Mahmoud Soliman Osman

The importance of intensifying feedbacks between drought and heat, however, depends on context and can be difficult to quantify (Miralles et al., 2019). Hypothesized drought-heat interactions include (a) surface energy partitioning effects, in which drought leads to enhanced sensible heat flux relative to latent heat flux, convectively warming the planetary boundary layer, (b) surface net radiation effects, in which drought alters incoming solar radiation or surface albedo; thus, changing the amount of energy that needs to be dissipated from the surface, (c) precipitation-mediated feedbacks related to planetary boundary layer processes or convective dynamics, and (d) broader impacts on atmospheric circulations (Seneviratne et al., 2010). Determining whether any of these processes were active in the record-setting events of June 2021 has implications on how we interpret projections of future climate extremes by global climate models (GCMs) that may or may not include such cascading dynamics. Given the growing evidence for cascading dry-hot hazard dynamics in theory and global analysis, the Southwest United States drought and heat extreme of 2021 offers an important opportunity to probe for the presence of hypothesized feedbacks. We do this through controlled numerical experiments with the Weather Research and Forecasting (WRF) model (Skamarock et al., 2021) applied at convection resolving scales, allowing us to consider how dynamics often described in GCMs and global scale analysis played out at local to regional scale during a sentinel event.

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.

The term "flash drought" is frequently invoked to describe droughts that develop rapidly over a relatively short timescale. Despite extensive and growing research on flash drought processes, predictability, and trends, there is still no standard quantitative definition that encompasses all flash drought characteristics and pathways. Instead, diverse definitions have been proposed, supporting wide-ranging studies of flash drought but creating the potential for confusion as to what the term means and how to characterize it. Use of different definitions might also lead to different conclusions regarding flash drought frequency, predictability, and trends under climate change. In this study, we compared five previously published definitions, a newly proposed definition, and an operational satellite-based drought monitoring product to clarify conceptual differences and to investigate the sensitivity of flash drought inventories and trends to the choice of definition. Our analyses indicate that the newly introduced Soil Moisture Volatility Index definition effectively captures flash drought onset in both humid and semi-arid regions. Analyses also showed that estimates of flash drought frequency , spatial distribution, and seasonality vary across the contiguous United States depending upon which definition is used. Definitions differ in their representation of some of the largest and most widely studied flash droughts of recent years. Trend analysis indicates that definitions that include air temperature show significant increases in flash droughts over the past 40 years, but few trends are evident for definitions based on other surface conditions or fluxes. These results indicate that "flash drought" is a composite term that includes several types of events and that clarity in definition is critical when monitoring, forecasting, or projecting the drought phenomenon.