The Impacts of Drought on Heatwave Duration, Intensity, and Exposure.

As the availability of moisture at the surface diminishes due to drought, air temperature conditions often increase as a greater share of surface energy goes into heating the atmosphere (sensible energy) than evaporating (latent heat) moisture. During compound events where droughts and heatwaves co-occur, the warming affects brought on by drought are thought to enhance the intensity of heatwaves and associated impacts with increased exposure to extreme heat. However, few studies have explored the impacts of soil moisture conditions on compound events. In this study, heatwave and drought events were identified at U.S. Climate Reference Network (USCRN) stations to compare differences in duration, intensity, and exposure of heatwaves that did and did not co-occur with drought. The USCRN is a sparely dense high quality climate network that monitors both surface and subsurface (i.e., soil moisture and temperature) conditions at an hourly frequency.

Overall, compound events were on average 1.1-days longer than heatwaves with 18% of stations exceeding 2-days. While there was little over all intensity differences, compound events had warmer air temperature anomalies than heatwaves from the late afternoon to early evening hours. The combination of warmer evenings and longer duration lead to an increase in heat exposure (temperatures above the 90^th percentile) with compound events having on average 16.9 ºC more extreme heat degree hours per event than heatwaves. Air temperature and heat index anomalies were found to be sensitivity to soil moisture conditions with drier soils associated with slight warmer day time anomalies and cooler mornings, which led to some rest bit during dry soil moisture heatwaves. For moist soil moisture conditions, air temperature and heat index anomalies remained 1 standardized departure above normal throughout the diurnal cycle, which may be more problematic to human health exposure.