Mesoscale soil moisture heterogeneity can locally amplify humid heat

TL;DR

This study uses high-resolution simulations to show that soil moisture heterogeneity on specific scales can locally intensify humid heat by up to 4°C, influenced by mesoscale circulation and background winds.

Contribution

It reveals the critical soil moisture length-scale (~50 km) that maximizes humid heat amplification and links soil moisture patterns to extreme heat prediction.

Findings

Humid heat can be amplified by 1-4°C due to soil moisture heterogeneity.

Maximum amplification occurs at a soil moisture length-scale of about 50 km.

Background wind and wet-dry contrast influence the relationship between soil moisture scale and heat amplification.

Abstract

Soil moisture is a key ingredient of humid heat through supplying moisture and modifying boundary layer properties. Soil moisture heterogeneity due to e.g., 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 ($\Delta x$=500 m), coupled land-atmosphere model wherein wet patches on length-scales $\lambda \in$ 25-150 km are prescribed. Compared to experiments with uniform soil moisture, humid heat is locally amplified by 1-4$^\circ$C, with maximum amplification for the critical soil moisture length-scale $\lambda_c=$ 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 $\lambda_c$ and the humid heat amplification. Based on observed soil moisture patterns, these results will help to predict extreme humid heat at city and county scales across the Tropics.