No abstract

A severe case of Saharan dust transport and deposition with rain over Italy and southern Europe has been investigated by combining all available data including meteorological information, analyses of air sampling filters and sampled dust with rain, and satellite information on cloud cover. This typical straight transport case is characterized by a trough oriented south‐north, a cold front on the mobilization area, and a warm advection from the south. Meteorological analysis included relative topography, maps of pseudo‐potential temperatures, and barotopographies of ϑp = 45°C. The conditions of dust mobilization have been investigated and strong updrafts and thermals, with and without concomitant convective clouds, proved to be responsible for it. The size distribution of particles in rainfall residue has shown two modes, with diameters around 0.2 and 1 μm. X ray diffractometric analysis and SEM pictures have shown that the two modes are also different in composition, the smaller particles being composed of clay platelets and the larger of quartz grains. The mass concentration of the dust layer has been evaluated in 154 μg/m3. Wet removal through in‐cloud scavenging processes was the effective mechanism of deposition of the dust, as muddy rain has been observed at the ground long before the dust was observed in air sampling filters at mountain stations.

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