Wet-bulb temperature extremes locally amplified by wet soils

Wet-bulb temperature extremes (WTEs) occur due to a combination of high humidity and temperature, and are hazardous to human health. Alongside favourable large-scale conditions, surface fluxes play an important role in WTEs; yet, little is known about how land surface heterogeneity influences them. Using a 10-year, pan-African convection-permitting model simulation, we find that most WTEs have spatial extents less than 2,000 km2. They occur preferentially over positive soil moisture anomalies (SMA) typically following rainfall. The wet-bulb temperature is locally amplified by 0.5–0.6 degrees C in events associated with smaller-scale SMA (50 km across) compared to events with larger-scale SMA (300 km across). A mesoscale cifrculation, resulting from stronger spatial contrasts of sensible heat flux, more efficiently concentrates moist, warm air in a shallower boundary layer. This mechanism could explain the underestimation of peak Twb values in coarser-resolution products. The role of antecedent SMA from recent rainfall may help issue localized early warnings.