Mesoscale soil moisture heterogeneity can locally amplify humid heat

Soil moisture is a key ingredient of humid heat through supplying moisture and modifying boundary layer properties. Soil moisture heterogeneity due to for example, antecedent rainfall, can strongly influence weather patterns; yet, its effect on humid heat is poorly understood. Idealized numerical simulations are performed with a cloud-resolving (Δx = 500 m), coupled land-atmosphere model wherein circular wet patches with diameter λ ∈ 25-150 km are prescribed. Compared to experiments with uniform soil moisture, humid heat is locally amplified by 1 to 4°C in experiments with heterogeneous soil moisture, with maximum amplification for the critical soil moisture length-scale λ_c = 50 km. Subsidence associated with a soil moisture-induced mesoscale circulation concentrates warm, humid air in a shallower boundary layer. Additional pairs of uniform-heterogeneous soil moisture simulations are performed to assess the influence of the background wind, the strength of the soil moisture contrast, and the vertical structure of the atmosphere, on the relationship between soil moisture length-scales and humid heat amplification. This study provides process-based insights into the effects of soil moisture heterogeneity on humid heat in various environments at fine time and space scales, challenging extreme humid heat outputs from coarser-resolution weather and climate models. Furthermore, these results will help to predict extreme humid heat at city and county scales across the Tropics based on observed soil moisture patterns.