Some studies of the growth of monsoon disturbances

Abstract

The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation (compositeof 15depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25 0 longitude and extends up to 300 mb and has the greatest intensity at 700 mb. It shows north-northeast to south-southwest 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-valuetechnique it is shown that the monsoon zonal current is not baroclinically 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.

High resolution simulation of the South Asian monsoon using a variable resolution global climate model

“…These disturbances generally form in the Bay of Bengal and move in a west-northwest direction along the quasi-stationary monsoon trough across north-central India (eg., Koteswaram and Rao, 1963, Rao, 1976, Sikka 2006. Studies have shown that the existence of combined barotropicbaroclinic instability of the mean monsoon flow is a necessary condition for the generation of monsoon disturbances (e.g., Keshavamurty et al, 1978, Mishra and Salvekar et al, 1980, Dash and Keshavamurty, 1982. On the other hand, the energetics of monsoon depressions appears to be primarily maintained by cumulus convection and moist processes (see .…”

Diskussionen über barokline und barotrope instabilitäten im Sommer über Indien

“…This suggests that there is no possibility of baroclinic growth of Indian monsoon depression. Possibly, due to the above reason, Keshvamurty et al [6] obtained real value of the baroclinic frequency with 20 level model. Now, let us consider the reverse problem.…”

“…The diabatic heating effect can explain the growth rate and phase velocity both. Keshvamurty et al [6] have shown that the growth of monsoon depression might be possible by the barotropic process but not by the baroclinic process. Again, Krishnamurti's [5] observational facts suggest that the axis of Indian monsoon depression is slightly tilted westward at the low level and, beyond 800 mb, the axis is slightly tilted eastward.…”

The stability of monsoon depression

“…The theoretical works [11,14] suggest that the CISK gives a possible dynamical reason of development of tropical disturbances. Thus, knowing that the monsoon depression has a wave-like structure and it grows due to the effect of diabatic heating, one considers that the monsoon depression is a pressure wave of 2000 km wavelength whereas the diabatic heating function may be treated as a wave of same wave length, propagating with the pressure wave with an equal velocity.…”

“…The maximum intensity of disturbance occurs at 700 mb level and so the discussion is restricted to 700 mb at 20 °N only. Krishnamurti [ 12] and Keshavamurty et al [ 11 ] have shown that its…”