Lee cyclones that detach from the lee of the Alps, gain in strength, but remain stationary for a longer time over the Mediterranean Sea (e.g., Gulf of Genoa), are feared for the large amount of precipitation they can discharge over land. Larger systems transfer moist air from the sea to the Apennines and the Alps, where the air rises and water is precipitated. Moisture transport is most intense in the late autumn when the ocean temperature is still high. Later in the year, low-pressure systems over the Mediterranean Sea cause heavy snowfall to occur in the southern part of the Alps.
Figure 1: Aqua Alta in Venice (Campo Erberia) on November 14, 2019. © jkb
In the summer and autumn seasons, cyclones over the Mediterranean Sea are often accompanied by severe thunderstorms. Because of the abundant supply of moisture from the relatively warm sea, these convective cells produce a high amount of precipitation when stationary. Mountain ranges like the Alps or the Apennines have an impact on the local distribution of convection and hence precipitation is unevenly distributed.
The loop below shows emerging convective cells over Italy and the density of cloud water droplets measured from ground-based radar stations.
Figure 2: IR10.8 loop (from July 3, 2020, 12:00 UTC to July 4 2020, 06:00 UTC) and radar reflectivity [dBZ].
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