Impact of the synoptic situation and topography on the regional atmospheric water budget for the COPS period using COSMO simulations

The atmospheric water budget, consisting of atmospheric water vapour, moisture advection, precipitation and evapotranspiration is closely connected with the climate system. On the global and even more on the regional scale, the partitioning and transfer rates of the water budget components are highly variable and not routinely known from observations. Hence there is a great demand for a reliable quantification. In this study the budget components are determined using the high-resolution regional model COSMO (termed COSMO-CLM when applied in the climate mode). The balance related approach advances a comprehensive understanding of the influences of weather situations and topography on the atmospheric water budget. The model validation using observation data focuses on the GPS technology which offers a powerful tool to explore the atmospheric water vapour content in a high spatial and temporal resolution.

The COPS campaign (Convective and Orographically-induced Precipitation Study) which took place in Southwest Germany and East France in summer 2007 provides the opportunity to compare simulated budget components with observations from GPS, precipitation and energy balance measurements for the whole COPS period. The balance components are determined for control volumes comprising the region of interest with dimensions of about 1000 to 10000 km² in the horizontal and 10 km in the vertical. The dependence of the atmospheric water budget on air mass characteristics and the inflow direction is discussed. To explore the impact of topography control volumes are set up in the Rhine Valley as well as in the Black Forest and Swabian Jura.

The volume related comparisons indicate good agreements between simulations and observations encouraging the use of COSMO simulations for water budget analysis. Comparing the data in consideration of all balance components helps to identify deficiencies of the model. Accordingly the overestimation of the atmospheric water vapour changes in COSMO is closely linked to the simulated precipitation amount.

During the summer season the main inflow is directed from the southwest. In such situations, we observe a preponderance of evapotranspiration and precipitation. In the case of a northwesterly inflow moisture divergence nearly balances evapotranspiration. Due to the relatively dry air, the contribution of precipitation is only minor. In the Rhine Valley and the region of the Black Forest and Swabian Jura the partitioning of evapotranspiration and precipitation is similar for the inflow from Southwest and Northwest; the contributions due to moisture convergence respectively divergence differ. We notice that the results are quite sensitive to the persistence of weather situations and the number of days for each inflow characteristic. Concluding the results for the COPS period are put into a climatic context.