Sensitivity of the European climate to aerosol forcing as simulated with a regional climate model

[1] Despite the recognized importance of atmospheric aerosols for the simulation of present and future climate, the quantification and understanding of their impacts are still poorly constrained. Difficulties arise especially on a regional scale, owing to the short atmospheric lifetime of the aerosol particles combined with the limited observational possibilities. In this study the sensitivity of the European climate to a change in its aerosol forcing is investigated using a regional climate model (RCM) and two aerosol distributions. The original RCM aerosol climatology of Tanre et al. (1984) and a modified version of the Global Aerosol Data Set are considered, while their direct radiative forcing, together with the induced climatic response, is simulated through two five-year integrations of the RCM. The comparison of both sensitivity experiments demonstrates that the uncertainties associated with the aerosol forcing, as expressed here through the use of two different data sets, significantly influence the modeled climate on a regional scale and thus the model biases (in our case the well-known cold bias over the Iberian Peninsula and the summer dry bias over the Danube region). The observed model climatic response can be related to the modification of the radiation budget, also affecting the water cycle. The latter leads to the release of several feedbacks, i.e., cloud and water vapor feedbacks as land-atmosphere interactions, which either intensify or counteract the expected aerosol forcing, depending on the atmospheric conditions, vegetation state, and soil water content.