Recycling of moisture in Europe: contribution of evaporation to variability in very wet and dry years

Abstract. Evaporation is a key parameter in the regional atmospheric water cycle. Precipitation recycling is defined as the contribution of water that evaporates from a region to precipitation within the same region. We apply a dynamic precipitation recycling model, which includes a dynamic moisture storage term, to calculate the warm season variability of the precipitation recycling over central Europe at a daily time scale for 2003 (dry) and 2006 (wet).

For the central part of Europe advection is the most important contributor to precipitation. In dry spells in both years 2003 and 2006, when moisture of advective origin diminishes, local evaporation becomes an important contributor to precipitation (negative feedback). In two dry periods (June 2003 and July 2006) where there is enough moisture storage in the soil to continue evaporation, precipitation recycling is enhanced. In case studies we follow the path of an air column for days with high precipitation recycling to discuss the role of moisture recycling in land-atmosphere interactions. For 2 days with enough moisture availability (28 May 2003 and 5 July 2006) moisture particles stay long in the study area due to weak winds. By following the paths we show that the air is transported over land for a very long distance before it precipitates. It thus takes a considerable amount of time to traverse the region and capture moisture of evaporative origin. However, we hypothesize that the precipitation falling on those days still originates (partly) from oceanic sources, but that the triggering of precipitation may itself be a result of enhanced instability induced by soils, which still have sufficient moisture storage. In dry periods with enough moisture available precipitation recycling acts as a mechanism to keep the precipitation at a stable level.

In August 2003 evaporation is affecting the precipitation recycling due to the lack of water availability caused by the dryness of the preceding spring and summer season. According to a Granger Causality test the evaporation in 2003 exerts the strongest causal impact on the precipitation recycling ratio. For the case study of 10 August 2003, the atmosphere is too dry to generate precipitation with exception of the mountainous regions due to orographical lifting.