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

Flash-droughts events are characterized by the fast depletion of soil moisture in the top soil layers. Given their rapid onset and intensification, flash droughts entail severe impacts on ecosystem productivity. Thus understanding their initialization mechanisms is important for improving the skill of drought forecasting systems. Here, we examined the role of antecedent meteorological conditions that lead to flash droughts across Europe over the last 70 years (1950--2020). We found two major flash-drought types based on sequence of development of antecedent hydro-meteorological conditions. The first one is characterized by a joint occurrence of two mechanisms, a continuous decline of precipitation in conjunction with an increase of the evaporative demand, both occurring before the onset of a flash drought event. The second type, on the contrary, is characterized by high precipitation preceding the start of the event followed by a sudden precipitation deficit combined with an increase in evaporative demand at the onset of the drought. Both drought types showed increased occurrence and higher spatial coverage over the last 70 years, the second drought type have increased at a much faster rate compared to the first one specifically over the Central Europe and the Mediterranean region. Overall our study highlights the differences between the two types of flash droughts, related to varying antecedent meteorological conditions, and their changes under recent climate warming.