Attribution of precipitation to cyclones and fronts over Europe in a kilometer-scale regional climate simulation

Abstract

This study presents a detailed analysis of the climatological distribution of precipitation in relation to cyclones and fronts over Europe for the 9-year period 2000-2008. The analysis uses hourly output of a COSMO (Consortium for Small-scale Modeling) model simulation with 2.2km grid spacing and resolved deep convection. Cyclones and fronts are identified as two-dimensional features in 850hPa geopotential, equivalent potential temperature, and wind fields and subsequently tracked over time based on feature overlap and size. Thermal heat lows and local thermal fronts are removed based on track properties. This dataset then serves to define seven mutually exclusive precipitation components: cyclonic (near cyclone center), cold-frontal, warm-frontal, collocated (e.g., occlusion area), far-frontal, high-pressure (e.g., summer convection), and residual. The approach is illustrated with two case studies with contrasting precipitation characteristics. The climatological analysis for the 9-year period shows that frontal precipitation peaks in winter and fall over the eastern North Atlantic and the Alps (>70% in winter), where cold frontal precipitation is also crucial year-round; cyclonic precipitation is largest over the North Atlantic (especially in summer with >40%) and in the northern Mediterranean (widespread >40%); high-pressure precipitation occurs almost exclusively over land and primarily in summer (widespread 30%-60%, locally >80%); and the residual contributions uniformly amount to about 20% in all seasons. Considering heavy precipitation events (defined based on the local 99.9th all-hour percentile) reveals that high-pressure precipitation dominates in summer over the continent (50%-70%, locally >80%); cold fronts produce much more heavy precipitation than warm fronts; and cyclones contribute substantially (50%-70%), especially in the Mediterranean in fall through spring and in northern Europe in summer.