Diagnostic Classification of Flash Drought Events Reveals Distinct Classes of Forcings and Impacts

Abstract

Recent years have seen 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 consider 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: “dry and demanding” events for which antecedent evaporative demand is high and soil moisture is low, “evaporative” events with more modest antecedent evaporative demand and soil moisture anomalies, but positive antecedent evaporative anomalies, and “stealth” flash droughts, which are different from the other two classes in that precursor meteorological anomalies are modest relative to the other classes. The three classes exhibit somewhat different geographic and seasonal distributions. 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.

Trends, Variability, and Drivers of Flash Droughts in the Contiguous United States

“…For most CONUS areas (Figure 3), EDFD has a low TPR prior to SMPD onset which supports the findings by Osman et al. (2022) showing that one pathway for SMPD development has no ED early warning signal and is referred to as a low severity “stealth” FD which is forced by precipitation deficits, whereas the other two potential pathways do contain excessive ED prior to drought onset. ED has potential for overestimation on the impact of crop yields because RZSM is not explicitly accounted for (Rigden et al., 2020), but research has identified that RZSM and ED co‐regulate stomatal conductance and plant stress is induced under high ED conditions despite having concurrent, high RZSM conditions (F. Zhang et al., 2021; J. Zhang et al., 2021).…”

“…The Evaporative Demand Drought Index (EDDI) identifies anomalous ED leading to surface moisture limitations and this index found similar interannual drought patterns as the United States Drought Monitor (USDM), evaporative stress index (ESI), and standardized precipitation index (SPI), and EDDI was able to identify drought development up to 2 months prior to the USDM (Hobbins et al., 2016; McEvoy et al., 2016). Research has identified three potential pathways for FD development which describe antecedent environmental conditions prior to FD onset and these pathways are dry & demanding, evaporative, or stealth (Osman et al., 2022). Both dry and demanding and evaporative FD pathways contain elevated ED prior to drought development, suggesting the potential use of EDDI as an early warning of FD events.…”

“…The canonical 2012 United States Midwest FD went from moderate to extreme drought conditions across nearly half of the country within 3 months and cost agricultural sectors nearly $34 billion in losses and over 120 lives were lost in the associated summer heatwave (NCEI, 2020). Potential FD drivers include high temperature and cloud‐limited skies, low precipitation, high wind speed, high vapor pressure deficit, low humidity, or associated land‐atmosphere feedbacks which increase drought severity, varied by geographic location and season (Anderson et al., 2013; Christian et al., 2021; Ford & Labosier, 2017; Hobbins et al., 2016; Osman et al., 2022; Otkin et al., 2014; Pendergrass et al., 2020). These rapid developing conditions induce water and heat stress, which inhibit plant productivity, undermine natural ecosystems, deplete water resources, increase wildfire risks, and decrease agricultural productivity (Banerjee et al., 2013; Gerken et al., 2018; McEvoy et al., 2020; NCEI, 2020; Reidmiller & Al., 2018).…”

Cascading Drought‐Heat Dynamics During the 2021 Southwest United States Heatwave

“…But overall, we find that soil moisture conditions converge rather than diverge in the period leading up to and through the heat maximum (Table 1)—perhaps because the drought had already driven moisture conditions so low—such that there is no clear indication that this increase in evaporative demand triggered a cascading drought‐heat‐drought feedback. We note that this result might not hold when severe heatwaves hit more humid regions or when there is not already a severe drought; indeed, some types of rapid‐onset “flash droughts” are understood to be triggered by elevated air temperatures (Osman et al., 2022).…”

On the role of antecedent meteorological conditions on flash drought initialization in Europe

“…Apart from the conditions occurring during the event, examinations of antecedent meteorological conditions leading to flash drought onset have received less attention (e.g. Ford and Labosier 2017, Osman et al 2022). This knowledge gap, combined with climate warming-which intensifies the flash drought occurrence (Wang et al 2016, Yuan et al 2019, Shah et al 2022increases the potential of future impacts.…”