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Title: Barotropic growth of monsoon depressions

Abstract

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 monsoonmore » depressions, but not their rapid growth rates. We speculate that condensational heating coupled to these waves is needed to obtain a positive growth rate.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]
  1. Univ. of California, Berkeley, CA (United States)
  2. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER); US Department of the Navy, Office of Naval Research (ONR); National Science Foundation (NSF)
OSTI Identifier:
1542355
Alternate Identifier(s):
OSTI ID: 1494214
Grant/Contract Number:  
AC02-05CH11231; AGS‐1746160; N00014‐15‐1‐2531; DE‐SC0019367
Resource Type:
Accepted Manuscript
Journal Name:
Quarterly Journal of the Royal Meteorological Society
Additional Journal Information:
Journal Volume: 145; Journal Issue: 719; Journal ID: ISSN 0035-9009
Publisher:
Royal Meteorological Society
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; barotropic instability; idealized numerical modelling; Indian monsoon; monsoon depression; monsoon low pressure system

Citation Formats

Diaz, Michael, and Boos, William R. Barotropic growth of monsoon depressions. United States: N. p., 2018. Web. doi:10.1002/qj.3467.
Diaz, Michael, & Boos, William R. Barotropic growth of monsoon depressions. United States. https://doi.org/10.1002/qj.3467
Diaz, Michael, and Boos, William R. Mon . "Barotropic growth of monsoon depressions". United States. https://doi.org/10.1002/qj.3467. https://www.osti.gov/servlets/purl/1542355.
@article{osti_1542355,
title = {Barotropic growth of monsoon depressions},
author = {Diaz, Michael and Boos, William R.},
abstractNote = {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.},
doi = {10.1002/qj.3467},
journal = {Quarterly Journal of the Royal Meteorological Society},
number = 719,
volume = 145,
place = {United States},
year = {Mon Dec 24 00:00:00 EST 2018},
month = {Mon Dec 24 00:00:00 EST 2018}
}

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Works referencing / citing this record:

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Monsoon depression amplification by moist barotropic instability in a vertically sheared environment
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