A moist-thermal quasigeostrophic model for monsoon depressions
Abstract
Monsoon depressions (MDs) are synoptic-scale storms that occur during the summer phase of the global monsoon cycle and whose dynamical mechanisms remain incompletely understood. To gain insight into the dynamics governing the large-scale structure of MDs, we formulate an idealised moist-thermal quasi-geostrophic model that includes distinct thermal and moisture fields in simple forms. A linear-stability analysis of the model, with basic states corresponding to typical monsoon conditions, shows three distinct mode classifications: thermal-Rossby modes, heavy precipitating modes, and a moist-thermal mode. In the linearised model, the presence of a background precipitation gradient strengthens thermal-Rossby modes by coupling the dynamics to latent heating. The separation of heavy precipitating modes from fast-propagating thermal-Rossby modes is further examined with numerical experiments of large-amplitude MDs. Wind-induced evaporation is found to amplify large-amplitude MDs in conditions analogous to those over the northern Bay of Bengal. An energetic analysis shows the pathways by which the MDs derive energy from the background state. A further series of experiments through a continuum of meridional temperature gradients demonstrates the sensitivity of large-scale MD dynamics to the background state and suggests a possible mechanism to explain variations in the propagation direction of MDs.
Citation
Chaudhri , A K , Byrne , M P & Scott , R K 2024 , ' A moist-thermal quasigeostrophic model for monsoon depressions ' , Quarterly Journal of the Royal Meteorological Society , vol. Early View . https://doi.org/10.1002/qj.4723
Publication
Quarterly Journal of the Royal Meteorological Society
Status
Peer reviewed
DOI
10.1002/qj.4723ISSN
0035-9009Type
Journal article
Description
Funding: AKC is supported by a St Leonard’s College Interdisciplinary Doctoral Scholarship awarded by the University of St Andrews.Collections
Items in the St Andrews Research Repository are protected by copyright, with all rights reserved, unless otherwise indicated.