Re-evaluation of moisture sources for the August 2002 extreme rainfall episode in central Europe: Evaporation from falling precipitation included in a mesoscale modeling system

- A new technique can evaluate and map moisture sources of a target rainfall.
- For the first time, both surface evaporation and virga can be estimated.
- Virga accounted for the 18% of the target rainfall in central European floods.
- Main fraction of virga contribution took place over land, near the target region.
- Time scales of surface evaporation (6-7 days) exceeded those of virga (2 days).

SummaryDiscriminating moisture sources with precision is an important requirement to better understand the processes involved in extreme rainfall episodes. In a previous contribution by Gangoiti et al. (2011b), an innovative technique was presented to assess surface moisture sources contributing to a target precipitation within a Lagrangian framework. The technique was based in transporting parcels of vapor, representing the target precipitation, across a set of nested grids covering a large area at different resolutions. A mesoscale model estimated the meteorological variables to transport and redistribute the vapor back into its original sources, all of them assumed to be at the surface. The sequence of extreme rainfall events, which occurred over central Europe on August 11-13, 2002, was chosen to put the methodology to test. An important innovation has now been introduced. This new advance allows discriminating not only the terrestrial and oceanic sources but also the evaporation from precipitation occurring below the clouds and falling either on land or on the open sea. It is also able to detect with greater precision the relative importance of remote versus local sources, together with the sequence of evaporation of a rainfall event. After its application to the same episode and targets, our results confirm a similar distribution and strength of surface terrestrial and marine sources. Furthermore, the estimated direct evaporation from precipitation columns contributes to the precipitation episode with a significant amount of moisture which averages around 18% of the total sources, with a main fraction evaporated over land and close to the target regions in central Europe. This contribution adds to the surface sources, and it is consistent with the existence of an important mechanism of positive feedback for the inland transport efficiency of moisture and precipitation, operating at the regional level for this type of episodes. Significant regional differences are found in the contribution to different rainfall targets, with a lower fraction of 14% for the southern target (Upper Austria), and 22% for the northern one (Bohemia).