ON CHANGES IN ZONAL MOMENTUM IN SHORT-RANGE NUMERICAL PREDICTION

Abstract

Different factors influencing the changes in the zonally averaged wind are investigated. It is evident from recent investigations of errors in the zonally averaged winds in the non-divergent, one-parameter model that the convergence of the meridional transport of zonal momentum concentrates too much momentum in the middle latitudes and predicts too small amounts of momentum in the low and high latitudes. The other factors influencing the changes of zonal momentum, i.e., mean meridional circulations, friction, and vertical transport of momentum, are investigated on an averaged basis in the, following sections: In section 3 it is shown that the divergent, one-parameter model will reduce the errors in zonal momentum predicted by the non-divergent model, due to an implied mean meridional circulation. The corrections to the predictions of changes in zonal momentum caused by mean meridional circulations in a two-parameter model are investigated in section 4 by the aid of operationally computed initial values of the vertical velocities. It, is shown that reductions in the errors of the non-divergent model with respect to zonal momentum can be expected with a careful arrangement of the information levels in a two-parameter model. Section 5 contains a similar investigation of the averaged contribution of vertical advection t o changes in zonal momentum. It is found that this contribution is smaller than the one resulting from mean meridional circulations, and further that the contribution from the vertical advection of momentum is not likely to reduce the errors found in the non-divergent predictions. The main conclusion from the study is that the contributions from mean meridional circulations and surface friction are the most important for the reductions of errors in the prediction of zonal momentum in the non-divergent model. Some reduction of the errors can he expected in the divergent, one-parameter model or in a two-parameter model with a proper arrangement of the information levels. In order t o incorporate surface friction in a realistic way, and further in order to avoid the artificial constraint of a non-divergent level appearing in a two-parameter model, it is most likely that more than two parameters ale needed for accurate forecasts of zonal momentum.