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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Quantification of moisture sources for an event of heavy precipitation in the western Mediterranean

Heini Wernli (University of Mainz); Andreas Winschall (University of Mainz, Germany), Harald Sodemann (NILU, Norway), Stephan Pfahl (University of Mainz, Germany)

Research towards a better understanding of the processes responsible for heavy precipitation events in the Mediterranean are at the heart of HYMEX activities. It is a key issue to better understand the pathways of atmospheric water vapour associated with such events: Where did the water evaporate that contributed to intense precipitation? On what time scales and over what distances is this water transported to the precipitation region? And how large is the case-to-case variability of these characteristics of the atmospheric water cycle?
In this study, a mesoscale model with a recently developed tagging technique is used to address the first two of these questions for the November 2002 heavy precipitation event in the Piedmont region. This tagging technique allows to quantitatively assess the relative importance of various moisture sources for the intense precipitation. Humidity evaporating from the North Atlantic, the Mediterranean and from land surfaces can be traced; but also, using a more refined tracer initialization, the importance of evaporation from the nearby Gulf of Lion can be quantified.
The results from 11-day hindcast experiments (using ECMWF analyses as initial and boundary conditions) demonstrate that the heavy precipitation event is fed by moisture from the Western Mediterranean (as expected), but also from the North Atlantic (which constitutes the largest moisture source), European land surfaces and inflow across northern Africa. Consideration is also given to the numerical consistency of the tagging approach and to the generality of the results from this first tagging case study of a Mediterranean heavy precipitation event.