Influence of Soil Moisture on the Evolution of Landfalling Tropical Cyclones during pre and post-monsoon seasons

Numerous cyclones develop in the Bay of Bengal during the pre-monsoon and post-monsoon seasons. The heavy rain associated with these cyclones causes devastating damage to life and property during landfall. The modern numerical weather prediction models and high temporal satellite observation data have significantly increased the accuracy of cyclone prediction in recent years. However, accurately predicting rainfall intensity and its dissipation after landfall is still challenging. Previous studies have indicated that land-based evapotranspiration plays an essential role in determining the intensity and decay of cyclones post-landfall. In this study, we quantify the contribution of land-based evapotranspiration to the rainfall associated with cyclones and the impact of land conditions on the speed and track of cyclones originating in the Bay of Bengal. For this purpose, we employed the Weather Research Forecasting (WRF) model upgraded with Eulerian water tagging capabilities to track evapotranspiration from land. The tagging model will tag the evapotranspiration originating on land and track it throughout the atmosphere till it precipitates or moves out of the domain. We simulated six cyclones of varying intensities, with three during the pre and three during the post-monsoon seasons. We conducted sensitivity experiments with dry and wet initial soil moisture conditions to determine the impact of perturbed soil moisture on TC. To account for the model's internal variability, we simulated an ensemble with four members for the control simulation. The ensemble is created by changing each member's model initialization time by six hours. This ensemble helped identify the magnitude of the model's internal variability, which was less than the variability due to soil moisture changes. The study revealed that soil moisture conditions prior to TC formation have an impact on its evolution. By analyzing the latent heat, temperature, and wind pattern, we found that the initial soil moisture during the pre and post-monsoon seasons alters the synoptic features over the Indian subcontinent, resulting in variations in the TC evolution. The relatively low-intensity TC tracks are more sensitive to the initial soil moisture conditions. The rainfall originating from land-based evapotranspiration is more significant as the cyclone approaches land. Therefore, land-based evapotranspiration plays a crucial role in the end phase of the cyclone (from just before landfall till its decay). For post-monsoon cyclones, the rainfall from land-based evapotranspiration is as high as 20% to 30% after landfall, whereas, for pre-monsoon cyclones, the land contribution is around 5% to 10%. In addition to soil moisture, factors such as proximity to land, track length over land, and TC intensity also have a role in determining the quantity of precipitation originating from the land for a TC.