A Moist Baroclinic Model for Monsoonal Mid-Tropospheric Cyclogenesis

View More View Less
  • 1 Department of Atmospheric Sciences, University of Illinois, Urbana 61801
© Get Permissions
Full 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