How important is intensified evaporation for Mediterranean precipitation extremes?

Abstract

The moisture sources for heavy precipitation events in the northwestern Mediterranean are investigated with a Lagrangian moisture source diagnostic, which is based on the analysis of moisture changes during 6 h time intervals along backward trajectories, originating from the location of the heavy precipitation events. We consider the 50 strongest events in each season in the region 0-15°E, 42-46°N during the years 1989-2009. The results of the moisture source diagnostic indicate that during precipitation extremes the Mediterranean Sea surface is only one of several source regions in all seasons, and typically matched by moisture from the North Atlantic in autumn and winter and land evapotranspiration in summer. The event-to-event variability is large. Individual events can be dominated by land evapotranspiration, moisture transport from the North Atlantic, or an export of moisture from the tropics. The time of maximum moisture uptake varies between a few hours to more than a week before the precipitation event. The results from the moisture source analysis are used for investigating whether intense surface evaporation occurs prior to heavy precipitation. Surface evaporation anomalies at the moisture sources are positive over the North Atlantic and the European and African land surface, whereas no signal is present over the Mediterranean. Therefore, for the Mediterranean moisture contribution, convergence from the background moisture reservoir is essential, whereas for the remote sources anomalously intense surface evaporation is required to foster the moisture supply for Mediterranean heavy precipitation events.