REGIONAL CLIMATE SIMULATION WITH A HIGH-RESOLUTION GCM - SURFACE RADIATIVE FLUXES

The ability of a high resolution (T106) version of the ECHAM3 general circulation model to simulate regional scale surface radiative fluxes has been assessed using observations from a new compilation of worldwi de instrumentally-measured surface fluxes (Global Energy Balance Archi ve, GEBA). The focus is on the European region where the highest densi ty of observations is found, and their use for the validation of globa l and regional climate models is demonstrated. The available data allo w a separate assessment of the simulated fluxes of surface shortwave, longwave, and net radiation for this region. In summer, the incoming s hortwave radiation calculated by the ECHAM3/T106 model is overestimate d by 45 W m(-2) over most of Europe, which implies a largely unrealist ic forcing on the model surface scheme and excessive surface temperatu res. In winter, too little incoming shortwave radiation reaches the mo del surface. Similar tendencies are found over large areas of the mid- latitudes. These biases are consistent with deficiencies in the simula tion of cloud amount, relative humidity and clear sky radiative transf er. The incoming longwave radiation is underestimated at the European GEBA stations predominantly in summer. This largely compensates for th e excessive shortwave flux, leading to annual mean net radiation value s over Europe close to observations due to error cancellation, a featu re already noted in the simulated global mean values in an earlier stu dy. Furthermore, the annual cycle of the simulated surface net radiati on is strongly affected by the deficiencies in the simulated incoming shortwave radiation. The high horizontal resolution of the GCM allows an assessment of orographically induced flux gradients based on observ ations from the European Alps. Although the model-calculated and obser ved flux fields substantially differ in their absolute values, several aspects of their gradients are realistically captured. The deficienci es iden tified in the model fields are generally consistent at most st ations, indicating a high degree of representativeness of the measurem ents for their larger scale setting.