Small-scale patterns of sulfate aerosol climate forcing simulated with a high-resolution regional climate model

A high-resolution, regional climate model (RCA2) is employed to evaluate direct and indirect radiative forcing patterns due to man-made sulfate aerosols over Europe and to examine the sensitivity of the results to the choice of model resolution. A simulation encompassing the whole year of 1993 is performed. The model includes an explicit parameterization of the atmospheric sulfur cycle where predicted cloud and precipitation parameters are utilized at each time step. The overall pattern of the monthly mean direct climate effect simulated by the regional climate model is similar to that obtained using global climate models. Calculations over 0.4 and 4.0° spatial resolution indicate that, for the climatic conditions simulated by the RCA2, correlations between small-scale variations of relative humidity and aerosol loading do not contribute substantially to the magnitude of the monthly mean optical thickness. For the monthly mean indirect climate effect, the finer grid spacing in the RCA2 results in a pronounced spatial variability, not visible in global climate model simulations. An interesting question is whether this variability affects the estimated magnitude of the indirect climate effect. Calculations of the effective droplet radius (re) for 0.4 and 4° spatial resolution indicate a minor importance over the RCA2 model domain (the difference in re is less than 7%). The model generally underestimates the sulfate concentration within the boundary layer, whereas the magnitude of the simulated CDNC and re agrees well with aircraft measurements. Despite an underestimate of the absolute magnitude, the regional pattern of the modeled re resembles that observed by satellite. A number of sensitivity simulations demonstrate that the magnitude of the indirect radiative forcing is highly uncertain. In order to reduce the uncertainty, different parameterizations of the indirect effect should be evaluated in more detail versus measurements of, e.g., aerosol concentration and properties, CDNC, and re at various locations.