A. Jansà

Western Mediterranean cyclones and heavy rain. Part 2: Statistical approach

This is the second part (statistical approach) of a work concerning the relationship between heavy rain and cyclonic centres in the western Mediterranean. Using a statistical approach we seek to verify the indirect role of the cyclone centres in locating, triggering or focusing heavy rain: a cyclonic centre –even if neither strong nor deep –may contribute to the low-level flow organisation and so to the creation or intensification of a low-level warm and wet current that can feed and sustain convective rain in favourable environmental conditions. The foundation for the statistical approach is several Several databases maintained and updated by the Spanish Institute of Meteorology (INM). These databases of cyclonic centres, heavy rain and strong wind events, and mesoscale convective systems (MCSs), all covering the western Mediterranean or part thereof, can be cross-referenced by looking for simultaneity between heavy rain events (or MCSs) and cyclone centres in the vicinity. We have found that in most of the heavy rain events (around 90%) there is a cyclonic centre in the vicinity, usually located so that its presence favours the creation or intensification of a feeding flow of Mediterranean air towards the area affected by heavy rain. The same occurs with the MCSs. Copyright

A catalogue and a classification of surface cyclones for the Western Mediterranean

The Mediterranean Sea is a region that possesses a high frequency of cyclones. Many of them are lee depressions and so, mesoscale and weak, but others are strong and cover a wide area. In this study some characteristics of surface cyclones in the Western Mediterranean are presented. First a database was built from hand analyses of sea-level pressure from 1992 to 1995. Next, a similar database was obtained from the LAM-INM (Limited Area Model of the Instituto Nacional de Meteorologia of Spain) objective analyses of the 1000 hPa geopotential field. Results show an important increase in the number of cyclones when a mesoscale analysis is carried out. Most of the Western Mediterranean cyclones are mesoscale and weak, and they are not uniformly distributed in space and in time. On the one hand, there are some areas with a high concentration of cyclones. The location of some of them, close to the main mountain ranges, suggests their possible orographic origin. On the other hand, in some areas the cyclones are present during the whole year, but in other areas are seasonally distributed. Finally, a study of typologies of the cyclones was conducted by using the cluster analysis technique. The classification was performed from the intensity of the cyclones and the shape of the sea-level pressure around the centre. Results are similar from both databases and so the existence of seven typologies have been identified. Copyright

Western Mediterranean cyclones and heavy rain. Part 1: Numerical experiment concerning the Piedmont flood case

Heavy rain events constitute a major operational problem in the Mediterranean area. The present work tries to contribute to a better understanding of this problem by considering the possible indirect influence of surface depressions in the triggering and location of the heavy rain. The idea is that any surface low contributes to the organisation of the low-level flow. This then affects the creation/intensification of a low-level feeding current of warm and wet air that can sustain heavy rain if there is convective instability. Two approaches have been used: (a) a numerical and synoptic approach, Part 1, and (b) a statistical approach, Part 2. In Part 1, the differences between a control run and a numerical experiment without the Iberian mountains for the ‘MAP Piedmont flood event’ are discussed. Among other effects, the Iberian orography seems to intensify a secondary low near Liguria as well as increase the mixing ratio convergence and the rainfall intensity near it. A few other cases of heavy rain are also briefly reviewed in order to identify the presence of a Mediterranean depression in the vicinity.