited understanding of the interaction among clouds, convection, and circulation. The variability of cloud hydrometeors (cloud ice and cloud water) in different time scales (3-7 days, 10-20 days and 30-60 days bands) are examined from re-analysis data during Indian summer monsoon (ISM). Here, we also show that the 'internal' variability of cloud hydrometeors (particularly cloud ice) associated with the ISM sub-seasonal (synoptic + intra-seasonal) fluctuations is partly predictable as they are found to be tied with slowly varying forcing (e.g., El Ni\~no and Southern Oscillation). The representation of deep convective clouds, which involve ice phase processes in a coupled climate model, strongly modulates ISMR variability in association with global predictors. The results from the two sensitivity simulations using coupled global climate model (CGCM) are provided to demonstrate the importance of the cloud hydrometeors on ISM rainfall predictability. Therefore, this study provides a scientific basis for improving the simulation of the seasonal ISMR by improving the physical processes of the cloud on a sub-seasonal time scale and motivating further research in this direction.

Reply to Comment by E. T. Swenson, D. Das, and J. Shukla on “Unraveling the Mystery of Indian Summer Monsoon Prediction: Improved Estimate of Predictability Limit”

Article

Full-text available

Oct 2020

Swenson et al. (2020, https://doi.org/10.1029/2020JD033037) (hereafter SN20) raise some technical issues on observed correlations between the synoptic variances and the seasonal mean of area averaged (all‐India or central India) Indian summer monsoon rainfall (ISMR) reported in Saha et al. (2019, https://doi.org/10.1029/2018JD030082) (hereafter SA19). SN20 did not comment on the other major finding of SA19 on model‐based prediction and predictability of the ISMR. Therefore, we focus in this reply only on the observational part of SA19 even though the modeling part is closely related to the observations. While we disagree with SN20 in all three aspects of their comments, we present additional analysis to clarify the scientific basis for our selection of area averaging, the selection of 30‐year period for the study and argue that the relevant probability density function of daily rainfall is not that of the individual stations (or grid points) but that of the daily averaged rainfall over a comparable area. On the debate of small area averaging (major point of SN20), we argue that it is physically meaningless in the context of ISMR.

Synoptic Scale Systems

Chapter

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Jun 2020

Decadal variability of precipitation and moisture source attribution over India

Article

Full-text available

Jan 2024
INT J CLIMATOL

The seasonal variability of precipitation has a profound impact on water demand for India's agricultural and socio‐economic sectors which are associated with continental and surrounding oceanic moisture sources controlled by dynamic and thermodynamic processes. In this study, moisture sources have been tracked by the Lagrangian trajectory approach over four homogeneous regions: Northwest India (NWI), Central India (CEI), Northeast India (NEI) and South Peninsular India (SPI). ERA‐Interim reanalysis data are used throughout the study for three decades from 1989 to 2018. The results indicate that the multidecadal variability of precipitation has strengthened towards the mid‐decade (1999–2008), mainly attributable to moisture source transport from the Arabian Sea. These enhancement patterns of moisture sources have been widespread over Central India for all three decades. Moisture availability in the boundary layer highly affects the precipitation and it is found that 80% of the moisture above the boundary layer contributes >10% of monsoonal precipitation occurrences. Further, the NEI drying trend in monsoonal rainfall is found to be related to the consequent weakening of the land–ocean temperature gradient (LOTG) during all three decades in this region and the absence of low cloud cover over the region throughout the study period. In addition, the moisture sources are regulated by prevailing large‐scale features such as frequency of La Niña, northward shifting of low‐level jet and strengthening of Hadley cell.

Climate change quadruples flood‐causing extreme monsoon rainfall events in Bangladesh and northeast India

Article

Full-text available

Jan 2024
Q J ROY METEOR SOC

Bangladesh and northeast India are the most densely populated regions in the world where severe floods as a result of extreme rainfall events kill hundreds of people and cause socio‐economic losses regularly. Owing to local high topography, the moisture‐carrying monsoon winds converge near southeast Bangladesh (SEB) and northeast Bangladesh and India (NEBI), which produces significant extreme rainfall events from May to October. Using observed data, we find an increasing trend of 1‐day extreme event (>$$ > $$150 mm·$$ \cdotp $$day −1$$ {}^{-1} $$) frequency during 1950–2021. The extreme rainfall events quadrupled over western Meghalaya (affecting NEBI) and coastal SEB during this period. Composite analysis indicates that warm Bay of Bengal sea‐surface temperature intensifies the lower tropospheric moisture transport and flux through the low‐level jet (LLJ) to inland, where mountain‐forced moisture converges and precipitates as rainfall during extreme events. To understand the role of climate change, we use high‐resolution downscaled models from Coupled Model Intercomparison Project phase 6 (CMIP6). We find that the monsoon extreme event increase is ongoing and the region of quadrupled events further extends over the NEBI and SEB in the future (2050–2079) compared with historical simulations (1950–1979). A quadrupling of the intense daily moisture transport episodes due to increased LLJ instability, a northward shift of LLJ, and increased moisture contribute to the increased future extreme events. This dynamic process causes moisture to be transported to the NEBI from the southern Bay of Bengal, and the local thermodynamic response to climate change contributes to the increased extreme rainfall events. The CMIP6 projection indicates that more devastating flood‐causing extreme rainfall events will become more frequent in the future.

Improving rainfall forecast at the district scale over the eastern Indian region using deep neural network

Article

Full-text available

Nov 2023
THEOR APPL CLIMATOL

Indian Summer Monsoon (ISM) rainfall is largely contributed by synoptic scale low-pressure systems over the Bay of Bengal and moves towards Indian landmass through eastern Indian states such as Odisha. These systems often cause heavy to very heavy rainfall localized events. The prediction of these events with high accuracy is still a major challenge for deterministic weather models. For the first time, this study has used machine learning (ML) and deep learning (DL) methods to improve the rainfall forecast using Weather Research and Forecast (WRF) forecasts output up to a lead time of 96 h (day 4) at the district scale of Odisha. Our findings demonstrate that the ML model improves the cumulative rainfall forecast (> 70%) but not more than the DL (multilayer perceptron (MLP) and convolutional neural network (CNN)) models, i.e., > 80%. Overall, on average, the DL model improved the rainfall prediction accuracy by 14% compared to ML models and 16% compared to the WRF model respectively. Results suggested that the CNN predicts rainfall with more than 70% for heavy and very heavy rainfall events for all days. It is also noted that WRF microphysics schemes are biased towards light rainfall class and the same has been effectively corrected by DL models. Furthermore, CNN shows promising results with more than 80% percent accuracy in forecasting rainfall for heavy rainfall events at the district scale. The inclusion of DL models in the Numerical Weather Prediction (NWP) model forecast output convincingly enhances the prediction skills. The findings of this study are highly significant for operational agencies, and disaster managers for effective planning, management, and preparedness at the district scale.

Contrasting controls on convection at latitude zones near and away from the equator for the Indian summer monsoon

Article

Full-text available

Oct 2023
ENVIRON RES LETT

Understanding controls on convection on various timescales is crucial for improved monsoon rainfall forecasting. Although the literature points to vertically homogeneous vorticity signatures preceding rainfall during the Indian summer monsoon, we show using reanalysis data that, for rainfall associated with northward propagating intraseasonal oscillations (ISOs), different controls are present at different latitude zones. For the latitude zone close to the equator (5°N-14°N) and including the southern Indian region, a conventional dynamical control on rainfall exists with barotropic vorticity leading ISO rainfall by about five days. In contrast, for the latitude zone away from the equator (15°N-24°N; covering the central Indian region), thermodynamic fields control ISO rainfall, with barotropic vorticity following rainfall by two days on average. Over central India, the pre-moistening of the boundary layer yields maximum moist static energy (MSE) about four days prior to ISO rainfall. Analyzing the statistics of individual events verifies these observations. Similar thermodynamic control is also present for the large-scale extreme rainfall events (LEREs) occurring over central India. These high rainfall events are preceded by positive MSE anomalies arising from the moisture preconditioning of the boundary layer. The resulting convection then leads to a maximum in barotropic vorticity 12 hours after the rainfall maximum. Characterizing these influences on convection occurring over various timescales can help identify the dominant mechanisms that govern monsoon convection. This can help reduce climate model biases in simulating Indian monsoon rainfall.

