Characterization of Extreme Wet-Bulb Temperature Events in Southern Pakistan

Abstract

We study extreme wet-bulb temperature (Tw) events in a regional hot spot of high Tw values in southwest Pakistan's Indus Valley. We focus on the May–June premonsoon months and employ a combination of station data, ERA-Interim reanalysis, and trajectory calculations to analyze the extreme events. We find that the events are associated with anomalous influx of marine air from the Arabian Sea. The air flows in a shallow layer below 950 hPa and is heated and moistened by diabatic processes over land, arriving at the hot spot region with very high Tw. We surmise that surface evaporation within the strip of well-irrigated land flanking the Indus River plays a major role in generating these extreme Tw values. Our results imply that studies of extreme Tw events in future, warmer climates must pay close attention to fine-scale diabatic processes within the boundary layer, especially evaporation over land.