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

Abstract

Abstract. This study presents a detailed analysis of the climatological distribution of precipitation in relation to cyclones and fronts over Europe for the nine-year period 2000–2008. The analysis uses hourly output of a COSMO (Consortium for Small-scale Modeling) model simulation with 2.2 km grid spacing and resolved deep convection. Cyclones and fronts are identified as two-dimensional features in 850 hPa 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 data set then serves to define seven mutually exclusive precipitation components: high-pressure (e.g., summer convection), cyclonic (near cyclone center), cold-frontal, warm-frontal, collocated (e.g., occlusion area), far-frontal, and residual. The approach is illustrated with two case studies with contrasting precipitation characteristics. The climatological analysis for the nine-year period shows that frontal precipitation peaks in fall and winter over the eastern North Atlantic, with cold frontal precipitation also being crucial year-round near the Alps; cyclonic precipitation is largest over the North Atlantic (especially in summer) and in the northern Mediterranean (except in summer); high-pressure precipitation occurs almost exclusively over land and primarily in summer; and the residual contributions uniformly amount to about 20 % in all seasons. Considering heavy precipitation events (defined based on the local 99.9th percentile) reveals that high-pressure precipitation dominates in summer over the continent; cold fronts produce much more heavy precipitation than warm fronts; and cyclones contribute substantially, especially in the Mediterranean in fall through spring and in Northern Europe in summer.

Empirical estimate of forestation-induced precipitation changes in Europe

“…Especially forests close to the coast might therefore hinder the propagation of frontal systems into the interior of the continent. As cyclones contribute more to the total precipitation during winter than during summer 45,46 , this can explain why forestation leads to a downwind increase of precipitation in areas close to the coast during winter, while the signal is neutral in Central and Western Europe and even negative in Northern Europe. Indeed, forestation is estimated to decrease the propagation speed of the calculated wind trajectories further away from the coasts, leading to a reduction of the rainfall amount (Supplement B).…”

The life cycle of upper-level troughs and ridges: a novel detection method, climatologies and Lagrangian characteristics

“…Further details can be found in Sect. 2.2 of Rüdisühli (2018). We use the tracking to identify young troughs or ridges, but it could also be used, for example, to study the change in the orientation of the troughs and ridges during their life cycle.…”

“…At the equatorial tip of a trough, where the geopotential isolines are closely aligned, a jet streak can form, and the conservation of the absolute vorticity and a region of diffluent flow predict forced upward motion in the jet exit region. It is therefore not too surprising that surface cyclogenesis (Petterssen and Smebye, 1971;Sanders, 1988; S. Schemm et al: Life cycle of upper-level troughs and ridges Lackmann et al, 1997;Graf et al, 2017), rapid cyclone intensification (Sanders and Gyakum, 1980;Wash et al, 1988;Uccellini, 1990;Wernli et al, 2002;Gray and Dacre, 2006) and enhanced precipitation (Martius et al, 2006;Massacand et al, 2001;Martius et al, 2013) usually take place in the region ahead of the upper-level trough axis.…”

Convection‐permitting modeling with regional climate models: Latest developments and next steps

“…This way, the size of the data to handle reduces dramatically from 3D fields (horizontal spatial dimension and time) to a few parameters of interest, without the loss of relevant information. As an interesting application of the tracking approach, Rüdisühli et al (2020) analyzed the climatological distribution of precipitation in Europe in relation to cyclones and fronts in a 9‐year European CPRCM simulation. Finally, a classification algorithm has been proposed to identify days with an elevated potential for extreme precipitation, or other variables for which added value is expected, in order to perform CPRCM simulations only when they are deemed the most useful, reducing the computational load (Beaulant et al, 2011; Meredith et al, 2018).…”