Water Vapor Sources of the October 2000 Piedmont Flood

摘要

Very intense mesoscale or synoptic-scale rainfall events can occasionally be observed in the Mediterranean region without any deep cyclone developing over the areas affected by precipitation. In these perplexing cases the synoptic situation can superficially look similar to cases in which very little precipitation occurs. An example is the major precipitation and flooding event that affected Piedmont, Italy, between 13 and 16 October 2000. The emphasis of this study is on the moisture origin and transport. Moisture balances are computed on different space and time scales, revealing that precipitation exceeds evaporation over an area inclusive of Piedmont and the northwestern Mediterranean region, on a time scale encompassing the event and about 2 weeks preceding it. This is suggestive of an important moisture contribution originating from outside the region. A synoptic and dynamic analysis is then performed to outline the potential mechanisms that could have contributed to the large-scale moisture transport. The central part of the work uses a quasi-isentropic water vapor back-trajectory technique. The moisture sources obtained by this technique are compared with the results of the balances and with the synoptic situation to unveil possible dynamic mechanisms and physical processes involved. It is found that moisture sources on a variety of atmospheric scales contribute to this event. First, an important contribution is caused by the extratropical remnants of former Tropical Storm Leslie. The large-scale environment related to this system allows a significant amount of moisture to be carried toward Europe. This happens on a time scale of about 5–15 days preceding the Piedmont event. Second, water vapor intrusions from the African intertropical convergence zone and evaporation from the eastern Atlantic contribute on the 2–5-day time scale. The large-scale moist dynamics appears therefore to be one important factor enabling a moderate Mediterranean cyclone to produce heavy precipitation. Finally, local evaporation from the Mediterranean, water vapor recycling, and orographically induced low-level convergence enhance and concentrate the moisture over the area where heavy precipitation occurs. This happens on a 12–72-h time scale.