Identifying the moisture sources for heavy precipitation events – results from tagging experiments

Scale interactions play an important role in the formation of meso-scale heavy precipitation events. The processes leading to the moistening of air masses (e.g. surface evaporation) and the formation of the precipitation are associated with small-scale features, while the transport of the moisture to the location of the heavy precipitation event might happen by large-scale advection. In this study the transport of moisture to a heavy precipitation event is investigated with a so-called “moisture tagging technique”, that has recently been implemented in two regional NWP models, the CHRM and the COSMO. With such an implementation of a secondary diagnostic water cycle it is possible to decompose the moisture in the model domain into different previously specified source regions and determine the contribution of each source to a heavy precipitation event. In this presentation, results will be shown from the application of this technique to two heavy precipitation events in the Mediterranean region and Eastern Europe.

After a brief introduction into the tagging technique and the setup of tagging simulations, results will be presented first of a heavy precipitation event in the Piedmont in November 2002. This late autumn event has been triggered by an approaching upper-level trough, which prior to reaching the European continent led to intense surface evaporation over the eastern North Atlantic. The tagging simulation shows that water from this evaporation hot spot was transported around the trough and contributed substantially to the precipitation event.

A second event that occurred in May 2010 in Eastern Europe illustrates the dramatic variability of moisture sources and transport processes associated with European heavy precipitation events. For this case, a cyclone that had developed over northern Africa induced the transport of moisture over the Mediterranean towards the Karpartian mountains, which led to heavy precipitation and major flooding in Eastern Europe between 16 and 18 May 2010. The tagging simulation in harmony with offline backward trajectory calculations reveal a surprisingly strong contribution of moisture transported from the tropical Western Africa (10-20°N) over the Sahara and the Mediterranean towards Eastern Europe on a time scale of 5 to 7 days. Additional moisture sources are evaporation from the land surface and moisture of North Atlantic origin. The moisture source over western Africa is connected to intense convection of an African Easterly wave system, which was present 5 to 7 days prior to onset of the heavy precipitation event in Eastern Europe. We conclude that long-distance meridional moisture export from the tropics played a considerable role for this particular heavy precipitation event, whereas a more zonal moisture pathway dominated for the Piedmont 2002 event.