The emergence of flash drought has attracted widespread attention due to its rapid onset. However, little is known about the recent evolution of flash droughts in terms of the speed of onset and the causes of such a rapid onset phase of flash droughts. Here, we present a comprehensive assessment of the onset development of flash droughts and the underlying mechanisms on a global scale. We find that 33.64-46.18% of flash droughts with 5-day onset of drying, and there is a significant increasing trend in the proportion of flash droughts with the 1-pentad onset time globally during the period 2000-2020. Flash droughts do not appear to be occurring more frequently in most global regions, just coming on faster. In addition, atmospheric aridity is likely to create a flash drought-prone environment, and the joint influence of soil moisture depletion and atmospheric aridity further accelerates the rapid onset of flash droughts. 

/pdf/accelerating-flash-droughts-induced-by-the-joint-influence-5u1y3dm8.pdf

Accelerating flash droughts induced by the joint influence of soil moisture depletion and atmospheric aridity

. 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.

/pdf/flash-drought-onset-over-the-contiguous-united-states-2q0yng85h1.pdf

Flash drought onset over the contiguous United States: sensitivity of inventories and trends to quantitative definitions

Flash droughts are characterized by an abrupt onset and swift intensification. Global surface soil moisture (θRS) from NASA's Soil Moisture Active Passive (SMAP) satellite can facilitate a near-rea...

Global Flash Drought Monitoring Using Surface Soil Moisture

Flash droughts are characterized by a period of rapid intensification over sub-seasonal time scales that culminates in the rapid emergence of new or worsening drought impacts. This study presents a new flash drought intensity index (FDII) that accounts for both the unusually rapid rate of drought intensification and its resultant severity. The FDII framework advances our ability to characterize flash drought because it provides a more complete measure of flash drought intensity than existing classification methods that only consider the rate of intensification. The FDII is computed using two terms measuring the maximum rate of intensification (FD_INT) and average drought severity (DRO_SEV). A climatological analysis using soil moisture data from the Noah land surface model from 1979–2017 revealed large regional and interannual variability in the spatial extent and intensity of soil moisture flash drought across the US. Overall, DRO_SEV is slightly larger over the western and central US where droughts tend to last longer and FD_INT is ~75% larger across the eastern US where soil moisture variability is greater. Comparison of the FD_INT and DRO_SEV terms showed that they are strongly correlated (r = 0.82 to 0.90) at regional scales, which indicates that the subsequent drought severity is closely related to the magnitude of the rapid intensification preceding it. Analysis of the 2012 US flash drought showed that the FDII depiction of severe drought conditions aligned more closely with regions containing poor crop conditions and large yield losses than that captured by the intensification rate component (FD_INT) alone.

Development of a Flash Drought Intensity Index

The topic of 'Flash Drought' is rapidly gaining attention within both the research and drought management communities. This literature review aims to synthesize the research to-date and provide a basis for future research on the topic. Specifically, our review is focused on documenting the range of definitions of 'flash drought' being proposed in the research community. We found that the term first appeared in the peer-reviewed literature in 2002, and by 2020 has become an area of active research. Within that 18-year span, 'flash drought' has been given 29 general descriptions, and 20 papers have provided measurable, defining criteria used to distinguish a flash drought from other drought. Of these papers, 11 distinguish flash drought as a rapid-onset drought event while eight distinguish flash drought as a short-term or short-lived, yet severe, drought event and one paper considers flash drought as both a short-lived and rapid onset event. Of the papers that define a flash drought by its rate of onset, the rate proposed ranges from 5 days to 8 weeks. Currently, there is not a universally accepted definition or criteria for 'flash drought', despite recent research that has called for the research community to adopt the principle of rapid-intensification of drought conditions.

/pdf/making-sense-of-flash-drought-definitions-indicators-and-479hnvwlib.pdf

Making sense of flash drought: definitions, indicators, and where we go from here

Flash droughts are extreme phenomena that have been identified using two different approaches. The first approach identifies these events based on unusually rapid intensification rates, whe...

/pdf/two-different-methods-for-flash-drought-identification-1bsx2uii58.pdf

Eric D. Hunt

Papers

Two Different Methods for Flash Drought Identification: Comparison of Their Strengths and Limitations

Development of a Flash Drought Intensity Index