A Lagrangian Analysis of the Atmospheric Branch of the Global Water Cycle. Part I: Method Description, Validation, and Demonstration for the August 2002 Flooding in Central Europe

Understanding and quantifying the relationships between evaporation of water in one region, precipitation in another, and the transport processes connecting them, is one of the key problems in hydrometeorology. However, to date few methods exist that are suitable for establishing these relationships. In this paper, a new Lagrangian technique is described that builds on methods that have been developed for investigating source–receptor relationships for air pollutants. It is based on meteorological analysis data and a particle dispersion model and uses a Lagrangian analog to the Eulerian budget method to diagnose the surface moisture flux. Because of its Lagrangian nature, regions of net evaporation are connected by trajectories with regions of net precipitation, and these trajectories can be used to examine how the two are related. The method is shown to yield estimates for the global distribution of the annual mean surface freshwater flux that are equally accurate as those obtained with the Eulerian budget method. It is then applied in a case study of an extreme precipitation event that occurred in central Europe in August 2002 and led to floodings with return periods of 100 to 300 yr in some river catchments. Again it is shown that the moisture fluxes obtained with the Lagrangian and Eulerian method, respectively, agree well with each other, and both agree well with observed precipitation patterns and short-range precipitation forecasts. Then the new method is used to determine where the water that became precipitation during the flooding event has evaporated. It is found that in addition to a strong Mediterranean source, much of the water evaporated from land. The strong extra evaporation over land was likely due to a wet spell the weeks before that left soils saturated with water in large parts of Europe and flooded in some smaller regions. It appears that precipitation forecasts suffered from predicting too little evaporation in these regions.