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Weather situation

The weather situation during the event of 9-13 August 2002 was characterized by a situation referred to as “Vb” in the van Bebber cyclone tracks classification. It consists of the formation of a low pressure area at ground level above the Gulf of Genoa, which then travels north-eastwards in direction of Eastern Europe. Since the air is advected in an anti-clockwise direction around such a system, warm and moist air is transported from the Mediterranean towards Central and Eastern Europe and can, in some cases, produce intense precipitation. Hence, Vb situations are sometimes responsible for the occurrence of large-scale summer floods in those regions.

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Swiss federal authoritiesSwiss federal authorities

In Switzerland, the precipitation event of 9-13 August 2002 began with a first phase of precipitation on 9 August. It was caused by the transport of moist air from the west towards Switzerland, in relation with a low pressure system centered over the British Isles and northern France.

On 10 August, a secondary low pressure area developed over the Gulf of Genoa at ground level. On its eastern side, warm and moist Mediterranean air was advected from the south towards northern Italy and Austria, while on its western side cold and moist air from the Altantic Ocean was advected from the north towards Western Europe. The confrontation of these two air masses led to the occurrence of precipitation along the confrontation zone. Switzerland, located below that zone, received precipitation both on the northern and southern side of the Alps, marking a second precipitation phase.

The following days, the low pressure area strengthened and moved slowly towards the north-east. On 11 August, its center was located over northern Italy, and moist Mediterranean air was advected around it in an anti-clockwise direction, impinging on the Swiss Alps from the north-east. This flow of moist air was forced to rise over the topography, leading to a third precipitation phase. In particular, the central and eastern parts of Switzerland received relatively large amounts of precipitation that day.

As the low-pressure area continued to move north-eastwards over Eastern Europe on 12 and 13 August, the moisture transport shifted towards Eastern Europe, generating intense rainfall, namely in Germany, Austria and the Czech Republic. Over Switzerland, on the other hand, precipitation receded and was mostly confined to the eastern part of the country.