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

Abstract

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

Contribution of large-scale midlatitude disturbances to hourly precipitation extremes in the United States

“…Likewise, hourly precipitation within 660-km of the centroid and within ± 1-day of the COL day was attributed to COL. This radius is consistent with previous studies pairing precipitation to midlatitude synoptic phenomenon (Porcù et al 2007;Abatzoglou 2016). The padding by 1-day addresses potential differences in the transit time of a COL or a jet stream event [e.g., Oakley and Redmond (2014)].…”

Toward an estimation of the relationship between cyclonic structures and damages at the ground in Europe

“…A lower amount of ERD events appears linked to CUT development although the small peak at around 750 km may be directly related to precipitation induced by the CUT which usually tends to occur close to CUT center (Porcù et al, 2007). Other ERDs cluster at distances larger than 2000 km from the CUT center: for these cases no direct link with the CUT can be supposed.…”

“…The detection of a cut-off low event (CUT) is carried out adopting a two-step strategy introduced in Porcù et al (2007) which is in turn a follow-up extension of an automatic objective detection algorithm (Nieto et al, 2005) based on the three main characteristics of the CUT conceptual model. It can be summarized according to subsequent stages, as follows:…”

“…The database obtained after the application of the automatic algorithm was manually post-processed, following Porcù et al (2007), according to a subjective analysis to overcome possible tracking problems due to the higher time resolution (6 h) used here, with respect to the daily data used in the original application of the automatic algorithm (Nieto et al, 2005).…”

<strong>Pseudo global warming simulations of the effects of climate change on extreme precipitation events in the Iberian Peninsula</strong>

“…Study [2] used 10 years of warm season ERA Reanalysis to study COLs in the Mediterranean region. They found that deep cut-off lows tend to produce the most intense and widespread rain episodes, mainly distributed to the south of the cut-off low center.…”