Resolution dependence of orographic torques

An evaluation of the performance of the subgrid orography parametrization scheme in the ECMWF Integrated Forecast System is made by examining the variation with resolution of the orographic torques (resolved and parametrized) in short-range global forecasts. As the spectral resolution is increased from T95 to T511, the magnitudes of the resolved torques increase while those of the parametrized torques decrease. In general the changes in the total orographic torques are reasonably small, but an important exception is between 20degreesN and 50degreesN in winter. Here the total orographic torque decreases significantly with increasing resolution. Through examination of errors in the modelled angular momentum budgets, it is suggested that the problems lie primarily at the low resolution end, where the parametrized orographic torques seem to be excessive. Furthermore, recent work with the NCEP reanalysis data suggests that this result may not be unique to the ECMWF system.