The Influence of the Tropics on the Prediction of Ultralong Waves. Part I: Tropical Wind Field

Abstract

The influence of tropical wind data on the numerical prediction of ultralong waves is examined. Two data assimilation experiments are performed using the GLAS fourth-order general circulation model. The two experiments are identical except that one utilizes tropical wind data while the other does not. Six forecasts are generated from the initial conditions provided by each experiment. After two days, a reduction in the extratropical wind error is found in the assimilation experiment with tropical wind data. For the six pairs of forecasts examined, the effect of tropical wind data on the 72 h planetary wave prediction is positive in four cases and negative in two cases over the western half of the Northern Hemisphere. Also, the 72 h planetary wave error appears in a predominantly barotropic mode. A detailed examination of the 0000 GMT 15 January 1979 case reveals that the planetary waves are more strongly affected by the wind data in the Northern Hemisphere than in the Southern Hemisphere. Examination of the velocity potential suggests the presence of stronger heating gradients in the no-tropical-wind forecast. The differences present in the initial divergent wind field remain largely restricted to the tropics after 72 h whereas significant differences may be seen in the rotational wind component at all latitudes after 72 h.