Dynamics and evolution of a case study monsoon depression in a high-resolution simulation of the Met Office Unified Model

Monsoon depressions (MD) are synoptic-scale cyclonic vortices that form over the Bay of Bengal and propagate north-westward 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.  Motivated by aircraft and ground-based observations made during the INCOMPASS field campaign in India in 2016, here we inspect the structure and dynamics of a MD case study (1-10 July 2016) using a variety of Met Office model simulations (1.5 km, 4.4 km and 17 km horizontal resolutions). 

The 1.5 km simulation proves effective at resolving intense rainfall caused by deep convection, convergence lines, and kilometre-scale orographic interactions.  The evolution of the case-study MD can be divided into two stages: initially the MD is completely embedded in a near-saturated environment up to the mid-troposphere.  Then, an intrusion of low-potential-temperature dry air from the west at low and mid-levels starts interacting with the MD.

Using Lagrangian trajectory analysis, we find that during the initial stage of the MD, high-θ_e air from mesoscale convective systems in the vicinity of the MD reaches its centre at low and mid-levels, enabling its growth.  During the second stage, the intrusions of stable and subsiding dry air bring low-θ_e, low-PV air at low and mid-levels towards the centre of the depression, hindering its development.

The 1.5-km simulation enables us to highlight the presence of individual vorticity towers or filaments embedded within the MD that were not otherwise resolved at coarser (17km) resolution.  We use analysis with Stokes' theorem to explore the aggregation of these filaments and their contribution to central vorticity as the MD develops.  The work paves new directions for theoretical understanding of growth of monsoon depressions.