Pradeep PhD thesis

Thesis

Full-text available

Oct 2022

Pradeep Kushwaha

Middle Tropospheric Cyclones (MTCs) are moist synoptic tropical systems with vorticity maxima in the middle troposphere and weak signature in the lower troposphere. We begin with a tropical survey of MTCs; in South Asia, manual tracking reveals that MTCs change character during their life, i.e., their track is composed of MTC and LTC (lower troposphere cyclone) phases. The highest MTC-phase density and least motion is over the Arabian Sea, followed by the Bay of Bengal and the South China Sea. An MTC-phase composite shows an east-west tilted warm above deep cold-core temperature anomaly with maximum vorticity at 600 hPa. In contrast, the LTC phase shows a shallow cold-core below 800 hPa and a warm upright temperature anomaly with a lower tropospheric vorticity maximum. Further, the systems with MTC-like morphology are observed over the west and central Africa and east and west Pacific in boreal summer. In boreal winter, regions that support MTCs include northern Australia, the southern Indian Ocean, and South Africa. The MTC’s kinematic and thermal structure exhibit remarkable similarity among different basins, suggesting a common underlying maintenance mechanism. Given that the Arabian Sea is a hot spot of devastating MTCs, their classification and genesis mechanisms in this region are explored. Both k-means and cyclone tracking approaches reveal four dominant weather patterns that lead to the genesis of these systems; specifically, re-intensification of westward-moving synoptic systems from Bay of Bengal (Type 1, 51%), in-situ formation with a coexisting cyclonic system over the Bay of Bengal that precedes (Type 2a, 31%) or follows (Type 2b, 10%) genesis in the Arabian Sea, and finally in-situ genesis within a northwestward propagating cyclonic anomaly from the South Bay of Bengal (Type 2c, 8%). Thus, a significant fraction of rainy middle tropospheric synoptic systems in this region form in association with cyclonic activity in the Bay of Bengal (BOB). While in-situ formation with a BOB cyclonic anomaly (Type 2a and 2b) primarily occurs in June, downstream development is more likely in the core of the monsoon season. Type 2a is associated with the highest rain rate and points towards the dynamical interaction between a low-pressure system over the BOBand the development of MTCs over western India and the northeast Arabian Sea. The frequent coexistence BOB lows during the Type 2a formation of MTCs is not merely a coincidence. Rather the BOB system induces an off-equatorial Gill type response which deepens the middle tropospheric trough and zonal shear over the Arabian Sea. In turn, this enhances the cyclonic vorticity and intensifies the middle troposphere anomalous easterlies north of 20◦N. This results in reduction of dry and warm desert air advection, depletion of low-level inversion and destabilization of the lower troposphere. Following which, the eddy-induced moisture flux convergence and advection of climatological moisture increases the saturation fraction. These favorable conditions within the middle troposphere trough region triggers the genesis of MTCs over the Arabian Sea. The proposed role of BOB lows in MTC formation is validated by numerical experiments using the state-ofthe-art Weather Research & Forecast (WRF) model. Twenty one ensemble members were generated through addition of balanced vortices to the climatological flow in the BOB. Consistent with observations, in simulations, the BOBlowdeepens the monsoon trough over western India, enhances the cyclonic shear, reduces the inversion, and increases the middle troposphere relative humidity; supporting the genesis of an MTC over the Arabian Sea within 2.5 − 4 days of model integration. During the first 24 − 36 hours of intensification, advection of absolute vorticity and tilting account for the entire vorticity tendency, while during the rapid intensification phase, vortex stretching is the dominant source of vorticity enhancement. Mechanism Denial Runs with cooling and drying of BOBwere then performed to show that this hinders the intensification of the low over the Bay and consequently, the MTC did not form over the Arabian Sea. This global survey, classification, identification of precursors, connection with cyclonic activity over the Bay of Bengal, and dependence on the large-scale environment provide an avenue for better understanding and predicting rain-bearing MTCs.

How do the characteristics of monsoon low pressure systems over India change under a warming climate? A modeling study using the NCAR CESM

Article

Full-text available

Jun 2023
CLIM DYNAM

In this study, using the NCAR Community Earth System Model (CESM1.2.2), we investigate the changes in the characteristics of the summer monsoon low pressure systems (LPS) over India in a twenty-first century climate change simulation corresponding to the RCP8.5 scenario. A slight weakening in monsoon circulation and an increase in mean summer monsoon precipitation over India are simulated in a warmer climate, consistent with several previous studies. The weakening of the monsoon circulation is associated with a pair of anticyclonic anomalies straddling the equator in the low level, weakening the cross-equatorial monsoonal flow from the southern hemisphere. These low-level circulation anomalies appear to be robust features in the equatorial Indian Ocean under climate change. An increase in moisture flux is also simulated over the Indian subcontinent. However, we find no significant change in the number of LPS or their spatial distribution under the RCP8.5 scenario. This is attributed to a small but non-significant decrease in the low-level meridional cyclonic shear in zonal winds. An increase in the intensity and frequency of extreme precipitation over India in a warmer world is simulated and is likely associated with an increase in moisture content. Our study shows that the fractional contribution of LPS to mean and extreme precipitation over India in a warmer world is likely unchanged, but the frequency and intensity of extreme events are larger. Because of the diversity in the results from single model studies on LPS characteristics in a warmer world, a future study that uses CMIP6 multi-model data would be valuable to assess the robustness and the uncertainty in changes in LPS activities under climate change.

Hydro-Meteorological Characteristics of the 1973 Catastrophic Flood in the Mahi Basin, India

Article

Full-text available

Apr 2023

The September 1973 flood in the Mahi Basin was one of the most catastrophic and widespread in the 20th century. However, the hydro-meteorological characteristics of the 1973 flood were not studied. Therefore, an attempt has been made to analyze the meteorological and hydrological processes that led to the 1973 flood. Accordingly, daily rainfall data, river discharge, and cross-section data were obtained for the analysis. The 1973 flood was associated with very heavy rainfall resulting from two successive low-pressure systems (LPS) from 26 to 31 August 1973 and 2 to 5 September 1973. The rainfall variability in the Mahi Basin was 24% (annual) and 25% (monsoon) in 1973. The analysis showed that out of 69 rainfall stations, 13 stations received 100% rainfall in the monsoon season in 1973. Under the influence of the second LPS (7 and 9 September 1973), 21 rain gauge stations recorded very heavy rainfall (124.5-244.4 mm) on 8 September. As a result, the maximum discharge of the Mahi River (40,663 m 3 /s) was observed at Wanakbori on 9 September. The flood hydrograph denoted two flood peaks of 28,125 m 3 /s and 33,097 m 3 /s magnitudes resulting from LPS at Kadana. A newly constructed bridge (in 1972) on the Mahi River at the Kailashpuri village washed out due to a large discharge of 21,553 m 3 /s magnitude on 7 September 1973. The hydro-meteorological analysis of the 1973 flood specified the significance of the LPS in a flash flood disaster in the Mahi Basin. This study will benefit hydrologists and civil engineers creating design floods for the construction of the hydraulic structures in the Mahi Basin, and will help to avoid any future catastrophic floods.

Role of Low Level Wind to the Heavy Rainfall on the East Coast of the Bay of Bengal during Low Pressure System

Article

Mar 2023
DYNAM ATMOS OCEANS

The Stirring Tropics. Part II: Theory of Moisture Mode−Hadley Cell Interactions

Preprint

Full-text available

Mar 2023

Interactions between large-scale waves and the Hadley Cell are examined using a linear two-layer model on an f-plane. A linear meridional moisture gradient determines the strength of the idealized Hadley Cell. The trade winds are in thermal wind balance with a weak temperature gradient (WTG). The mean meridional moisture gradient is unstable to synoptic-scale (horizontal scale of ∼1000 km) moisture modes that are advected westward by the trade winds, reminiscent of oceanic tropical depression-like waves. Meridional moisture advection causes the moisture modes to grow from “moisture-vortex instability” (MVI), resulting in a poleward eddy moisture flux that flattens the zonal-mean meridional moisture gradient, thereby weakening the Hadley Cell. The amplification of waves at the expense of the zonal-mean meridional moisture gradient implies a downscale latent energy cascade. The eddy moisture flux is opposed by a regeneration of the meridional moisture gradient by the Hadley Cell. These Hadley Cell-moisture mode interactions are reminiscent of quasi-geostrophic interactions, except that wave activity is due to column moisture variance rather than potential vorticity variance. The interactions can result in predator-prey cycles in moisture mode activity and Hadley Cell strength that are akin to ITCZ breakdown. It is proposed that moisture modes are the tropical analog to midlatitude baroclinic waves. MVI is analogous to baroclinic instability, stirring latent energy in the same way that dry baroclinic eddies stir sensible heat. These results indicate that moisture modes stabilize the Hadley Cell, and may be as important as the latter in global energy transport.

Deciphering the relationship between vegetation and Indian summer monsoon rainfall

Article

Full-text available

Mar 2023
ENVIRON RES LETT

Land surface utilization in the Indian subcontinent has undergone dramatic transformations over the years, altering the region’s surface energy flux partitioning. The resulting changes in moisture availability and atmospheric stability can be critical in determining the season’s monsoon rainfall. This study uses fully coupled global climate model simulations with idealized land cover to elucidate the consequences of land surface alterations. We find that an increase in forest cover, in general, increases precipitation in India. However, precipitation is not a linear function of forest-covered-area due to the spatially heterogeneous nature of the impact. A fully forest-covered India receives less precipitation than when the forest covers only the eastern side of India, occupying just about half the area. This signifies the importance of the east-west gradient in vegetation cover observed over India. Using an energy balance model, we diagnose that the diverse nature of this precipitation response results from three different pathways: evaporation from the surface, the net energy input into the atmosphere, and moist stability. Evaporation exhibits a linear relationship with forest-covered-area and reveals minimal spatial heterogeneity. On the contrary, the influence through the other two pathways is found to be region specific. Rainfall modulation via changes in net energy input is dominant in the head Bay of Bengal region, which is susceptible to convective systems. Whereas impact through stability changes is particularly significant south of 20 ∘ N. In addition, we find that moisture advection modulates the significance of these pathways over northwest India. Thus, the impact of land cover changes act via three effective mechanisms and are region dependent. The findings in this study have broader ramifications since the dominant region-specific mechanisms identified are expected to be valid for other forcings and are not just limited to the scenarios considered here.

