2 results
27 - Growth of monsoon disturbances over western India
-
- By B. N. Goswami, Physical Research Laboratory, India, V. Satyan, Physical Research Laboratory, India, R. N. Keshavamurthy, Physical Research Laboratory, India
- James Lighthill, R. P. Pearce
-
- Book:
- Monsoon Dynamics
- Published online:
- 05 November 2011
- Print publication:
- 19 March 1981, pp 415-428
-
- Chapter
- Export citation
-
Summary
Active monsoon epochs over western India are often associated with cyclonic disturbances most marked in the midtroposphere. In order to understand the mechanism of their growth, linear barotropic, baroclinic and combined barotropic–baroclinic stability analyses of the mean flow in this region have been carried out. The potential vorticity of the mean flow shows extreme values in the region of the midtropospheric shear zone. The analyses yield barotropically unstable modes (i) at 700, 600, 500 mb and (ii) at 200 mb, with wavelengths of 20 to 30 degrees of longitude and doubling times of 3 to 4 days. Combined barotropicbaroclinic stability analyses using a 2-level quasi-geostrophic model yield essentially the same upper- and lower-tropospheric barotropically unstable modes, with only a marginal effect due to the presence of baroclinicity.
Introduction
There seem to be certain parts of the monsoon region from southeast Asia to western India which are particularly cyclogenetic. Shukla (1977) and Keshavamurty et al. (1977) have carried out a general stability analysis of the monsoon zonal flow. It turns out that the zonal wind profile over western India shows some slightly different features compared to that, say, over southeast Asia. In the former longitudes, the heat low over northwest India–Pakistan is overlain by anticyclonic flow in the midtroposphere, so that there is an east–west shear zone in the middle levels. This feature is not so well marked over other longitudes in the monsoon region.
26 - The stability of the monsoon zonal flow with a superposed stationary monsoon wave
-
- By V. Satyan, Physical Research Laboratory, India, R. N. Keshavamurthy, Physical Research Laboratory, India, B. N. Goswami, Physical Research Laboratory, India
- James Lighthill, R. P. Pearce
-
- Book:
- Monsoon Dynamics
- Published online:
- 05 November 2011
- Print publication:
- 19 March 1981, pp 403-414
-
- Chapter
- Export citation
-
Summary
Using a two-level quasi-geostrophic model, the effect of the inclusion of a long stationary monsoon wave (basic meridional wind) on the combined barotropicbaroclinic instability of the monsoon current has been investigated. Both barotropic and baroclinic interactions of the perturbation with the monsoon wave and the zonal flow are included. This analysis, using realistic velocity profiles, yields fast-growing westward-moving upper-tropospheric waves corresponding to easterly waves, and lower-tropospheric modes corresponding to monsoon disturbances.
Introduction
The stability of the monsoon zonal flow has been studied by Shukla (1977) and Keshavamurty et al. (1977). They found that the monsoon atmosphere is not baroclinically unstable. Shukla found that the CISK mechanism can lead to a growth of disturbances of the same scale as the observed monsoon disturbances, but that there was no preferred scale with a fastest rate of growth. Keshavamurty et al. found that the monsoon zonal flow is barotropically unstable in the lower and midtroposphere and that this instability can yield disturbances of reasonable growth rate and scale. Mak (1975) studied the effect of the meridonal motion on baroclinic instability of a monsoon flow, but found that the meridonal components required for growth were very large compared to those obtained from observed winds. Lorenz (1972) studied the barotropic instability of Rossby waves and found that shorter waves can be barotropically unstable. The observed monsoon flow is seen to have an appreciable meridional component.