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 wet patches on length‐scales λ∈ 25–150 km are prescribed. Compared to experiments with uniform soil moisture, humid heat is locally amplified by 1–4◦C, with maximum amplification for the critical soil moisture length‐scale λc = 50 km. Subsidence associated with a soil moisture‐induced mesoscale circulation concentrates warm, humid air in a
shallower boundary layer. The background wind and the magnitude of the wet‐dry contrast control the relationship between λc and the humid heat amplification. Based on observed soil moisture patterns, these results will help to predict extreme humid heat on city and county scales across the Tropics.