Details of Low Latitude Medium Range Numerical Weather Prediction Using a Global

Abstract

This study addresses the formation of a tropical depression over the northern Bay of Bengal. A medium range numerical wheather prediction experiment is carried out with the observations from the Global and the Monsoon Experiments during 1979. The global spectral model contains 11 vertical levels; horizontally it is described by 29 waves via the so-called rhomboidal truncation. In the present experiment all 29 waves of the nondivergent wind and only 5 waves of the irrotational wind define the initial state. The model physics include air-sea interaction, detailed radiation, cumulus parameterization, large scale condensation, dry convective adjustment and the effects of mountains via the usual sigma frame. The initial state for the case study selected here is on July 1, 1204GMT1979. A tropical depression forms by July 5, 1979 over the northern Bay of Bengal. The global model shows considerable skill in predicting its formation and subsequent westward motion. The predicted fields of the total motion field and the streamfunction appear reasonable to day 8 of prediction. On the other hand, the divergent part of the motion field exhibits considerable error beyond day 2, especially on the planetary scale. These errors on the scale of the depression are much smaller since mass and moisture convergence move westward with the stream function minimum. In this region Kuo's scheme prescribes very reasonable rainfall rates for the monsoon depression.Single-level models, run at 700mb up to 48 hours starting on day 3, fail to predict the formation of the monsoon depression. These are respectively based on the conservation of absolute and potential vorticity. In order to understand further the formation of this depression, calculations of detailed energetics of the multi-level spectral forecast are carried out over a limited domain. They show that during the formative stage the barotropic process was most important while the rapid growth and its maintenance is largely attributable to the role of cumulus convection. The storm forms at upper levels (near 500mb) and descends down to the surface in the initial period; this is shown by observation and is reasonably simulated by the prediction.The initial barotropic growth appears to occur over a deep lower tropospheric layer and vertical coupling seems to be an important feature that is not described by the single level experiments.Finally, we present a short summary of results obtained for this experiment over other parts of the globe as well as for other experiments, where some results of data sensitivity studies are addressed.