Sensitivity of initial soil moisture in simulating a heatwave event over India

The amount of moisture in the soil can significantly influence hot extremes, such as heatwaves and droughts, through various direct and indirect mechanisms. This study examines the impact of soil moisture initialisation on the simulations of the 2009 heatwave event over India using the Weather Research and Forecasting model (WRF) model. Multiple initial conditions were produced by modifying actual soil moisture in ERA-5 reanalysis. The impact of soil moisture on the intensity and spatial extension of heatwave is studied, and physical mechanisms are examined in detail. The control simulations have reasonably simulated spatial patterns of soil moisture and maximum temperature when compared with gridded observations. Our results show that an increase (decrease) in initial soil moisture simulates a heatwave with reduced (enhanced) magnitude and spatial extent. Interestingly, an increase in soil moisture does have more influence on heat waves than a decrease in soil moisture. In wet simulations, we see a greater increase in latent heat flux when compared to a decrease in sensible heat. Additionally, wet simulations shift soil moisture-temperature coupling from a dry regime to a transitional regime. An overall increase in net radiation is observed in wet simulations through water vapour and cloud feedback. Reduced soil moisture enhances the high-pressure system associated with heatwaves through positive feedback on geopotential height in the mid-troposphere.