Atmospheric preconditions investigation of wet-cold compound events in Greece between 1980 and 2004

Abstract This study investigates the origins and the atmospheric circulation patterns that led to the occurrence of Wet-Cold compound events (WCCEs) recorded by the Hellenic National Meteorological Service (HNMS) between 1980 and 2004. The study employed two methods to provide insights into the issue. The first method involved identifying clusters from backward trajectories (BTs) at three heights above the arrival point (500, 1500 and 5500 metres). The second method aimed at detecting weather patterns (WPs) that result from the clustering of atmospheric pressure at sea level (SLP) and the geopotential height (GH500) at 500hPa from ERA5. To detect clusters for both methods, k-means clustering is applied. The analysis of backward trajectories produced seven clusters of BTs at each height level. Most of these clusters originated from the Northwest or North, with medium, short, and long-distance clusters being observed at 500m, 1500m and 5500m height levels, respectively. Despite analysing the temporal data, no clear connection was established between the clusters and months. From the cluster of GH500 and SLP variables for 94 different dates, we derived three main WPs. All WPs showed high GH500 values in Western Europe and lower GH500 values in Eastern Europe. Although SLP values differed significantly among the clusters, they helped to identify distinct weather patterns. Finally, we identified that on 7 out of the total of 94 different dates with WCCEs, large amounts of moisture are transported to Greece through the atmosphere over long distances by atmospheric rivers (ARs).