Mechanisms Driving the Diurnal Cycle of Orographic Precipitation and Monsoon Rainfall Modes over the West Coast of India

Abstract: The Indian monsoon rainfall exhibits large spatial variability associated with orography and surface temperature gradients. The Western Ghats (WG) mountains along the west coast of India is prone to extremely heavy rainfall (e.g., giving rise to the Kerala floods of 2018) and hence it is of immense importance to understand the mechanisms driving different modes and time scales of rainfall variability around this region to improve prospects for prediction. In this study, we use convection-permitting fully-coupled regional simulations of the Met Office model with the latest RAL3 science configuration to investigate the offshore-to-coastal regime transition of monsoon rainfall over the west coast of India, with a focus on the mechanisms driving diurnal variability of orographic precipitation. We also conduct ocean-atmosphere coupled experiments with and without land surface irrigation to explore the impact of irrigation on surface fluxes and orographic precipitation. Our findings reveal that both land-surface initialization and science settings in the model influence diurnal variability of orographic precipitation, with the latter exerting a more significant effect. During the offshore phase, heat and radiation flux diurnal amplitudes intensify over the WG compared to the coastal phase. The simulations indicate a drier mid-troposphere and a moister lower troposphere over the west coast of India during the coastal phase relative to the offshore phase, indicating a strengthening of the mid-tropospheric dry-air intrusion during the coastal phase. Our ongoing investigation aims to elucidate the influence of atmosphere-ocean coupling and irrigation on the mechanisms governing diurnal variability of orographic precipitation and monsoon rainfall modes near the Western Ghats. Future research will further explore the role of irrigation in soil moisture-induced mesoscale circulations and convective initiation.

Biography: Arathy works as a senior scientist at the Met Office in the Momentum® Partnership team. In her current role, she provides scientific and technical support for the use of regional atmosphere and land configurations of the Met Office model at the partner sites. She is a fellow of the Royal Meteorological Society. Her research interest is on high resolution modelling of the monsoon and understanding the physical processes associated with the monsoon.