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

Abstract

Our report makes the case that monsoon depressions over South Asia can form from a variant of moist barotropic instability. Along with an idealized numerical framework in which the atmosphere is partitioned into a basic state and a perturbation, we simulate vortices resembling monsoon depressions that draw energy from the meridional shear of the monsoon trough and amplify when they interact with precipitating ascent. The influence of the basic state vertical shear on the vortex induces upward velocity, which couples precipitation with a Rossby-wave-like mode arising from dry barotropic growth, allowing the vortex to intensify. Sensitivity experiments reveal that both the sheared basic state and latent heating are necessary to achieve positive growth rates and that this process requires a sufficiently large initial perturbation. Trajectory analyses suggest that the combined flow of the vortex and the large-scale monsoon transport diabatically generated potential vorticity from southwest of the vortex into the vortex center, thus enabling growth. In contrast with tropical cyclones, this mechanism does not require a feedback between surface wind speed and surface heat and moisture fluxes, though this feedback does ultimately result in a slightly stronger vortex.

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 Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23); National Science Foundation (NSF)
OSTI Identifier:
1574333
Alternate Identifier(s):
OSTI ID: 1543184
Grant/Contract Number:  
AC02-05CH11231; SC0019367; AGS‐1746160; award DE‐SC0019367
Resource Type:
Accepted Manuscript
Journal Name:
Quarterly Journal of the Royal Meteorological Society
Additional Journal Information:
Journal Volume: 145; Journal Issue: 723; Journal ID: ISSN 0035-9009
Publisher:
Royal Meteorological Society
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; barotropic instability; idealized numerical modeling; Indian Monsoon; monsoon depression; monsoon low‐pressure system

Citation Formats

Diaz, Michael, and Boos, William R. Monsoon depression amplification by moist barotropic instability in a vertically sheared environment. United States: N. p., 2019. Web. doi:10.1002/qj.3585.
Diaz, Michael, & Boos, William R. Monsoon depression amplification by moist barotropic instability in a vertically sheared environment. United States. doi:10.1002/qj.3585.
Diaz, Michael, and Boos, William R. Wed . "Monsoon depression amplification by moist barotropic instability in a vertically sheared environment". United States. doi:10.1002/qj.3585.
@article{osti_1574333,
title = {Monsoon depression amplification by moist barotropic instability in a vertically sheared environment},
author = {Diaz, Michael and Boos, William R.},
abstractNote = {Our report makes the case that monsoon depressions over South Asia can form from a variant of moist barotropic instability. Along with an idealized numerical framework in which the atmosphere is partitioned into a basic state and a perturbation, we simulate vortices resembling monsoon depressions that draw energy from the meridional shear of the monsoon trough and amplify when they interact with precipitating ascent. The influence of the basic state vertical shear on the vortex induces upward velocity, which couples precipitation with a Rossby-wave-like mode arising from dry barotropic growth, allowing the vortex to intensify. Sensitivity experiments reveal that both the sheared basic state and latent heating are necessary to achieve positive growth rates and that this process requires a sufficiently large initial perturbation. Trajectory analyses suggest that the combined flow of the vortex and the large-scale monsoon transport diabatically generated potential vorticity from southwest of the vortex into the vortex center, thus enabling growth. In contrast with tropical cyclones, this mechanism does not require a feedback between surface wind speed and surface heat and moisture fluxes, though this feedback does ultimately result in a slightly stronger vortex.},
doi = {10.1002/qj.3585},
journal = {Quarterly Journal of the Royal Meteorological Society},
number = 723,
volume = 145,
place = {United States},
year = {2019},
month = {6}
}

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