Soil Moisture Drying Rate Regulated by Land-Atmosphere Coupling and its Implications in Flash Drought

Flash droughts are characterized by the sudden onset and rapid intensification of drought conditions, and have drawn increasing attention due to their substantial agricultural and economic consequences over the past decade, particularly in the Central United States. Climate projections suggest an increasing risk of flash droughts, which poses a significant challenge for agriculture in this region. However, persistent summertime dry and warm biases over the Central US have been an issue in many generations of global and regional climate models. The projected changes in flash droughts based on the multimodel ensemble are significantly affected by these biases. In this study, we evaluate biases in temperature, precipitation, evapotranspiration (ET), soil moisture, and land-atmosphere coupling strength from multiple CMIP6 models. We quantify the soil moisture drying rate using observational datasets and different climate models, and investigate related land-surface and atmospheric conditions during the drying periods. Results show that the biases in soil moisture drying rate are closely associated with land-atmosphere coupling strength, which affects the evaporative cooling effect during the drying period, and consequently leads to temperature biases in some climate models. There is a clear relationship between the estimated frequency of flash droughts and the average drying rate among the climate models. We further investigate uncertainties in flash drought measurements, contributing to a robust assessment of future drought risk in the Central United States.