Structure and dynamics of a case-study monsoon depression in high-resolution numerical simulations using the Met Office Unified Model

Monsoon depressions (MD) are synoptic-scale cyclonic vortices that form over the Bay of Bengal and propagate northwestward through the monsoon trough onto the Indian subcontinent, bringing substantial amounts of rainfall to central and northern India. Despite their importance, key questions on the mechanisms driving their generation and development are still open. In this study we inspect the structure and dynamics of a MD case study (1-10 July 2016) using a set of high-resolution simulations performed within the INCOMPASS project. The simulations are performed at a grid spacing of 17 km, 4.4 km and 1.5 km (with parametrised convection for the former experiment and explicit convection for the latter two). Initial results of this study show that the two higher-resolution simulations are more effective in resolving intense rainfall caused by deep convection, convergence lines and orographic enhancement. The evolution of the case-study MD can be divided into two stages: initially the MD is completely embedded in a close-to-saturated environment up to mid-troposphere, whilst in the following stage the intrusion of low-potential-temperature dry air at low- and mid-levels starts interacting with the MD. During this latter stage, the dry-air intrusion brings in low PV-air towards the centre of the depression. Further analysis of the case study takes advantage of a system-relative framework to look into detail at the time evolution of dynamic and thermodynamic parameters around the storm centre and at its small- and meso-scale structure. For example, the 1.5 km-spacing simulation enables us to highlight the presence of individual vorticity towers embedded within the MD. In summary, using a suite of high-resolution numerical simulations of a case-study MD, we are able to achieve a detailed understanding of its structure and dynamics, highlighting the processes driving its evolution.