Fidelity of the latest high-resolution CORDEX-CORE regional climate model simulations in the representation of the Indian summer monsoon precipitation characteristics

Article

Full-text available

Nov 2022
CLIM DYNAM

Namendra Kumar Shahi

This study evaluates the fidelity of the latest high-resolution CORDEX-CORE individual model (i.e., REMO2015, COSMO-crCLIM-v1-1, and RegCM4.7) simulations and their multi-model mean (MMM) in simulating the spatio-temporal characteristics of the Indian summer monsoon (ISM) precipitation during 1980–2015. The present study focuses on the mean, extreme, onset date, and intraseasonal variability and its propagation characteristics of the ISM. In addition, the evaluation of the recent IMDAA reanalysis is also presented. The results show that the MMM produced a more realistic representation of the spatio-temporal distribution of seasonal mean precipitation and wet-day frequency/intensity than any individual CORDEX model. It is noted that all simulations (except for the REGCM model) largely captured the observed spatial patterns of extreme precipitation indices (i.e., R99p and Rx1day) with some variation in precipitation spatial variability. The skill of simulations in representing the observed frequency of most severe precipitation events is relatively low, although their performance for precipitation intensity is considerable. On the other hand, the IMDAA reanalysis has shown good skill with overestimation of observed precipitation over the regions of northeast India and Indo-Gangetic plains, which may be associated with the increased atmospheric instability due to orographic lift. Furthermore, the observed intraseasonal variability of the summer monsoon (i.e., active and break spells) and it’s northward and eastward propagation characteristics have been well captured by the IMDAA as well as the CORDEX models (except for the REGCM). The REGCM has shown a northward rather than a westward stretch of monsoon precipitation during the active and break phases and hence the propagation characteristics are also unclear in the REGCM. It has been observed that the low-pressure system (with a slightly northward gradient) is confined to the region of central India in the REGCM which explains the northward shift of monsoon trough activity and precipitation. The observed variability of monsoon onset has been well reproduced by the IMDAA. Overall, the IMDAA has shown better performance than the CORDEX models in all cases/aspects, and the COSMO model has been found to be the best performing model among the three CORDEX models and is in line with the skills of IMDAA and MMM, although COSMO has an early onset of monsoon.

Role of cloud microphysics and energetics in regulating different phases of the monsoon low‐pressure systems over the Indian region

Article

Full-text available

Dec 2022
Q J ROY METEOR SOC

Monsoon low‐pressure systems (LPS) are the major contributor to these heavy rainfall events and pose a significant challenge to operational forecasting agencies in terms of prediction accuracy with adequate lead time, particularly at a district scale. The present study investigates the role of microphysical parameterizations associated with these LPS during the intensification phases, that is, depressions (MD) and deep depressions (DD), using the Weather Research and Forecasting (WRF) model. A total of 130 simulations are carried out (12 MD and 14 DD) using five cloud microphysical parameterizations, that is, WSM6, WDM6, Thompson, Milbrandt, and Aerosol Aware Thompson, up to 96 hr. The study aims to interlink rainfall vulnerability and LPS intensity both at the district scale for the state of Odisha (India). Results suggest that Mayurbhanj is the most vulnerable district in terms of rainfall. The WDM6 has the best skills in terms of rainfall. The analysis of storm energetics is carried out to provide a possible clue about the mechanism facilitating the intensification of specific LPS to DD. Results suggest that DDs are more thermodynamically efficient than MDs to convert the latent energy to kinetic energy, facilitating its intensification process through higher kinetic energy generation and moisture consumption. Further, it is found that deep vertical updraft with the strong inward flow in the lower troposphere supported by intense tangential wind within the 300 km radial periphery is distinct in DD compared to MD. The findings of the study will have direct implications on localized forecast, policy planning, disaster preparedness, mitigation, and adaptation.

A curious case of the Indian Summer Monsoon 2020: The influence of Barotropic Rossby Waves and the monsoon depressions

Preprint

Full-text available

Oct 2022

Summer monsoon rainfall over India depends on synoptic-scale phenomena like lows and depressions. India's summer monsoon season 2020 saw above-average rainfall. North Indian Ocean had 12 LPAs in summer 2020 in JJAS season. This monsoon season has the highest all-India rainfall since 1976 in August 2020. 50% of seasonal LPAs emerged in August 2020, but none intensified into a monsoon depression (MDs). This study explores the potential factors behind LPAs not intensifying/concentrating as MDs. Warming over the Northern Parts of the Arabian Sea (NPAS) boosted convection in August 2020, resulting in substantial low-level convergence. Strong northwesterly winds from central Asia also combined with the cross-equatorial monsoon flow. This powerful flow across the Arabian Sea split into two branches: one extending to NW India following the monsoon trough, and another diverging into an anticyclone over the south Bay of Bengal (SBOB), reducing horizontal shear there (Barotropic Instability). Due to weak barotropic instability over the head BOB, LPAs cannot become MDs. In August 2020, the 200 hPa barotropic Rossby wave remains stationary over South Central Asia and retrogresses northeast to southwest. It affected all-India rainfall by increasing rainfall in the NW and Western Ghats (WG). Despite unfavourable dynamical conditions, the barotropic Rossby wave and low-level anticyclone over the WNP caused above-normal rainfall over India in August 2020. We also confirmed these mechanisms in Community Earth System Model Large Ensemble (CESM-LE) model simulations. Analysis showed that model has limited skill to mimic monsoon rainfall and associated circulation and failed to reflect mid-latitude circulation impact on ISM rainfall.

A curious case of the Indian Summer Monsoon 2020: The influence of Barotropic Rossby Waves and the monsoon depressions

Article

Oct 2022
ATMOS RES

The seasonal summer monsoon rainfall over India has substantially depended on the synoptic-scale systems such as monsoon lows and depressions. India has received above-average rainfall during the 2020 summer monsoon season. Total 12 Low Pressure Areas (LPAs) formed in the north Indian Ocean during summer 2020 (in JJAS season). The significance of this monsoon season is that August 2020 received the highest all-India rainfall in the past 44 years since 1976. This is accompanied by around 50% of the total seasonal LPAs formed in August 2020, none of which intensified into a monsoon depression (MDs). This study attempts to understand the characteristic features of monsoon rainfall during August 2020 and explore the plausible mechanisms behind the LPAs not intensifying/concentrating as MDs. It is noted that the anomalous warming over the Northern Parts of the Arabian Sea (NPAS) resulted in increased convection over this region in August 2020, as a result, strong low-level convergence is observed. In addition to this convergence, strong northwesterly winds emanating from central Asia merged with the enhanced cross-equatorial monsoon flow. However, this strong flow over the Arabian Sea sheared/dissociated into two branches: one extending up to northwest (NW) India along the monsoon trough, another one diverging into an anticyclone over the south Bay of Bengal (SBOB), which reduced the horizontal shear there (Barotropic Instability). This anticyclone strength over the SBOB and its westward shift is determined by the western north Pacific (WNP) anticyclone. Our analysis suggests that due to the poor barotropic instability over the head BOB, LPAs could not develop into MDs. Additionally, upper level (200 hPa) barotropic Rossby wave in August 2020 remains stationary over South Central Asia and retrogressed with a northeast to southwest orientation. It determined the path of movement of the low-level disturbance beneath and affected the all-India rainfall by virtue of enhanced rainfall over NW & Western Ghats (WG) regions. The interplay of the barotropic Rossby wave alongside low-level anticyclone over the WNP accompanied by local conditions caused the above normal rainfall over India in August 2020, even though there are adverse dynamical conditions. We have also verified these mechanisms in Community Earth System Model Large Ensemble (CESM-LE) model simulations, analysis shows that model has a limited skill to simulate the changes in the monsoon rainfall and associated circulation and failed to capture the mid-latitude circulation impact on ISM rainfall.

Simulation of Monsoon Depression over the Bay of Bengal and Its Features using Weather Research and Forecasting Model

Article

Full-text available

May 2019

An attempt has been made to simulate the monsoon depression over the Bay of Bengal during 13-16 June, 2008 and its associated rainfall using Weather Research and Forecasting Model. The model was run on a single domain of 10 km horizontal resolution using Morrison 2-moment microphysics with Kain-Fritsch cumulus parameterization scheme and Yonsei University planetary (YSU) boundary layer scheme, MM5 surface layer physics scheme, Unified Noah LSM land surface physics, Rapid Radiative Transfer Model (RRTM) for longwave and Dudhia scheme for shortwave scheme are used in version 3.9.1 for the simulation. The NCEP high resolution FNL 6-hourly data is used for initial and lateral boundary conditions. GrADS is used to visualize the different graphics. The model predicting capability is evaluated by analyzing Mean Sea Level Pressure (MSLP), wind pattern, vorticity, vertical wind shear, reflectivity, temperature and rainfall distribution. The model has successfully captured the system, its initial condition, propagation, landfall time and location reasonably well. The model has simulated rainfall, wind and rh sensibly well compared with the observed data by BMD and Tropical Rainfall Measuring Mission (TRMM). It can be concluded that the WRF model with the accurate arrangement of the domain, horizontal resolution and the appropriate parameterization schemes is proficient to simulate and forecast the monsoon depressions over the Bay of Bengal and its associated rainfall over Bangladesh up to 96-hours advance reasonably well.

