Role of baroclinic instability in the development of monsoon disturbances

A linear baroclinic stability analysis of the zonal wind representing the mean monsoon situation over India is performed by the use of a multi-level quasi-geostrophic numerical model. An initial value approach is chosen to determine the instability characteristics of the wind. The dependency of the growth rate spectrum on the number of levels in the vertical and on the presence of vertical walls is studied. It is shown that 20 levels in the vertical are sufficient to realize the baroclinic instability of the monsoon mean wind. A shorter unstable wave of wavelength 1500 km and a longer unstable wave of wavelength 4750 km are found to be the most preferred growing waves from the growth rate spectrum. The shorter unstable wave is essentially confined below 500 mb, whereas the longer unstable wave is above 500 mb. It is also shown that the removal of wind shear below (above) the level of the westerly (easterly) jet from the wind profile, shifts the shorter (longer) unstable wave toward higher wavelengths by ~1000 km, with a significant decrease in the growth rates. The horizontal scale (1500 km), level of nondivergence (900 mb), and level of maximum intensity (825 mb) associated with the shorter unstable wave are in close agreement with the observed values, obtained from a composite monsoon depression. The computed phase velocity of the unstable wave is opposite to the observed westward motion. The computed levels of cold core, warm core and top of the wave are at 900, 800 and 500 mb, respectively, which are ~100 mb lower than the observed levels. The computed phase velocity of the longer unstable wave (-23 m s-1) is found to be very close to the observed value for disturbances along the easterly jet level. The longer unstable wave has a level of nondivergence at 200 mb which is supported from the results of barotropic studies obtained by others