Intercomparison of aerosol climatologies for use in a regional climate model over Europe

Abstract

Many regional climate models (RCM) and numerical weather prediction (NWP) models use old aerosol data sets to calculate the direct aerosol effect. In this study, three multi‐year simulations with COSMO‐CLM using different aerosol climatologies and a simulation with monthly mean aerosol optical properties, stemming from a COSMO‐CLM simulation with coupled aerosol microphysics and transport, are evaluated. The climatologies of Tegen et al. (1997), the AEROCOM‐climatology for present‐day conditions, and the monthly mean optical properties from Zubler et al. (2011), with realistic patterns of aerosol optical depth (AOD), lead to an increase of downward surface shortwave radiation (SSR) of 35 W m−2 (20%) in the Mediterranean region in comparison with the climatology of Tanré et al. (1984). The former is known to strongly overestimate AOD over Europe. The associated bias in SSR exceeds the observed variations of the recent decades by up to a factor 5. Despite an annual mean temperature increase of 0.5 K above Southern European land surfaces owing to enhanced SSR, the newer climatologies yield colder temperatures in the mid‐troposphere because of a reduction of the shortwave absorption by desert dust. This reduced heating destabilizes the atmosphere relative to the simulation with the climatology of Tanré et al. (1984), enhancing cloud formation and precipitation in these simulations. It is recommended that the RCM community uses updated aerosol information for radiative transfer calculations.