Precipitation Characteristics of Cyclonic Disturbances over South Asia Region as Revealed by TRMM and GPM

Article

Full-text available

May 2022
J CLIMATE

The relative contributions of cyclonic disturbances (CDs) (low pressure systems, depressions and cyclonic storms) and non-CDs to annual and seasonal rainfall are studied using 22 years of TRMM and GPM measurements during the passage of 864 CDs in the South Asia Region (SAR). The changes in stratiform and convective precipitation within the cyclonic storm and in different CDs are also examined. The rainfall in the wettest regions of the SAR, west coasts of India and Myanmar and slopes of the Himalayas is of non-CD origin, while CD rainfall peaks in the eastern parts of monsoon trough and north Bay of Bengal (BOB). The CD rain fraction (RF) of annual and seasonal rainfall exhibits large spatial variation in the range of 4% - 55%. The land-ocean dichotomy exhibited by CD RF is not uniform across India. Large CD RF is confined to the coast in some regions due to topographical barriers, but extends to 800-1000 km inland from the coast in the monsoon trough region. The low-pressure systems contribute more to annual rain than depressions and cyclonic storms in the monsoon trough and northern BOB (~40%), particularly during the monsoon, mainly due to their frequent occurrence. The stratiform RF and occurrence are higher in CDs than in non-CDs with its greatest contribution in central India (>80%), whereas the non-CDs are characterized by having higher convective RFs. The stratiform rain occurrence increases with intensification of CD over both land and ocean, indicating its importance in the intensification of CDs and organizing large-scale systems.

Classification of Middle Tropospheric Systems over the Arabian Sea and Western India

Preprint

Full-text available

Mar 2022

The formation of Middle Tropospheric Cyclones (MTCs) that are responsible for a large portion of annual precipitation and extreme rainfall events over western India is studied using an unsupervised machine learning algorithm and cyclone tracking. Both approaches reveal four dominant weather patterns that lead to the genesis of these systems; specifically, re-intensification of westward moving synoptic systems from Bay of Bengal (Type 1, 51%), in-situ formation with a coexisting cyclonic system over the Bay of Bengal that precedes (Type 2a, 31%) or follows (Type 2b, 10%) genesis in the Arabian Sea, and finally in-situ genesis within a northwestward propagating cyclonic anomaly from the south Bay of Bengal (Type 2c, 8%). Thus, a large fraction of rainy middle tropospheric synoptic systems in this region form in association with cyclonic activity in the Bay of Bengal. The four variants identified also show a marked dependence on large-scale environmental features with Type 1 and Type 2a formation primarily occurring in phases 4 and 5, and Type 2b and Type 2c in phases 3 and 4 of the Boreal Summer Intraseasonal Oscillation. Further, while in- situ formation with a Bay of Bengal cyclonic anomaly (Type 2a and 2b) mostly occurs in June, downstream development is more likely in the core of the monsoon season. Out of all categories, Type 2a is associated with the highest rain rate (60 mm/day) and points towards the dynamical interaction between a low pressure system over the Bay of Bengal and the development of MTCs over western India and the northeast Arabian Sea. This classification, identification of precursors, connection with cyclonic activity over the Bay of Bengal and dependence on large-scale environment provides an avenue for better understanding and prediction of rain-bearing MTCs over western India.

Rainfall extremes on the rise: Observations during 1951–2020 and bias-corrected CMIP6 projections for near- and late 21st century over Indian landmass

Article

May 2022
J HYDROL

In this study, we provide a comprehensive analysis to identify and quantify spatial patterns of heavy, very heavy and extremely heavy rainfall and their trends that have emerged during the last seven decades (1951 to 2020) of monsoon months (June to September) under warming scenario as well as to project these extreme rainfall counts during the near- (2036–2060) and late-21st century (2075–2099) w.r.t. historical period (1990–2014) using bias-corrected Coupled Model Intercomparison Project-6 (CMIP6) multi-model ensemble method. On the basis of daily maximum rainfall occurrences, the Central India, North-East India, Western Ghats and Eastern Ghats are found to be susceptible to extreme rainfall zones over the Indian landmass. The trend distribution during 1951–2020 suggests an increase of 42–63 heavy rainfall events over Orissa, Chattisgarh and parts of Madhya Pradesh and a declination over Uttar Pradesh, Kerala and hilly regions of North-East India. Our study suggests the causal theory for the rise of monsoon rainfall extremes in terms of both dynamics and local-scale thermodynamics over the sub-continent. Moreover, the bias-corrected dataset is developed using Empirical Quantile Mapping (EQM) for historic and projected climate for the three scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5) using output from 20 General Circulation Models (GCMs) from CMIP6. Hence, the bias-corrected projections suggest that most susceptible places for increasing heavy rainfall extremes likely to be Mumbai, Pune, Panaji in the Western coasts of India (Western Ghats), Itanagar and Shillong in the North-East India, Raipur and Bhopal in the Central India in near- and late-21st century under all scenarios in a warming climate. Thus, the bias-corrected projections from CMIP6-GCMs can be used for hydroclimate impact assessment in Indian region under the changing atmospheric circulation dynamics and warming induced by greenhouse gases.

Statistical study of the low pressure systems during summer monsoon season over the Indian region

Article

Full-text available

Jan 2022
MAUSAM

The occurrence of a closed low pressure area due to low, depression or cyclonic storm is termed as Low Pressure System (LPS). LPS plays an important role in the distribution of rainfall during the southwest monsoon season. Lows produce widespread rainfall as compared to depressions and cyclonic storms which yield concentrated rainfall over a smaller area. The distribution of rainfall depends upon the track and life span of LPS. Most of the LPS formed over the Bay of Bengal travel in northwest direction strengthening the rainfall activities over the large parts of the country. In this study, the LPS formed during the monsoon season, June to September for the period 1891-2000 over the region covering India, Pakistan, Bangladesh, Bay of Bengal and Arabian Sea are taken into account. The duration of LPS is also studied in terms of LPS Days during the monsoon season. The statistical analysis of LPS and LPS Days is carried out for the monsoon months and for the monsoon season as a whole, for the period 1891-2000. It is seen that the frequency of LPS during any of the monsoon months does not exceed six but three LPS in a month are more common. Total number of LPS during the monsoon season ranges from 9 to 18. In June and July maximum total duration of LPS lies between 10-14 LPS Days while in August and September, it is between 15-19 LPS Days. August is the month having maximum number of LPS and LPS Days. The frequency analysis shows that monthly and seasonal LPS and LPS Days are normally distributed. It is also observed that even though season's total number of LPS has no significant increase or decrease, the LPS Days have significantly increased during the decades, 1971-80 and 1981-90.

Summer monsoon low pressure systems over the Indian region and their relationship with the sub-divisional rainfall

Article

Full-text available

Jan 2022
MAUSAM

S. K. JADHAV

In the present paper performance of the monthly sub-divisional summer monsoon rainfall is studied in association with the position of the Low Pressure System (LPS) over the Indian region. Existence of the LPS over a particular location increases the rainfall activities in certain parts of the country while decreases in some other parts. For this study, the Indian region (5°-35° N and 60° -100° E) is divided into 5° Lat. ´ 5° Long. grids. The duration of LPS is taken in terms of LPS days with respect to the location of LPS in a particular grid. Monthly total number of LPS days in each of the grids are computed during the summer monsoon season, June to September for the period 1891 – 1990. Maximum number of LPS days (more than half of the total) are observed in the latitude belt between 20°-25°N. The percentages of total LPS days in this area are higher in July and August which are peak monsoon months as compared to June and September. When there is a LPS are in the area 20°-25° N and 80°-90° E, there is significant increase in the rainfall activities in the sub-divisions along mean monsoon trough while northeast India and southeast peninsular India experience significant decrease in rainfall in the months of July and August. Owing to the movement of LPS from east to west through central India, most parts of the country, excluding northeast India and south peninsular India get good rainfall activity. Correlation coefficients between monthly LPS days over the different grids and monthly sub-divisional rainfall are computed to study the relationships. The performance of sub-divisional rainfall mostly related with the occurrence of LPS in certain grid- locations. The correlation field maps may give some useful information about rainfall performance due to LPS in a particular grid locations.

