Tracking Flash Drought with Land-Atmospheric Feedbacks

Deficits in precipitation are the primary cause of drought development at monthly and longer time scales, but other climatic factors such as temperature, solar radiation, wind, and humidity play a central role in rapid onset droughts, recently termed “flash drought.” The evolution of flash drought often begins with ample soil moisture during the spring and early summer, leading to high, energy-limited actual evapotranspiration (Ea) rates. Although precipitation may remain near normal, this situation nonetheless leads to rapid decreases in soil moisture, forcing the land-surface energy balance from energy-limited to water-limited conditions. During such transitions, feedbacks between the land and the near-surface boundary layer accelerate and intensify the evaporative demand (Eo) though increases in sensible heat, and in the buoyancy, turbulence, and vapor pressure deficit of the over-passing air. These complex feedbacks and the resulting evolution of a flash drought are not fully captured with precipitation and temperature-based indicators alone. This observation motivates the development of a physically metric that is sensitive to changes in solar radiation, temperature, humidity, and wind through the use of the ASCE Standardized Penman-Monteith equation to quantify weekly Eo anomalies, termed the Evaporative Demand Drought Index (EDDI). Consistent with the complementary relationship between regional Ea and Eo, we derive weekly Eo anomalies that characterize drought conditions as anomalously high Eo, and wet conditions as anomalously low Eo. Daily Eo is computed from the North American Land Data Assimilation System Phase 2 (NLDAS-2) 1/8th-degree grid over the CONUS from 1979-2012. Comparisons of the EDDI to the United States Drought Monitor (USDM) over the Midwestern U.S. for 2011 and 2012 indicate that weekly Eo anomalies presage rapid onset droughts weeks before they appear in the USDM. This capability suggests that the EDDI could prove to be a very useful, and easy-to-implement, operational early warning drought monitoring tool.