Analysis of planetary boundary layer fluxes and land-atmosphere coupling in the regional climate model CLM

Land-atmosphere interactions and associated boundary layer processes are crucial elements of the climate system and play a major role in several feedback processes, in particular for extreme events. In this article, we provide a detailed validation of land surface processes and land-atmosphere interactions in the climate version of the Lokal Modell (CLM), a regional climate model that has been recently developed and is now used by a wide research community. For the evaluation of the model, we use observations from the FLUXNET network and meteorological data. Moreover, we also compare the performance of the CLM with that of its driving data set, the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis, and simulations of the Inter-Continental Transferability Study (ICTS). The results show that most of the land-atmosphere coupling characteristics are consistent in CLM and the observations. Nonetheless, the analysis also allows identification of specific weaknesses of the CLM such as an underestimation of the incoming surface shortwave radiation due to cloud cover overestimation, leading to an underestimation of the sensible heat flux. The comparisons with the ECMWF operational analysis and the ICTS models suggest, however, that all models have biases of comparable magnitude. This study demonstrates the utility of flux observations for diagnosing biases in land-atmosphere exchanges and interactions in current climate models and highlights perspectives for our improved understanding of the relevant processes.