Flood hydrometeorological situations associated with monsoon floods on the Par River in Western India

Article

Full-text available

Aug 2021
MAUSAM

The objective of present work is to understand flood hydrometeorological situations associated with monsoon floods on the Par River, therefore, the analyses of synoptic conditions connected with large floods was carried out. This encompasses analysis of interannual rainfall variability and associated floods, analysis of storm tracts, investigation of normalized accumulated departure from mean (NADM) and evaluation of the relation between El Niño and monsoon rainfall. In order to accomplish above analyses, the annual rainfall data of the Par Basin have been obtained for 118 years from India Meteorological Department (IMD), Pune and Chennai. The annual maximum series (AMS)/stage data were procured for a gauging site namely Nanivahial for 45 years from Irrigation Department of Gujarat State, Ahmedabad. The results indicate that the interannual variability was characterized by increased frequency and magnitude of floods on the Par River primarily after 1930s. Majority of the large floods in the basin were connected with low pressure systems. It is observed that most of the floods were associated with positive departure from mean rainfall in the basin. The NADM graph shows epochal behaviour of high and low rainfall of the basin and floods. The analysis of El Niño and Southern Oscillation indicates that the probability of the occurrence of the floods in the Par Basin is high during the average SST index and majority of the floods in the basin have occurred during above normal conditions of rainfall. The present study can, therefore, prove to be a significant contribution towards the Par-Tapi-Narmada link project of the Government of Gujarat and water divergent projects of the Government of Maharashtra in association with Government of India.

Downstream and In Situ Genesis of Monsoon Low-Pressure Systems in Climate Models

Article

Full-text available

Sep 2021

The monsoon low-pressure systems (LPS) are a major contributor to the rainfall over India. The genesis of LPS in climate models is not well understood. Here, we track the LPS activity in 11 coupled climate models using an automated tracking algorithm and classify their genesis mechanism broadly into two categories-in situ and downstream. We find that the in situ genesis mechanism dominates in all models, with an average of 56% systems categorized under this category, while 63% of the observed LPS had in situ genesis. The average downstream genesis in the models is 32%, closer to the observed 30%. About 12% and 7% of the LPS genesis could not be attributed to either of the categories in the models and observations, respectively, due to the presence of both types of genesis mechanisms. Although the bulk statistics of the in situ and downstream LPS genesis across the models in boreal summer is comparable to that of observations, substantial inter-model variability is observed. Also, we find significant differences in the temporal distribution of downstream LPS genesis in models. Although the models realistically capture the percentage of downstream LPS for the whole monsoon season, they tend to simulate a higher number of genesis in the early phase of monsoon as opposed to the observed peak in August and September, which is linked to a stronger Rossby wave activity in the models in June.

Simulation of Monsoon Low Pressure System and its Associated Rainfall Over Bangladesh Using WRF Model

Article

Jan 2018

In this paper an effort has been made to simulate the monsoon Low Pressure System (LPS) and its associated rainfall event of 16-20 August, 2013 using Weather Research and Forecasting (WRF) model. The model was run for 24-h, 48-h and 72-h in a single domain of 10 km horizontal resolution using The National Centre for Environmental Prediction (NCEP) high-resolution Global Final (FNL) Analysis 6-hourly data using initial and lateral boundary conditions. WRF double-moment 5 class micro physics scheme, Kain–Fritsch (new Eta) cumulus physics scheme,Yonsei University planetary boundary layer scheme, Revised MM5 surface layer physics scheme, Unified Noah LSM as land surface model, Rapid Radiative Transfer Model (RRTM) for long-wave and Dudhia scheme for short-wave scheme are used for the simulation. The performance of the model is evaluated analyzing Mean Sea Level Pressure (MSLP), Wind Pattern, Vorticity, Vertical Wind Shear and Rainfall Distribution. The model successfully captured the low pressure system, initial condition, propagation, landfall time and location reasonably well. The model simulated rainfall amount and associated areas sensibly well compared with the observed data by BMD and Tropical Rainfall Measuring Mission (TRMM). Dhaka Univ. J. Sci. 66(1): 29-35, 2018 (January)

Influence of cloud microphysical parameterization on the characteristics of monsoon depressions over the Indian region

Article

Full-text available

Jun 2021
INT J CLIMATOL

Indian summer monsoon (ISM) low‐pressure systems are considered as the lifeline for seasonal monsoon rainfall over the Indian region. However, the current models have limitations in predicting their characteristics and rainfall. This study assesses the influence of five cloud microphysical parameterization Schemes (MP) on simulations of 14 monsoon deep depressions (DD) using the weather research and forecasting (WRF) model. The simulations are carried out with a lead time up to 96 hr in a nested domain resolution of 27, 9, and 3 km. The five cloud microphysical parameterizations, that is, WRF Double moment (WDM6), WRF single moment (WSM6), Milbrandt (MIL), Thompson (THOM), and Aerosol Aware Thompson (AAT), are considered in this study, leading to a total of 70 simulations. The results are validated through composite analysis at a radial distance of 300 km from the respective storm centres. The choice of MP significantly impacts the key characteristics of the monsoon DDs such as rainfall, wind, temperature, moisture, humidity, hydrometeors, and associated convective processes. In terms of rainfall, it is found that WDM6 (AAT) has the best (worst) performance with a lead time up to day‐4. Besides, WDM6 has simulated the best result in terms of temperature and specific humidity. Examining the hydrometeors distribution around the storm intense convective region (i.e., 300 km), it is noted that frozen hydrometeors (i.e., ice, snow, and graupel) are mainly modulating the rainfall. There is a general tendency to overestimate snow and graupel among MP except WDM6. Further, ice hydrometeors are well represented in WDM6 compared with others leading to better rainfall forecast. The moisture flux convergence and absolute vorticity are two major mechanisms determining the convection within the storm's core zone, and WDM6 stands out the best among these schemes. The findings of this study are relevant and have direct consequences to the operational applications.

Observed Trends in the South Asian Monsoon Low‐Pressure Systems and Rainfall Extremes Since the Late 1970s

Article

Full-text available

Apr 2021
GEOPHYS RES LETT

Plain Language Summary The South Asian synoptic‐scale low‐pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time‐mean low‐level westerlies, produce more than half of the summer rainfall extremes over the densely populated central India. Given the vulnerability of societies in this region to rainfall extremes, investigating the connection between LPSs and extreme rainfall regarding their long‐term trends has important implications for climate adaptation. Using two different tracking algorithms and reanalyses‐derived LPS tracks, we find that the trends of extreme rainfall and LPS activity exhibit a strong coherence during the post‐1979 satellite era. Specifically, the LPSs prefer to propagate into south‐central India than north‐central India over time, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward‐propagating component due to horizontal nonlinear adiabatic advection and the southwestward‐propagating component due to diabatic heating, we notice that the LPSs migrating through the south‐central India have stronger updrafts on their west‐southwestern flank compared to those passing through north‐central India. Our results indicate that the increasing number of LPSs propagating into south‐central India is likely due to a strengthened cross‐equatorial moisture transport, which provides a wetter environment and favors stronger storm ascents.

The four regional varieties of South Asian monsoon low‐pressure systems and their modulation by tropical intraseasonal variability

Article

Full-text available

May 2021
Weather

We carry out the first‐ever investigation of four regional varieties of South Asian monsoon low‐pressure systems (LPSs) that occur during June‐September 1979–2018. We find that long‐lived Bay of Bengal (BoB) LPSs are most intense, whereas those over Sri Lanka are least intense. While Arabian LPSs are least frequent, short‐lived BoB LPSs are most frequent and bring most precipitation to east India. We also find that tropical intraseasonal variability modulates genesis of these four LPS varieties. image

Dual response of Arabian Sea cyclones and strength of Indian monsoon to Southern Atlantic Ocean

Article

Full-text available

Apr 2021
CLIM DYNAM

Variability and trends of the south Asian monsoon at different time scales makes the region susceptible to climate-related natural disasters such as droughts and floods. Because of its importance, different studies have examined the climatic factors responsible for the recent changes in monsoon strength. Here, using observations and climate model experiments we show that monsoon strength is driven by the variations of south Atlantic Ocean sea surface temperature (SASST). The mechanism by which SASST is modulating the monsoon could be explained through the classical Matsuno-Gill response, leading to changes in the characteristics of vertical wind shear in the Arabian Sea. The decline in the vertical wind shear to the warming of SASST is associated with anomalous lower (upper)-level easterlies (westerlies). This further leads to a strong increase in the frequency of the Arabian Sea cyclones; and also prohibits the transport of moisture to the Indian landmass, which eventually reduces the strength of monsoon. The conditions in the SASST which drove these responses are aggravated by greenhouse gas emission, revealing the prominent role played by anthropogenic warming. If, with proper mitigation, these emissions are not prevented, further increases in the SASST is expected to result in increased Arabian sea cyclones and reduced monsoon strength.

A Global Tropical Survey of Mid-Tropospheric Cyclones

Preprint

Nov 2020

Mid-Tropospheric Cyclones (MTCs) are moist synoptic systems with distinct mid tropospheric vorticity maxima and weak signatures in the lower troposphere. Composites and statistics of MTCs over the tropics are constructed and compared with monsoon lows and depressions (together, lower troposphere cyclones; LTCs). We begin with South Asia, where tracking reveals that MTCs change character during their life, i.e., their track is composed of MTC and LTC phases. The highest MTC-phase density and least motion is over the Arabian Sea, followed by the Bay of Bengal and South China Sea. A MTC-phase composite shows an east-west tilted warm above deep cold-core temperature anomaly with maximum vorticity at 600 hPa. While the LTC-phase shows a shallow cold-core below 800 hPa and a warm upright temperature anomaly with lower tropospheric vorticity maximum. Apart from South Asia, systems with similar morphology are observed over the west and central Africa, east and west Pacific in boreal summer. In boreal winter, regions that support MTCs include northern Australia, the southern Indian Ocean, and South Africa. Relatively, the MTC fraction is higher equatorward, where there is a cross-equatorial low-level jet that advects oppositely signed vorticity. Whereas the LTCs are more prevalent further poleward. Finally, a histogram of differential vorticity (difference between middle and lower levels) versus the height of peak vorticity for cyclonic centers is bimodal. One peak, around 600 hPa, corresponds to MTCs while the second, around 900 hPa, comes from LTCs. Thus, moist cyclonic systems in the tropics have a natural tendency to reside in either the MTC or LTC category preferentially

Flood hydrometeorological situations associated with monsoon floods on the Par River in Western India

Article

Full-text available

Oct 2020
MAUSAM

Comment on “Unraveling the Mystery of Indian Summer Monsoon Prediction: Improved Estimate of Predictability Limit” by Saha et al.

