Winter averaged forecast error of the 72 hour forecast by the semi-operational version of the JMA spectral model has been studied.
The zonally averaged tropospheric errors are characterized by the low level warming in the polar regions (north of 60N), and cooling in the subtropics and tropics. Tonal westerlies weaken in high latitudes and in the subtropics. The subtropical jet tends to strengthen but shifts slightly northwards.
The geographical distributions of the errors are characterized by (1) the warming in the polar region at 850hPa over the eastern Siberia and northern Canada, (2) the lowering of the surface pressure over the major continents north of 45N, (3) the exitation of positive 500hPa height error that propagates westward, and (4) the cooling in the subtropical region at 850hPa in the neighborhood of the large-scale mountains.
The temperature errors in the polar region and in the subtropics are examined with respect to the error in the heat transport by the transient disturbances. It was found that the error over the eastern Siberia is mainly due to the overprediction of meridional winds, while the error over the northern Canada is explained by the errors of the positions and the tracks of the disturbances. The errors in the subtropics are caused by the overprediction of cold surges at the eastern edge of the Tibetan Plateau, and by the eastward shift of the permanent trough to the east of the Rockies.