Some studies of the growth of monsoon disturbances |
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| Authors: | R N Keshavamurty G C Asnani P V Pillai S K Das |
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| Institution: | (1) Meteorological Office, Pune;(2) Present address: Physical Research Laboratory, 380 009 Ahmedabad;(3) Department of Meteorology, University of Nairobi, Nairobi |
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| Abstract: | The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation
(composite of 15 depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25° longitude
and extends up to 300 mb and has the greatest intensity at 700 mb. It shows north-north-east to south-south-west tilt in the
lower levels indicating that it may draw upon zonal kinetic energy for its growth. The disturbance has lower temperatures
to its west and tilts westwards with height indicating that eddy available potential energy is not converted from zonal available
potential energy by large scale advection. There appears to be a reduction of vertical shear at the time of formation of monsoon
depressions and this possibly aids cumulus convection.
The profiles of potential vorticity indicate extremes (i) in the upper troposphere and (ii) at several midtropospheric levels
in the region of the monsoon trough indicating the possibility of combined barotropic-baroclinic instability. Using multi-level
quasi-geostrophic model and employing the eigen-value technique it is shown that the monsoon zonal current is notbaroclinically unstable. A barotropic stability analysis is also done for monsoon zonal current in the lower and middle tropospheres. It
yields very slowly growing unstable modes at lower tropospheric levels with wave lengths of 2500 km and 5000 km. |
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| Keywords: | Monsoon disturbances Eddy kinetic energy static stability Coriolis parameter perturbation potential perturbation stream function barotropic-baroclinic instability |
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