Article

Full-text available

Oct 2020

In a recent study, Saha et al. (2019, https://doi.org/10.1029/2018JD030082) examine the correlation between boreal summer seasonal mean rainfall over India and rainfall variance as a function of subseasonal time scale and find that the correlation has a local maximum (exceeding a value of 0.6) for synoptic time scales (2–5 day periods). They claim these results to be a major advancement in understanding monsoon predictability but do not provide a clear physical explanation. Here we examine the sensitivity of this relationship to the details of the analysis and only consider the observed correlation identified by Saha et al. (2019, https://doi.org/10.1029/2018JD030082). There is large sensitivity in the correlation maximum between spatially averaged seasonal mean rainfall and synoptic scale rainfall variance averaged over the same domain. The correlation maximum is weaker over the longer period of 1901–2015, and more notably it is highly sensitive to the domain of prior averaging. A correlation peak is not found outside central India and is neither found within central India when averaging over a smaller domain. Averaging over a larger domain results in a disproportionate reduction in synoptic variance (that is of most interest). The peak in correlation between seasonal mean and 2–5 day variance only emerges after first averaging over a sufficiently large region; thus, the maximum appears to be an artifact of spatial averaging. It is further pointed out that a positive mean‐variance relationship is an intrinsic property of rainfall, and thus, its existence alone does not necessarily imply any physical, causal, and/or predictive connection between time scales.

Global track dataset of monsoon low pressure systems

Article

Jun 2020

This dataset contains the tracks and intensities of low pressure system (LPS) in the global tropics (35ºS-35ºN), as identified in five atmospheric reanalyses (ERA5, ERA-Interim, JRA55, MERRA2, and CFSR) using the algorithm described in the paper titled Assessing historical variability of South Asian monsoon lows and depressions with an optimized tracking algorithm. Tracking of LPS was performed using an automated Lagrangian pointwise feature tracker, TempestExtremes (Ullrich & Zarzycki, 2017), with criteria chosen to best match a subjectively analyzed LPS dataset while minimizing disagreement between four atmospheric reanalyses. A full description of the algorithm and dataset is described in the preprint (https://www.essoar.org/doi/10.1002/essoar.10502946.1)

Assessing Historical Variability of South Asian Monsoon Lows and Depressions With an Optimized Tracking Algorithm

Article

Full-text available

Aug 2020

Cyclonic low‐pressure systems (LPS) produce abundant rainfall in South Asia, where they are traditionally categorized as monsoon lows, monsoon depressions, and more intense cyclonic storms. The India Meteorological Department (IMD) has tracked monsoon depressions for over a century, finding a large decline in their number in recent decades, but their methods have changed over time and do not include monsoon lows. This study presents a fast, objective algorithm for identifying monsoon LPS and uses it to assess interannual variability and trends in reanalyses. Variables and thresholds used in the algorithm are selected to best match a subjectively analyzed LPS data set while minimizing disagreement between four reanalyses in a training period. The stream function of 850 hPa horizontal wind is found to be optimal in this sense; it is less noisy than vorticity and represents the complete nondivergent wind, even when flow is not geostrophic. Using this algorithm, LPS statistics are computed for five reanalyses, and none show a detectable trend in monsoon depression counts since 1979. Both the Japanese 55‐year Reanalysis (JRA‐55) and the IMD data set show a step‐like reduction in depression counts when they began using geostationary satellite data, in 1979 and 1982, respectively; the 1958–2018 linear trend in JRA‐55, however, is smaller than in the IMD data set, and its error bar includes 0. There are more LPS in seasons with above‐average monsoon rainfall and in La Niña years, but few other large‐scale modes of interannual variability are found to modulate LPS counts, lifetimes, or track length consistently across reanalyses.

The Stirring Tropics: Theory of Moisture Mode–Hadley Cell Interactions

Article

Full-text available

Dec 2023

Interactions between large-scale waves and the Hadley cell are examined using a linear two-layer model on an f plane. A linear meridional moisture gradient determines the strength of the idealized Hadley cell. The trade winds are in thermal wind balance with a weak temperature gradient (WTG). The mean meridional moisture gradient is unstable to synoptic-scale (horizontal scale of ∼1000 km) moisture modes that are advected westward by the trade winds, reminiscent of oceanic tropical depression–like waves. Meridional moisture advection causes the moisture modes to grow from “moisture-vortex instability” (MVI), resulting in a poleward eddy moisture flux that flattens the zonal-mean meridional moisture gradient, thereby weakening the Hadley cell. The amplification of waves at the expense of the zonal-mean meridional moisture gradient implies a downscale latent energy cascade. The eddy moisture flux is opposed by a regeneration of the meridional moisture gradient by the Hadley cell. These Hadley cell–moisture mode interactions are reminiscent of quasigeostrophic interactions, except that wave activity is due to column moisture variance rather than potential vorticity variance. The interactions can result in predator–prey cycles in moisture mode activity and Hadley cell strength that are akin to ITCZ breakdown. It is proposed that moisture modes are the tropical analog to midlatitude baroclinic waves. MVI is analogous to baroclinic instability, stirring latent energy in the same way that dry baroclinic eddies stir sensible heat. These results indicate that moisture modes stabilize the Hadley cell and may be as important as the latter in global energy transport. Significance Statement The tropics are characterized by steady circulations such as the Hadley cell as well as a menagerie of tropical weather systems. Despite progress in our understanding of both, little is known about how the mean circulations and the weather systems interact with one another. Here we show that tropical waves can grow by extracting moisture from the Hadley cell, thereby weakening it. They also transport moisture to higher latitudes. Our results challenge the notion that the Hadley cell is the sole transporter of energy out of the tropics and instead favor a view where tropical waves are also essential for the global energy balance. They dry the humid regions and moisten the drier regions via stirring.

Recent changes in the climatological characteristics of daily contiguous rain areas over India

Article

Full-text available

Nov 2023

This study documents the climatological feature (1951–1980) and recent changes (1981–2020) in rainfall characteristics considering the observed nearly full spectrum of rain event sizes (daily contiguous rain area (CRA) events) in all seasons over India. It is found that the low frequency very large CRA (~synoptic scale) from monsoon season contributes ~50% of annual rainfall. However, the small-sized CRA (isolated thunderstorms) are the most frequent daily rain events (~70% of annual frequency) and hence are important for rain-fed agricultural practices. The well-documented widespread drying trend in the monsoon season has manifested in the annual rainfall trend but with reduced magnitude illustrating the compensatory effect from other seasons. Spatial aggregated annual statistics show that there is no significant change in rainfall amount and frequency of occurrence of rain events in the recent past compared to the base period. However, seasonally the pre-monsoon rainfall amount has increased significantly. Annually, the number of extremely heavy CRA (EHR) events have significantly increased by ~55% owing to a significant increase in pre-monsoon and monsoon rainfall. In all seasons, small-sized extremely heavy CRA has intensified substantially by 50–200% as compared to the base period. Additionally, the rain events from areal category large (~Mesoscale Convective Complexes (MCC)) have intensified in all seasons except winter. Thus, to decrease the uncertainty in rain-fed agricultural practices and better prediction of EHR to develop effective climate change mitigation strategies; process studies beyond monsoon season and processes other than synoptic scales are also required.

Classification of mid‐tropopsheric cyclones over the Arabian Sea and western India

Article

Full-text available

May 2023
Q J ROY METEOR SOC

The formation of mid‐tropospheric cyclones (MTCs), responsible for a large portion of annual precipitation and extreme rainfall events over western India, is studied using an unsupervised machine learning algorithm and cyclone tracking. Both approaches reveal four dominant weather patterns that lead to the genesis of these synoptic systems. Specifically, re‐intensification of westward‐moving synoptic systems from the Bay of Bengal (type 1, 51%), in‐situ formation with a coexisting cyclonic system over the Bay of Bengal that precedes (type 2a, 31%) or follows (type 2b, 10%) genesis in the Arabian Sea, and finally in‐situ genesis within a northwestward‐propagating cyclonic anomaly from the south Bay of Bengal (type 2c, 8%). Thus, a large fraction of this region's rainy middle tropospheric synoptic systems form in association with cyclonic activity in the Bay of Bengal. The four variants identified also show a marked dependence on large‐scale environmental features. In particular, type 1 and type 2a MTC formation primarily occurs in phases 4 and 5, and type 2b and type 2c MTCs form mainly in phases 3 and 4 of the boreal summer intraseasonal oscillation. Further, though in‐situ formation with a Bay of Bengal cyclonic anomaly (types 2a and 2b) mostly occurs in June, downstream development is more likely in the core of the monsoon season. Out of all categories, type 2a is associated with the highest composite rain rate (60 mm·$$ \cdotp $$day −1$$ {}^{-1} $$) over western India and points towards the dynamic interaction between a low‐pressure system over the Bay of Bengal and the development of MTCs over western India and the northeast Arabian Sea. This classification, identification of precursors, connection with cyclonic activity over the Bay of Bengal, and dependence on a large‐scale environment provide an avenue for a better understanding of rain‐bearing MTCs over western India.

