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 9-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 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.

Measurement report: Spatial variability of northern Iberian rainfall stable isotope values – investigating atmospheric controls on daily and monthly timescales

“…Therefore, as we follow the typical displacement of an Atlantic front on its way to the IP from west to east, we find progressively more negative winter δ 18 O p values (see values for January-February-March in Table 2) going from El Pindal (−6.0 ‰), to Ortigosa de Cameros (−8.1 ‰) to Borrastre (−9.8 ‰) and, finally, to Molinos (−10.0 ‰). This pattern is not as evident in other seasons when the entrance of Atlantic fronts is not the dominant synoptic situation that generates rainfall in the transect (Rüdisühli et al, 2020). In addition to this continental effect, which accounts for only a small gradient in δ 18 O p (about 0.002 ‰ km −1 in Europe as described in Rozanski et al, 1993), the higher annual mean air temperature in both Mallorca and Barcelona as well as Oviedo and El Pindal compared to the other stations (Table 1), may help explain their similar δ 18 O p values.…”

“…In northwestern and north-central Iberia, precipitation is mainly controlled by the presence of westerly winds and the passage of Atlantic fronts, especially during November-April (Martín-Vide and Olcina Cantos, 2001;Rüdisühli et al, 2020). During the rest of the year, the subtropical Azores high-pressure system shifts northward, which blocks the westerly circulation and moisture inflow from Atlantic sources (Archer and Caldeira, 2008), thus favoring stable atmospheric conditions and reducing precipitation.…”

“…The role of geographic factors (continental and elevation effects) and atmospheric processes (moisture origin and type of rainfall) is evaluated. Additionally, this study will serve to improve the interpretation of oxygen isotope paleorecords from the northern IP that depend on δ 18 O p (Bartolomé et al, 2015;Domínguez-Villar et al, 2017;López-Blanco et al, 2016;Pérez-Mejías et al, 2019;Sancho et al, 2018Sancho et al, , 2015.…”

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).…”

Extreme wet seasons – their definition and relationship with synoptic-scale weather systems

“…For instance, the west coast of the US, the Iberian Peninsula and the north African coast, as well as Chile and eastern Australia, all experience primary extreme wet seasons in winter. This suggests that such regions are influenced by systems that made landfall, such as extratropical cyclones 76 E. Flaounas et al: Extreme wet seasons -their definition and relationship with synoptic-scale weather systems and atmospheric rivers (Rutllant and Fuenzalida, 1991;Leung and Qian, 2009;Lavender and Abbs, 2012;Flaounas et al, 2017). Other exceptions from the dominant summer occurrence of extreme wet seasons over land are several regions in the Northern Hemisphere where extreme seasons occur in spring, in contrast to summer for their neighboring continental areas.…”