Satellite-based climatology of Mediterranean cloud systems and their association with large-scale circulation.

The variability of Mediterranean cloud systems is investigated using 8.5 years (from January 1987 to June 1995) of TIROS-N Operational Vertical Sounder (TOVS) observations acquired aboard the National Oceanic and Atmospheric Administration (NOAA) series of operational polar satellites. Cloud systems and troughs are automatically detected with the retrievals of the cloud top pressure (CTP) and the temperature of lower stratosphere (TLS). Observed cloud systems have a typical size of few hundred kilometres with a larger occurrence between March and October. A threefold cloud system typology reveals the presence of an upper-level anomaly for about 30% of the cloud systems in winter, 26% in spring and 7% in autumn (but 23% in October). During summer, in contrast, the forcing is very likely local, and according to the composite analysis, weakly related to upper-level anomaly. During the cold seasons (15 October to 15 April), more cloud systems are found during negative North Atlantic Oscillation (NAO) phase when the north Atlantic storm track takes its southernmost position. Consistently, more systems are observed during the Greenland Anticyclone and the Atlantic Ridge regimes, compared to the Zonal and Blocking regimes. Finally, severe precipitation events over the Alpine region are associated with a warm TLS anomaly upstream the cloud system, showing once more the impact of the upper levels on the weather over this area