Abstract
We have investigated some effects of an envelope orography on the forecast fields by a hemispheric spectral model. For that, a series of 29 experiments is conducted for February 1984.
The use of envelope orography improves the daily forecasts. In particular, it remarkably reduces the systematic forecast errors of zonal mean and planetary wave fields. This suggests that the envelopetype enhancement of mountain forcing is very effective in the maintenance of planetary scale stationary fields. In the zonal mean field, the forecast error of latitudinal mass distribution that the surface pressure increases (decreases) in lower (higher) latitudes with increasing forecast period, is reduced. That is, the excessive zonal mean equatorward flow in the lower troposphere at middle latitudes is effectively reduced. This flow change also modifies the predicted zonal mean temperature in the lower troposphere. With regard to the stationary waves, the response of the model atmosphere to the envelope orography varies largely with the latitude. In higher latitudes (60N), the improvement of the wave number 3 pattern is most remarkable. Their temporal variation suggests a remote control of the mountain forcings originating from middle latitudes. In middle latitudes (40N), a significant difference between the two forecasts with standard and envelope orographies appears around 80E. Its temporal behavior suggests that the difference is directly caused by the difference of mountain forcings around the Himalayas.
Temperature differences between two forecasts occur mainly through the horizontal advection of sensible heat. The forecasts of cold surge events are also modified by the envelope orography.
