A Moist Baroclinic Model for Monsoonal Mid-Tropospheric Cyclogenesis

Mankin Mak Department of Atmospheric Sciences, University of Illinois, Urbana 61801

Search for other papers by Mankin Mak in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

This study investigates the effect of condensational heating on the formation of a monsoonal mid-tropospheric cyclone (MTC) by applying the heating parameterization of Mak (1982) to the dynamic model of Brode and Mak (1978) except that the beta-effect is also included. It is found that the observed basic baroclinic flow with turning provides a dynamic framework conducive for a MTC-like disturbance to interact with the embedded moist convection. The joint processes give rice to an unstable disturbance that has a large but finite growth rate and an intermediate length scale. The theoretical e-folding time is of the order of one day under a wide range of parametric heating conditions. The key structural features of the observed MTC are reproduced in the most unstable model disturbance. The main influence of the beta-effect is on the phase speed of the model disturbance. In the presence of condensational heating, the beta-effect only has a secondary influence on the growth rate. The moist processes and the unique baroclinic processes associated with a turning baroclinic flow are therefore both indispensible in the genesis of MTC. To understand whether or not the instability results might be strongly controlled by the low-level basic shears, the results computed with or without the latter am compared. It is found that such low-level shears have only a stabilizing influence and therefore do not play an essential role in the instability process.

Abstract

This study investigates the effect of condensational heating on the formation of a monsoonal mid-tropospheric cyclone (MTC) by applying the heating parameterization of Mak (1982) to the dynamic model of Brode and Mak (1978) except that the beta-effect is also included. It is found that the observed basic baroclinic flow with turning provides a dynamic framework conducive for a MTC-like disturbance to interact with the embedded moist convection. The joint processes give rice to an unstable disturbance that has a large but finite growth rate and an intermediate length scale. The theoretical e-folding time is of the order of one day under a wide range of parametric heating conditions. The key structural features of the observed MTC are reproduced in the most unstable model disturbance. The main influence of the beta-effect is on the phase speed of the model disturbance. In the presence of condensational heating, the beta-effect only has a secondary influence on the growth rate. The moist processes and the unique baroclinic processes associated with a turning baroclinic flow are therefore both indispensible in the genesis of MTC. To understand whether or not the instability results might be strongly controlled by the low-level basic shears, the results computed with or without the latter am compared. It is found that such low-level shears have only a stabilizing influence and therefore do not play an essential role in the instability process.

Save