Analysis of the Relationship Between Soil Moisture and Precipitation Across Heat Stress Categories

Extreme heat stress events are marked by significant deviations in surface air temperature that surpass the typical climatological range, coupled with increased atmospheric humidity. These events are characterised by their intensity, duration, and spatial extent, often crossing thresholds critical for both human and terrestrial ecosystem functioning. At the local scale, land-atmosphere interactions during these heat extremes modulate stress on soil and vegetation by altering energy partitioning, boundary layer feedbacks, and soil moisture memory. During these episodes, evapotranspiration is constrained due to low soil moisture (SM) conditions, leading to increased sensible heat and temperatures, which serve as the primary thermodynamic pathway for heat amplification. In conditions characterized by high soil moisture (SM), light precipitation (P) occurs, with an increase in latent heat flux may elevate atmospheric humidity and exacerbate heat stress, underscoring the nonlinear and stress-dependent nature of SM–P interactions. Despite the centrality of these processes, the relationship between SM and P across diverse heat stress regimes in South Asia remains insufficiently explored.

In this study, the Weather Research and Forecasting (WRF) model is employed to simulate an extreme heat stress event that occurred in May 2015 in the Indo-Gangetic Plains of India, utilizing initial and boundary conditions from the ERA5 dataset. To examine the SM-P feedback relationship, the initial SM is perturbed by 25% and 50% to represent a full spectrum of heat stress conditions (no stress, caution, danger, and extreme danger). Under no-stress conditions, the SM-P feedback exhibits a typical convex-concave relationship on the E[PSM] curve. However, as the heat stress intensifies, this relationship is broken. Extremely hot and deeply mixed boundary layers inhibit the development of moist convection, raising the lifting condensation level (LCL). Although cloud formation may still occur, the environmental conditions are insufficient to trigger heavy precipitation. The presence of upper-level anticyclones during this time period further suppresses vertical motion, reinforcing atmospheric stability and preventing convective initiation. Overall, the analysis highlights that an intermediate soil moisture range of approximately 0.25–0.35 m³/m³ maximizes land–atmosphere coupling strength in the IGP during extreme heat events. Within this range, the surface is sufficiently moist to sustain strong evapotranspiration yet dry enough to produce high surface temperatures, creating a feedback loop that exacerbates heat stress. These findings underscore the importance of accurately representing soil moisture dynamics in regional climate models to improve predictions of heat extremes in South Asia.