Quantifying Flash Droughts Over China From 1980 to 2017

Abstract

Drought can develop rapidly over a short period, the so‐called “flash drought”. The widely used standardized precipitation evapotranspiration index (SPEI), with traditional time scales longer than 1 month, cannot easily capture flash drought signals. Here, the SPEI with a 5‐day (pentad) time scale was proposed to investigate flash droughts in China from 1980 to 2017. New criteria for flash droughts based on the high‐resolution index were also developed. Flash droughts were stronger and longer in western and northern China. The intensity and duration of mild, moderate, and severe flash droughts increased during the study period, while severe flash droughts showed the largest increase. The high‐resolution SPEI permits us to examine the process of a drought event, that is, how a mild flash drought develops into a severe flash drought. The contribution of precipitation deficit to the duration of mild flash droughts was 50%. As the intensity of flash droughts increased, the contribution of high temperature and increased net radiation to flash drought duration increased from 50% (mild flash droughts) to 74% (severe flash droughts). When flash droughts occurred, Northwestern China and Southwestern China were dominated by unusually weak water vapor transport, while Northeastern China, Northern China, Southern China ,and Middle‐Lower Yangtze were mainly dominated by abnormal downward motion. The downward motion was generally accompanied by clear sky conditions with fewer clouds, higher air temperature, and higher surface solar radiation, which accelerated and amplified flash droughts triggered by initial precipitation deficits.

Contributions of Local Land‒Atmosphere Coupling and Mesoscale Atmospheric Circulation to the 2013 Extreme Flash Drought and Heatwave Compound Event Over Southwest China

“…The precipitation deficit gradually intensified from south to north and from east to west, with the most notable dry condition identified in SWC, where flash droughts could be triggered due to the lack of precipitation. As a result of soil water content depletion, evaporation decreased and temperatures rose, which could potentially lead to heat waves (Fu & Wang, 2022; Miralles et al., 2014). Figure 2b presents the daily soil moisture content and the maximum temperature conditions (black dots) in the normal state of the 1980–2020 summer in SWC (the green box in Figure 2a), while the red dot represents the 9‐day average soil moisture and maximum temperature conditions during the 2013 FDHW event.…”

“…In recent years, researchers have used the combination term “maximum air temperature‐evapotranspiration‐soil moisture” to evaluate the compound events of flash droughts and heat waves (Li et al., 2020; Mo & Lettenmaier, 2015; Ran et al., 2020; H. Zhang et al., 2019). Nevertheless, soil moisture observations have limited area coverage, and both satellite and model products have large uncertainties (Fu & Wang, 2022). Therefore, in this study, we utilized the pentad‐SPEI combined with the maximum air temperature percentile to define FDHW events.…”

“…Zhang et al, 2019). Nevertheless, soil moisture observations have limited area coverage, and both satellite and model products have large uncertainties (Fu & Wang, 2022). Therefore, in this study, we utilized the pentad-SPEI combined with the maximum air temperature percentile to define FDHW events.…”

“…We have presented the occurrence of a flash drought event according to the definition by Fu and Wang (2022), as follows: (a) during the 4-9 months (the growing season), the pentad-standardized precipitation evapotranspiration index (SPEI) decreased from the 40th percentile or above to the 20th percentile or below; (b) the flash drought reached its maximum intensity (minimum pentad-SPEI percentile) without allowing for more than one pentad with a decline rate (difference in pentad-SPEI percentile between two neighboring pentads) of less than 5th; (c) the average pentad-SPEI percentile was no more than 30th; (d) if the declined pentad-SPEI rises back up to the 20th percentile, the drought terminates, and the flash drought lasts for at least three pentads (Figure 1c). In this study, we used the pentad-SPEI from 1980 to 2020 to identify flash droughts in SWC.…”

“…In the past 41 years, the frequency, duration, and intensity of heat waves in SWC have shown a generally linear increasing trend (Y. Zhang et al., 2020). Fu and Wang (2022) found that the intensity and duration of flash droughts have increased in SWC. Additionally, anthropogenic warming in the coming decades may exacerbate FDHW events, especially in humid subtropical regions (Wang et al., 2016), which would place SWC at risk of experiencing an FDHW event.…”