Forecast experiments on a medium-scale disturbance associated with a long-lived cumulonimbus cluster in the Asian subtropical humid region are made wing a 6-level 77km-mesh primitive equation model. The purposes of the present study are to ascertain whether the medium-scale disturbance is predicted by using the present fine-mesh primitive model or not, and to inspect thermodynamic influence of cumulus convections on the development of the disturbance.
We make five experiments (Exp. 1 with standard moisture data, Exp. 2 with moist moisture bogus data as indicated by satellite IR cloud imagery, Exp. 3 with low-level moist moisture bogus data, Exp. 4 with artificial dry moisture bogus data and Exp. 5 without moist convective adjustment scheme) for each of the four development stages of the disturbance: formation, Cb cluster, Cb cluster-depression and depression stage. We also compare the results with prediction by 6L FLM (6-level 150km-mesh primitive model of JMA).
In the formation stage, the disturbance is located over the China Continent and the Cb cluster is apparently found on satellite IR cloud imagery. However, the amplitude of the disturbance is very weak, and the baroclinicity in the lower layers around the disturbance is very weak. For this period, all experiments can not simulate formation nor development of the disturbance.
For Cb cluster stage, Exp. 2 and 3 simulate the development of the disturbance while Exp. 1, 4, 5 and 6L FLM can not.
For Cb ciuster-depression and depression stages, the predictions of all experiments are accurate to same extent. However, the precipitation in Exp. 4 and 6L FLM are much smaller.
The present study indicates the significant influence of cumulus convections on the development of the disturbance in Cb cluster stage. The accuracy of precipitation prediction in Cb cluster, Cb cluster-depression and depression stage is evidently improved by the increase of the grid resolution and the improve of the initial analysis of the moisture field.
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