Idealized Simulations of Monsoon Depressions

Monsoon depressions are synoptic low pressure systems embedded in the baroclinic zones of monsoon circulations. They play a significant role in the meteorology of the tropics by producing a large fraction of monsoon precipitation. The energetics governing the growth of monsoon depressions are notably different from those of tropical cyclones and despite their significance, monsoon depressions are not completely understood.

This study examines the dynamics of monsoon depressions in idealized simulations. Monsoon depressions form and intensify in regions characterized by a poleward temperature gradient and strong vertical easterly shear of the zonal wind (i.e. monsoon westerlies in the lower troposphere and the tropical easterly jet in the upper troposphere). This mean state is replicated in the idealized simulations. An idealized axisymmetric vortex, representative of monsoon depressions observed near the time of genesis, is introduced into the center of the domain containing the described mean state. The numerical simulations are performed using the Weather Research and Forecast (WRF) numerical model, with a horizontal resolution of 3 km resolving convection. The domain has a longitudinal extent of 50°, a latitudinal extent from the equator to 45°N and planetary rotation represented by a Β plane. The lateral boundary conditions are zonally periodic and the initial mean state is zonally homogeneous.

The behavior of the vortex is analyzed to obtain better insight into the growth and propagation of monsoon depressions. The potential vorticity (PV) structure of the vortex is examined and compared with the PV structures of monsoon depressions in reanalysis datasets. The role of adiabatic and diabatic processes in shaping the PV structure is investigated. The mechanisms responsible for the intensification of the vortex are explored and the sensitivity of growth rate to the mean wind shear and moisture are measured.