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

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Abstract
This is a continuation of a recent study on medium range numerical weather prediction utilizing a global spectral model and FGGE/MONEX observations. In the present study the impact of diabatic initialization and steep orography over the monsoon region are examined in a number of medium range numerical prediction experiments. The steep orography is the so-called 'Envelope Orography' which is almost a kilometer higher than the conventional orography over most major global mountain chains. The physical initialization, proposed here, contains a reconstruction of the humidity field such that a close balance between the advective and the radiative forcing is achieved over most of the rain-free areas. Over the tropical rain areas the humidity analysis is structured to a cumulus parameterization scheme of the global spectral model and the observed rain. The initial observations of the rain come from a mix of satellite and surface based observations. The proposed physical initialization recovers a substantial part of the observed rain. Two interesting prediction experiments on tropical cyclogenesis-one on the formation of the onset vortex and the other on the formation of a monsoon depression are described in the context of the physical initialization and steep orography. In both cases realistic forecasts on the medium range time frame are obtained. A realistic track (or motion) of the Arabian Sea onset vortex was obtained with the steeper orography. The storm was noted to move too far inland when a smoothed orography was used. The experiments on the Bay of Bengal depression show a major improvement with the physical initialization. The formation and track up to 6 days is in close agreement with observations. The major result of this study is on the prediction of the time averaged motion field or the stationary components. Results of four medium range prediction experiments from the winter and summer FGGE periods are illustrated. These 7 to 10 day averaged motion fields contain details on many spatial scales, most of these details are reasonably simulated by the prediction. This success in the prediction of the stationary component of the flow field is attributable to the improved physical parameterization in the model.

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