engleski

Seasonal dynamical downscaling with ERA-40 data: A sensitivity study

The Regional Climate Model (RegCM) with 50 km horizontal resolution was used for seasonal dynamical downscaling of ECMWF ERA-40 data over central and southern Europe and the northern Mediterranean region for one winter and one summer season. Various configurations in initial conditions (ICs) and lateral boundary conditions (LBCs) as well as in the model vertical domain were tested. First, the horizontal resolution of ICs and LBCs was increased from T42 to T159 and then a gradual “degradation” was conducted: the frequency of the LBCs update was reduced from 6-hourly to 12-hourly intervals, the model top was lowered from 100 to 200 hPa and the number of vertical levels was reduced from 18 to 14. The latest, most “degraded”, configuration is the closest to that available in the ECMWF seasonal forecast archive. The limited number of experiments (a single experiment per configuration and per season) did not allow a thorough statistical assessment of model responses ; however, it can be concluded that, though the differences between various configurations and resolutions are generally small, they are far from being negligible. The increase in the ICs and LBCs horizontal resolution yields a reduction in geopotential, in both the upper-air and surface temperature (cooling) and a reduction in the summer convective precipitation. The reduced, 12-hourly, frequency of the LBCs update mostly renders the opposite result, i.e. an increase in geopotential and temperature. However, it was not possible to fully establish how actually detrimental this reduced frequency in LBCs is, because the input ERA-40 upper-air data “oscillate” between consecutive 6 hours due to the difference in the processing of temperatures from satellites and from radiosondes within the ECMWF 3D-Var assimilation system. The model upper troposphere winds are strengthened when the model top is lowered ; however, this effect is partly offset when the number of model levels is reduced. Changes in the model vertical configuration cause on average much weaker effects on surface fields than changes in LBCs. The Arakawa-Schubert closure in the parameterisation of convection reduces the amount of summer convective precipitation over the northern European lowlands when compared with the Fritsch-Chappel closure. However, in a large portion of the integration domain precipitation is still too high in respect of the CRU observational data. The experience with the RegCM downscaling of ERA-40 data is encouraging and will enable to better utilise and downscale the ECMWF ensemble seasonal forecasts.

dynamical downscaling; RegCM; ERA-40

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