An assessment study on the impact of land use land cover on monsoon depressions over India

Article

Jul 2022
ATMOS RES

This study investigates the influence of LULC representation on surface meteorological conditions associated with monsoon depressions (MDs) in the Advanced Research Weather Research and Forecasting (WRF) model. A total of 18 MDs were considered during 2007–2018, with a life period of at least two days. Two simulations are performed at a 5-km grid-spacing, ingesting the LULC from the United States Geological Survey (USGS) and National Remote Sensing Centre (NRSC) for each MD case. The urban area has been increased from 0.13% in USGS to 1.21% in NRSC over the north-central and east coastal states, reducing the soil moisture (SM) errors by 0.015 m³ m⁻³ (25%) in the NRSC run. The SM in the NRSC run has a correlation of 0.53, which is 15% higher than that in the USGS run. The NRSC has a larger forest area (22%) than the USGS (7%) over the north-western parts and some parts of Maharashtra, which helps increase latent heat flux (LHF) by 30 Wm⁻². Verifying with ERA analysis, the USGS and the NRSC simulations underestimate the LHF in most parts of India and overestimate in orographic areas. The NRSC-simulation shows fewer improvements (<5%) in LHF across the central and southern areas, unlike in the East and West parts (>13%). The surface temperature, moisture, and rainfall have been noticeably improved in the NRSC run for day-2 and day-3 simulation, unlike in USGS, while they are comparable in day-1. The spatial error of rainfall increases with forecast length, with NRSC having 10–15% less error than the USGS run. Further, the NRSC run could identify the rainfall peaks of 3 mm h⁻¹ with a mean error of 1.5 mm h⁻¹ as compared to 2.2 mm h⁻¹ error in the USGS. This study demonstrates the positive impact of NRSC-developed LULC in the MD simulations over India.

The structure of strong Indian monsoon low‐pressure systems in subseasonal‐to‐seasonal prediction models

Article

Full-text available

Jun 2022
Q J ROY METEOR SOC

The structure of strong Indian monsoon low‐pressure systems (LPSs) up to forecast lead times of 15 days in 11 models of the Subseasonal‐to‐Seasonal (S2S) Prediction Project is analysed. Strong LPS (SLPS) tracks are obtained from a catalogue of LPSs tracked in all ensemble members of the S2S models during a common reforecast period of June–September 1999–2010. SLPSs, which have a minimum intensity equal to at least the upper‐quartile intensity of all LPSs, are then composited to generate horizontal and vertical structures of several dynamic and thermodynamic fields. The evolution of fields with forecast lead time and during LPS lifecycle is analysed. Furthermore, the simulation of the lower‐tropospheric monsoon circulation, precipitation biases, and the precipitation contribution of LPSs are analysed. All S2S models and the multimodel mean simulate the lower‐tropospheric monsoon circulation, but prominent dry biases are observed in the Australian Bureau of Meteorology and Environment and Climate Change Canada models. The precipitation contribution of LPSs to the summer mean precipitation is smaller in all S2S models than in tracks derived from ERA‐Interim reanalysis. The location and amplitude of the lower‐tropospheric cold core and the location of maximum precipitation are not well simulated by many models, particularly by the Hydrometeorological Centre of Russia model, in which the cold core is missing altogether. The structure of relative vorticity anomaly in all S2S models and the multimodel mean is shallower and weaker than in ERA‐Interim and MERRA‐2 reanalyses. Though the cold core intensifies through the LPS lifecycle in all models, the warm core features a midlife maximum, except in models such as Australian Bureau of Meteorology and China Meteorological Administration. These results demonstrate the potential for S2S models at simulating the structure of SLPSs, benefiting stakeholders that use S2S models for forecasting.

Synoptic Weather Regimes over Western India and the Arabian Sea

Poster

Full-text available

Jan 2022

Western India, home to millions of people, and northeast Arabian sea, the entrance of monsoon jet to the Indian subcontinent, is a region that witnesses some of the world's heaviest rain spells. This region has been exhibiting a significant increase in frequency of extreme rainfall events and an increase in annual rainfall over the past few decades. These extreme rain events are related to different kinds of weather patterns and synoptic systems over the northeast Arabian Sea and western India. Despite their importance, the formation mechanisms and precursors of these systems are poorly understood. In this work, the characteristics of rainy days of western India are explored. An unsupervised machine learning algorithm (K-means) is used to identify dominant weather regimes which is followed by tracking so as to classify systems into various physically distinguishable groups. A composite of rainy days over western India suggests that they are associated with anomalous cyclonic circulation in the middle troposphere. Rainfall is highly correlated with the middle-level potential vorticity, relative humidity, and deep layer convergence and only weakly with the zonal wind. A K-means classification based on daily outgoing longwave radiation (OLR) anomalies shows four dominant weather regimes during rainy days in this region --- the first regime consists of wave-like westward moving monsoon systems; the second is an in-situ intensification of northward moving OLR anomalies that co-exist with systems further east (over the Bay of Bengal); the third shows OLR anomalies over western India with a weak signature over the Bay of Bengal and the fourth category consists of in-situ formation of an independent localized cyclonic anomaly in the Arabian Sea without any signature in the Bay of Bengal. This classification is followed by cyclone tracking using 600 hPa height and vorticity fields (from ERA-5 reanalysis) of 191 rainy systems during 1998--2019 from June through September. Tracking also suggests that there are two broad categories of genesis in this region; namely, westward moving downstream formation (Type-1, 98 systems, 51 %) and those born locally (Type-2, 92 systems, 49 %). Based on the co-existence of monsoon systems to the east, the Type-2 formation is further composed of three subgroups; first is the formation of a cyclone that follows anomalies in Bay of Bengal (Type-2a, 68 systems, 74 %); second is formation preceding a Bay of Bengal system (Type-2b, 14 systems, 15 %), and lastly, development in the absence of Bay of Bengal systems (Type-2c, 10 systems, 11 %). Further, Type-1 formation dominates during the core of the monsoon in July and August; Type-2a is prominent in June, Type-2b and Type-2c in June and July, respectively. In all, the categorization in this study clearly shows that most of cyclonic disturbances that develop over the Arabian Sea (about 90 %) during the summer monsoon are in the presence of systems in the Bay of Bengal. Not only does this point to the inter-dependence of conditions over the Arabian Sea and Bay of Bengal for the development of synoptic systems, it also opens up potential avenues for predictability of certain classes of heavy rainfall events in western India.

A Global Tropical Survey of Mid-Tropospheric Cyclones

Article

Full-text available

Jun 2021

Mid-Tropospheric Cyclones (MTCs) are moist synoptic systems with distinct mid-tropospheric vorticity maxima and weak signatures in the lower troposphere. Composites and statistics of tropical MTCs are constructed and compared with monsoon lows and depressions (together, lower troposphere cyclones; LTCs). We begin with South Asia, where tracking reveals that MTCs change character during their life, i.e., their track is composed of MTC and LTC phases. The highest MTC-phase density and least motion is over the Arabian Sea, followed by the Bay of Bengal and the South China Sea. An MTC-phase composite shows an east-west tilted warm above deep cold-core temperature anomaly with maximum vorticity at 600 hPa. In contrast, the LTC-phase shows a shallow cold-core below 800 hPa and a warm upright temperature anomaly with a lower tropospheric vorticity maximum. Globally, systems with MTC-like morphology are observed over the west and central Africa, east and west Pacific in boreal summer. In boreal winter, regions that support MTCs include northern Australia, the southern Indian Ocean, and South Africa. MTC fraction is higher equatorward where there is a cross-equatorial low-level jet that advects oppositely signed vorticity. Whereas LTCs are more prevalent further poleward. Finally, a histogram of differential vorticity (the difference between middle and lower levels) versus the height of peak vorticity for cyclonic centers is shown to be bimodal. One peak, around 600 hPa, corresponds to MTCs, while the second, at approximately 900 hPa, comes from LTCs. Thus, moist cyclonic systems in the tropics have a natural tendency to reside in either the MTC or LTC category.

Monsoons, ITCZs, and the Concept of the Global Monsoon

Article

Full-text available

Dec 2020
REV GEOPHYS

Earth's tropical and subtropical rainbands, such as Intertropical Convergence Zones (ITCZs) and monsoons, are complex systems, governed by both large‐scale constraints on the atmospheric general circulation and regional interactions with continents and orography, and coupled to the ocean. Monsoons have historically been considered as regional large‐scale sea breeze circulations, driven by land‐sea contrast. More recently, a perspective has emerged of a global monsoon, a global‐scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation. This results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone. Regional subsystems are embedded in this global monsoon, localized by surface boundary conditions. Parallel with this, much theoretical progress has been made on the fundamental dynamics of the seasonal Hadley cells and convergence zones via the use of hierarchical modeling approaches, including aquaplanets. Here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings. After summarizing the dynamical and energetic balances that distinguish an ITCZ from a monsoon, we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets. Limitations of current theories are discussed, including the need for a better understanding of the influence of zonal asymmetries and transients on the large‐scale tropical circulation.

