Study of Dynamical Structure of an Unusual Monsoon Depression formed over the Bay of Bengal during August 2006

Nagar, S. G.; Bawiskar, S. M.; Chinthalu, G. R.; Kulkarni, J. R.

doi:10.1007/s00024-009-0446-9

Cited by 4 publications

(3 citation statements)

References 15 publications

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“…Most of these events result from low‐pressure systems (depressions and cyclones with wind speeds of ≥8.5 m/s) in BOB, which transports moisture to the central Indian subcontinent, but a declining trend in the variability of these depressions has been noted in recent years (Naidu et al ., ; Roxy et al ., ; Sandeep et al ., ). The MLLJ acts as a major source of moisture feeding the monsoon depression formed over the BOB (Nagar et al ., ; Walker et al ., ; Roxy et al ., ), and therefore, variations in the MLLJ also has a strong influence on the formation of depressions over BOB and the occurrence of extreme rain events over central India (Roxy et al ., ).…”

Section: Resultsmentioning

confidence: 99%

Variability of monsoon low‐level jet and associated rainfall over India

Viswanadhapalli

Dasari

Dwivedi

et al. 2019

Intl Journal of Climatology

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The structure and climatology of the monsoon low‐level jet (MLLJ) is studied based on dynamically downscaled simulations over a 37‐year period (1980–2016) using the Weather Research Forecasting (WRF) model. The simulations are conducted by adopting a continuous initialization method with daily re‐initializations using ERA‐Interim data as initial and boundary conditions. Validation of the downscaled fields with radiosonde data shows that the model has reasonable skill in reproducing MLLJ characteristics. Analysis of the simulations suggests that the MLLJ exhibits systematic diurnal variation: maximum winds of the synoptically induced large‐scale monsoon jet occur during the daytime, and the orographic channelled winds through the mountains of East Africa, Hejaz, and Western Ghats in the night‐time. These diurnal changes in monsoon winds modulate the moisture convergence process and the associated evolution of rainfall over India. Seasonal and monthly climatology of monsoon winds shows that the model accurately reproduces the spatial pattern of winds and slightly overestimates (2–3 m/s) the mean monthly winds over the Bay of Bengal and Arabian Seas. Analysis of wind variability and the trends using 37 years simulations suggests that the MLLJ exhibits an increasing trend in wind speed on both seasonal and monthly scales, except for August which shows a decreasing trend. The weakening of the MLLJ in August has a profound influence on the number of monsoon depressions forming over the Bay of Bengal (which are decreasing), and on the number of break days (which are increasing) and associated precipitation reduction over the central Indian region.

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Section: Resultsmentioning

confidence: 99%

Variability of monsoon low‐level jet and associated rainfall over India

Viswanadhapalli

Dasari

Dwivedi

et al. 2019

Intl Journal of Climatology

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“…; Nitta and Masuda, ; Nagar et al. ). Recently, Mishra () determined that the monsoon trough becomes increasingly barotropically unstable preceding monsoon depression formation.…”

Section: Introductionmentioning

confidence: 99%

Barotropic growth of monsoon depressions

Diaz

Boos

2019

Quart J Royal Meteoro Soc

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Although monsoon depressions are a principal synoptic-scale element of the South Asian monsoon, producing extreme rainfall over India and surrounding regions, there exists no widely accepted mechanism explaining their occurrence. This study presents a hierarchy of numerical experiments aimed at finding such an explanation. Using a perturbation-basic state decomposition, we derive an anelastic system of equations that can represent disturbances growing in the complex, three-dimensional monsoon basic state. We find that modal solutions to these equations linearized about this basic state can explain many features of observed monsoon depressions, including their warm-over-cold core structure, westward propagation, and lower-tropospheric wind maximum. For the zonally symmetric case, these modes are barotropically unstable, drawing energy from the meridional shear of the monsoon trough. For the zonally varying basic state, modal solutions still derive energy from barotropic conversion, but fail to achieve positive net growth rates when dissipative processes are included. For the nonlinear equation set, these modes can be excited by a heating impulse, and their energy then remains roughly constant over several days as barotropic energy transfers oppose dissipative losses. Our results support the idea that the general concept of barotropic instability can explain the structure, propagation, and geographic distribution of monsoon depressions, but not their rapid growth rates. We speculate that condensational heating coupled to these waves is needed to obtain a positive growth rate.

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“…The MLLJ originates from cross-equatorial flow and acts as an important driver for transporting moisture toward the Indian region, controlling the monsoon inversion layers over the west central Arabian Sea (Dwivedi et al, 2016(Dwivedi et al, , 2021. The MLLJ further feeds moisture to the monsoon depressions formed over the Bay of Bengal (BoB) (Nagar et al, 2009;Walker et al, 2015) and modulate the rainfall over India (Roxy et al, 2017;Viswanadhapalli et al, 2020).…”

Section: Monsoon Circulationmentioning

confidence: 99%

Enhanced Simulation of the Indian Summer Monsoon Rainfall Using Regional Climate Modeling and Continuous Data Assimilation

et al. 2022

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This study assesses a Continuous Data Assimilation (CDA) dynamical-downscaling algorithm for enhancing the simulation of the Indian summer monsoon (ISM) system. CDA is a mathematically rigorous technique that has been recently introduced to constrain the large-scale features of high-resolution atmospheric models with coarse spatial scale data. It is similar to spectral nudging but does not require any spectral decomposition for scales separation. This is expected to be particularly relevant for ISM, which involves various interactions between large-scale circulations and regional physical processes. Along with a control simulation, several downscaling simulations were conducted with the Weather Research and Forecasting (WRF) model configured over the Indian monsoon region at 10 km horizontal resolution using CDA, spectral (retaining different wavenumbers) and grid nudging for three contrasting ISM rainfall seasons: normal (2016), excess (2013), and drought (2009). The simulations are nested within the global NCEP Final Analysis data available at 1 × 1° horizontal resolution. The model outputs are evaluated against the India Meteorological Department (IMD) gridded precipitation and the fifth generation ECMWF atmospheric reanalysis (ERA-5). Compared to grid and spectral nudging, the simulations using CDA produce enhanced ISM features over the Indian subcontinent including the low-level jet, tropical easterly jet, easterly wind shear, and rainfall distributions for all investigated ISM seasons. The major ISM processes, in particular the monsoon inversion over the Arabian Sea, tropospheric temperature gradients and moist static energy over central India, and zonal wind shear over the monsoon region, are all better simulated with CDA. Spectral nudging outputs are found to be sensitive to the choice of the wavenumber, requiring careful tuning to provide robust simulations of the ISM system. In contrast, control and grid nudging generally fail to well-reproduce some of the main ISM features.

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Study of Dynamical Structure of an Unusual Monsoon Depression formed over the Bay of Bengal during August 2006

Cited by 4 publications

References 15 publications

Variability of monsoon low‐level jet and associated rainfall over India

Variability of monsoon low‐level jet and associated rainfall over India

Barotropic growth of monsoon depressions

Enhanced Simulation of the Indian Summer Monsoon Rainfall Using Regional Climate Modeling and Continuous Data Assimilation

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