On the Mechanism of the Monsoonal Mid-Tropospheric Cyclone Formation

Abstract

The radiosonde data collected prior to the development of a well-documented mid-tropospheric cyclone (MTC) over the west coast of India is used to determine the actual basic state. Such observed basic state is in turn used as the basis of a general quasi-geostrophic instability eigenvalue analysis, whereby one can check whether or not the hypothesis suggested in Mak's (1975) analysis is relevant in a real setting. We will discuss the degree of similarities and differences between the observed basic state and the idealized basic state. The incompleteness of Mak?s analysis is noted and a supplementary analysis is given. The results using the observed state show that there does exist a dominantly most unstable mode which in its overall structure and characteristics resembles that of the observed MTC. This lends further support to the idea that MTC initially arises from a baroclinic instability of the broad southwest monsoonal flow, of which the direction as well as the magnitude vary significantly with height. The energetics of this genesis mechanism are clarified. The dynamical roles of each element in this basic state are also investigated. The main shortcomings of the model results are that the growth rate is somewhat too small and the level of maximum intensity of the disturbance is somewhat too low. Such discrepancy strongly suggests the plausible importance of moist convection.