The Dynamics of the Global Monsoon: Connecting Theory and Observations

Article

Feb 2020

Earth's monsoons are complex systems, governed by both large-scale constraints on the atmospheric general circulation and regional interactions with continents and orography, and coupled to the ocean. Monsoons have historically been considered as distinct regional systems, and the prevailing view has been, and remains, an intuitive picture of monsoons as a form of large-scale sea breeze, driven by land-sea contrast. However, climate dynamics is seldom intuitive. More recently, a perspective has emerged within the observational and Earth system modeling communities of a global monsoon that is the result of a seasonally migrating tropical convergence zone, intimately connected to the global tropical atmospheric overturning and localized by regional characteristics. Parallel with this, over the past decade, much theoretical progress has been made in understanding the fundamental dynamics of the seasonal Hadley cells and Intertropical Convergence Zones via the use of hierarchical modeling approaches, including highly idealized simulations such as aquaplanets. Here we review the theoretical progress made, and explore the extent to which these theoretical advances can help synthesize theory with observations and understand differing characteristics of regional monsoons. We show that this theoretical work provides strong support for the migrating convergence zone picture, allows constraints on the circulation to be identified via the momentum and energy budgets, and lays out a framework to assess variability and possible future changes to the monsoon. Limitations of current theories are discussed, including the need for a better understanding of the influence of zonal asymmetries and transients on the large-scale tropical circulation.

A statistical study of frequency of depressions cyclones in the Bay of Bengal

Article

Full-text available

Jul 1958
MAUSAM

A statistical study of frequency of depressions cyclones in the Bay of Bengal

A monsoon storm as studied on 5 day mean charts

Article

Full-text available

Jul 1960
MAUSAM

The storm in question followed a long track. It crossed the coast near Puri on the morning of 22 August 1957 and finally broke up over the Kashmir Himalayas on 26 August 1957. Under its influence, heavy to very heavy rain occurred in the central parts of the country, the West Coast, coastal Andhra Pradesh, Gujarat, Rajasthan, the Punjab (I) and Jammu and Kashmir. According to press reports the Kashmir Valley from Anantnag to Sepore - a 75 mile stretch was completely under water excepting the small areas comprising the city of Srinagar and some neighbouring villages. The Godavari near Bhadrachalam and the Narbada near Indore were in high spate, dislocating rail and road traffic in those areas. The river Ravi was also in spate threatening the city of Amritsar.

Statistical distribution of pentad rainfall over India during monsoon season

Article

Full-text available

Apr 1970
MAUSAM

Statistical distribution of pentad rainfall during southwest and northeast monsoon seasons, at representative stations in India, has been studied" From the histograms it is seen that these distributions are right (positive) skewed" Gamma distribution function has been fitted to rainfall data and the guess of fit of the distribution to the rainfall data has been tested by Chi-square tests. These tests show that pentad rainfall may be described by Gamma distribution.

Rainfall around monsoon depressions over India

Article

Full-text available

Jan 1957
MAUSAM

Rainfall that occurrs for 2 to 3 days within a radius of 350 miles from the centre of a depression along its track has been combined using the data of all raingauge stations within that area. Such 'composite' charts of rainfall were prepared for a sample of three mid-monsoon depressions in the year 1944. Falls of heavy rain of 3 and above in 24 hours are confined to an area lying to the left hand side of the track. On any particular morning, the heavy rainfall area extends to about 300 miles ahead and to about 300 miles behind the centre of the depression on that morning, measured respectively along the expected and past track of the depression. The width of the area is about 250 miles and extends to the left of the track. It is further noticed that out of this belt, about 30 per cent of the area is almost the maximum over which there may be rainfall of 3" or more in 24 hour's. The results are discussed from a forecaster's point of view.

Air-Masses and fronts in the Monsoon Depression in India

Article

Full-text available

Oct 1951
MAUSAM

From a synoptic and aerological studt of the data of radiosonde acents at Vizagapatam, Cuttack, Culcutta and Akyab during a depression in the Bay of Bengal in the beginning of July 1945, it is observed that different airmasses took part in both in the formation stage of the depression and during its subsequent intensification into a storm and movement westnorth westwards. The study supports the mechanism of the formation of depression in the north bay of Bengal during the monsoon season suggested by Desai in an earlier paper.

On the Formation of Monsoon Depressions in the Bay of Bengal

Article

Full-text available

Jan 1958
MAUSAM

The role of the upper tropospheric perturbations in the formation of monsoon depressions in the Bay of Bengal has been examined. From a study of these flow patterns, it is found that cyclonic development at sea level occurs when and where an area of positive vorticity advection in the upper troposphere becomes superimposed upon a preexisting trough at Sea level Other factors affecting cyclogenesis at sea level during the southwest monsoon period are also pointed out.

Remarks on Correlation Methods in Geophysics

Article

May 1960

James E. McDonald

From consideration of several studies of the sampling properties of the product-moment correlation coefficient r it is concluded first that undue concern has been expressed for the problem of non-normality in correlation studies in geophysics, and second, that use of geo-physically adequate significance tests and confidence limits for r can almost always be achieved through use of the simple standard error of r . A growing tendency in climatology and hydrology to employ unnecessarily elaborate methods appears to stem from unrealistic emphasis which mathematical statisticians frequently place upon theoretical refinements, emphasis that loses sight of the limits of accuracy inherent in the very type of data usually subjected to statistical analysis. DOI: 10.1111/j.2153-3490.1960.tb01296.x

Indian tropical storms and zones of heavy rainfall

Article

The Indian southwest monsoon and the structure of depressions associated with it

Article

Influence of cyclonic storms on monsoon rainfall in India

Article

Jun 1967

K. Raghavan

THE MEAN UPPER-WIND CIRCULATION AROUND MONSOON DEPRESSIONS IN INDIA

Article

Feb 1960

All available rawin observations taken within 400 mi from the centers of monsoon depressions in the Indian area have been used to obtain a picture of the average upper-wind circulation around the depressions. Cyclonic circulation is found to persist in the southwestern sector up to a height of 9 km, while in the other sectors it disappears at lower levels. Rainfall patterns associated with the depressions have been compared with the circulation in the upper levels. The inflow and outflow of air from the system have been assessed. The application of the concept of “steering” has been examined.

Weather Analysis and Forecasting

Article

Jan 1956

Sverre Petterssen

Tracks of Storms and Depressions in the Bay of Bengal and the Arabian Sea, 1877–1960. New Delhi

Article

Jan 1979

New Delhi (India) India Meteorological Dept

maps

Handbook of Statistical Methods i n Meteorology, Her Majesty's Stationery Office

Jan 1953
220

Charles Ernest Brooks
Pelham
N Carruthers

Brooks, Charles Ernest Pelham, and Carruthers, N., Handbook of Statistical Methods i n Meteorology, Her Majesty's Stationery Office, London, England, 1953, 413 pp. (See p. 220.)

On the Development and Structure of Monsoon Depressions in India

Jan 1951
217-228

B N Desai

Desai, B. N., "On the Development and Structure of Monsoon Depressions in India," Memoirs of the India Meteorological Department, Vol. 28, Part 5, New Delhi, 1951, pp. 217-228.

A Study of Rainfall Distribution Around the Tracks of Tropical Disturbances Over Coastal Orissa

Dec 1971
102-111

N Dhar
P R Mhaiskar

Dhar, 0. N., and Mhaiskar, P. R., " A Study of Rainfall Distribution Around the Tracks of Tropical Disturbances Over Coastal Orissa, " Proceedings of the Symposium on Flood Forecasting, Control, and Flood Damage Protection, New Delhi, India, November 28, 1970, Central Board of Irrigation and Power, Publication No. 107, New Delhi, Nov. 1970, 210 pp. (See pp. 102-111.)

Five Day Normals of Pressure, Mazimum and Minimum Temperature and Humidity

Aug 1965

India Meteorological

India Meteorological Department, Five Day Normals of Pressure, Mazimum and Minimum Temperature and Humidity, New Delhi, Aug. 1965, 321 pp.

Structure and Movement of Cyclones in the Indian Seas

Jan 1930
224-234

S C Roy
A K Roy

Roy, S. C., and Roy, A. K., "Structure and Movement of Cyclones in the Indian Seas," Beitrtige zur Physik der freien Atmosphate, Vol. 26, No. 3, Leipzig, Germany, 1930, pp. 224-234.

On the Physical Characteristics of Fronts During the Indian Southwest Monsoon

Jan 1933
37-50

N K Sur

Sur, N. K., "On the Physical Characteristics of Fronts During the Indian Southwest Monsoon," Memoirs of the India Meteorological Department, Vol. 26, Part 111, Poona, 1933, pp. 37-50.

Some Aspects of Indian Monsoon Depressions and the Associated Rainfall

Abstract

No full-text available

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