A study on cut-off low vertical structure and precipitation in the Mediterranean region

Cut-off lows are common features of Mediterranean meteorology in warm months and are often related to severe weather. The present work introduces a classification of cut-off episodes, based on the vertical extension of the depression and the presence of a linked surface vortex, also analyzing precipitation patterns. Ten years of warm-season ERA-40 reanalysis, available every six hours on a 2.5 degrees x 2.5 degrees grid, are processed to extract a database of cut-off lows and surface cyclones, along with the related total and convective precipitation at the ground. The high temporal resolution of the dataset permits a detailed characterization of short lasting events, so far poorly analyzed. The results show the relative abundance (41% of the total) of cut-off events lasting less than 24 hrs, sharing most of the characteristics of longer living cut-off in terms of structures and precipitation pattern. A large part of the 273 events identified in our database, about 54%, appear as high level signatures of depressions extending through a large portion of the troposphere, and in 38% of cases a well defined cyclonic structure is found at the ground. Most of these events carry precipitation, with relatively high rain-rates over wide areas, with well developed frontal rain bands. Among the cut-off events without a deep vertical structure (46%), about half do not produce precipitation, while the others produce relatively high rain-rates, although confined to small areas, indicating the presence of convective systems developing beneath the cut-offlow system. Such precipitation patterns are also confirmed at smaller scales by cloud resolving model runs. Finally, cut-off lows characterized by relatively high potential vorticity values in the mid-upper troposphere seem to have the potential for precipitation.