Diagnostic classification of flash drought events reveals distinct classes of forcings and impacts

Authors: OSMAN, M. - Johns Hopkins University; ZAITCHIK, B.F. - Johns Hopkins University; BADR, H. - Johns Hopkins University; OTKIN, J. - University Of Wisconsin; ZHONG, Y. - University Of Wisconsin; LORENZ, D. - University Of Wisconsin; Anderson, Martha; KEENAN, T. - University Of California; MILLER, D. - University Of California; HAIN, C. - Nasa Marshall Space Flight Center; HOLMES, T. - National Aeronautics And Space Administration (NASA) - Johnson Space Center

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2021
Publication Date: 2/1/2022
Citation: Osman, M., Zaitchik, B., Badr, H., Otkin, J., Zhong, Y., Lorenz, D., Anderson, M.C., Keenan, T., Miller, D., Hain, C., Holmes, T. 2022. Diagnostic classification of flash drought events reveals distinct classes of forcings and impacts. Journal of Hydrometeorology. 23(2):275–289. https://doi.org/10.1175/JHM-D-21-0134.1.
DOI: https://doi.org/10.1175/JHM-D-21-0134.1

Interpretive Summary: Rapid onset drought events, also known as “flash droughts”, can cause significant damage to crop yields and often do not allow time for effective mitigation once they have begun. Therefore, it is important to be able to improve our understanding of these events – what causes them, and under what conditions they are most likely to occur. This study creates an inventory of flash droughts that have occurred in the contiguous United States during the period 1979-2018 and attempts to classify these events into three general categories based on meteorological and land-surface conditions leading up to the drought onset. The resulting categories include “primed” droughts, where conditions were already dry and susceptible to rapid degradation; “hot and evaporative” droughts, where high temperatures and dry air conditions quickly bake the moisture out of the soil; and “stealth” events, where conditions leading up to the drought were not very unusual. The geography and seasonal timing of these different classes of flash drought are also explored. A better understanding of these flash drought classes will improve our ability to forecast them and more effectively respond to them.

Technical Abstract: In recent years there has been growing appreciation that rapidly intensifying “flash droughts” are significant climate hazards with major economic and ecological impacts. This has motivated efforts to inventory, monitor, and forecast flash drought events. Here we address the question of whether the term “flash drought” comprises multiple distinct classes of event, which would imply that understanding and forecasting flash droughts might require more than one framework. To do this, we first extend and evaluate a soil moisture volatility-based flash drought definition that we introduced in previous work and use it to inventory the onset dates and severity of flash droughts across the Contiguous United States (CONUS) for the period 1979-2018. Using this inventory, we examine meteorological and land surface conditions associated with flash drought onset and recovery. These same meteorological and land surface conditions are then used to classify the flash droughts based on precursor conditions that may represent predictable drivers of the event. We find that distinct classes of flash drought can be diagnosed in the event inventory. Specifically, we describe three classes of flash drought: “primed” events for which antecedent soil moisture anomalies are already strongly negative, “hot and evaporative” events in which pre-drought temperatures are very high and evaporation is enhanced, and a third class that we refer to as “stealth” flash droughts, in that precursor anomalies in soil moisture and in meteorological conditions other than precipitation are modest relative to the other two classes. The three classes have characteristic geographic preferences and occur with different seasonal timings. We conclude that soil moisture “flash droughts” are indeed a composite of distinct types of rapidly intensifying droughts, and that flash drought analyses and forecasts would benefit from approaches that recognize the existence of multiple phenomenological pathways.