REGIONAL CLIMATE MODEL OF THE ARCTIC ATMOSPHERE

A regional climate model of the whole Arctic using the dynamical packa ge of the High-Resolution Limited Area Model (HIRLAM) and the physical parameterizations of the Hamburg General Circulation Model (ECHAM3) h as been applied to simulate the climate of the Arctic north of 65 degr ees N at a 50-km horizontal resolution. The model has been forced by t he European Centre for Medium-Range Weather Forecasts (ECMWF) analyses at the lateral boundaries and with climatological or actual observed sea surface temperatures and sea ice cover at the lower boundary. The results of simulating the Arctic climate of the troposphere and lower stratosphere for January 1991 and July 1990 have been described. In bo th months the model rather closely reproduces the observed monthly mea n circulation. While the general spatial patterns of surface air tempe rature, mean sea level pressure, and geopotential are consistent with the ECMWF analyses, the model shows biases when the results are examin ed in detail. The largest biases appear during winter in the planetary boundary layer and at the surface. The underestimated vertical heat a nd humidity transport in the model indicates the necessity of improvem ents in the parameterizations of vertical transfer due to boundary lay er processes. The tropospheric differences between model simulations a nd analyses decrease with increasing height. The temperature bias in t he planetary boundary layer can be reduced by increasing the model sea ice thickness. The use of actual observed sea surface temperatures an d sea ice cover leads only to small improvements of the model bias in comparison with climatological sea surface temperatures and sea ice co ver. The validation of model computed geopotential, radiative fluxes, surface sensible and latent heat fluxes and clouds against selected st ation data shows deviations between model simulations and observations due to shortcomings of the model. This first validation indicates tha t improvements in the physical parameterization packages of radiation and in the description of sea ice thickness and sea ice fraction are n ecessary to reduce the model bias.