A joint spatio-temporal characterization of the major meteorological droughts in Europe

Drought can be considered the most severe and the most complex weather-related natural hazard. With impacts that may extend to large areas and log time spans, and the capability to occur in all climatic zones, drought is the first hazard for the number of people affected. Due to the transboundary nature of drought events, an effective monitoring of their evolution must properly account for the full spatio-temporal structure. This characterization is a key step for a proper attribution of the related impacts. In addition, understanding common features in major droughts is of utmost importance for both monitoring and forecasting activities. In this work, we introduce a set of tools used to summarize the main properties of major droughts in Europe, with the goal of subdividing the events in groups characterized by similar properties. We used a European dataset of meteorological droughts (from 1981 to 2020) that detects events based on the Standardized Precipitation Index using an event-oriented spatio-temporal clustering algorithm. Spatio-temporal characteristics of major droughts were summarized using Normalized Area - Time Accumulation curves to follow their expansion/contraction as a function of time and analyzing the main direction of expansion of the events. A clustering algorithm was applied to classify events. We identified three groups: a first group comprised of warm-season events, characterized by a longer duration, a shorter early growing phase, and a longer exhaustion phase; a second group, less numerous, comprised by droughts occurring during the cold season, that tend to have a shorter duration, a longer early growing phase and a shorter exhaustion phase; and a third group comprised of droughts occurring across the two periods. This last class is characterized by a longer duration and a high variability in most of the other characteristics.