IMPROVEMENT OF THE NCEP GLOBAL-MODEL OVER THE TROPICS - AN EVALUATIONOF MODEL PERFORMANCE DURING THE 1995 HURRICANE SEASON

An evaluation of the performance of the National Centers for Environme ntal Prediction Medium-Range Forecast Model was made for the large-sca le tropical forecasts and hurricane track Forecasts during the 1995 hu rricane season. The assessment of the model was based on changes to th e deep convection and planetary boundary layer parameterizations to de termine their impact on some of the model deficiencies identified duri ng previous hurricane seasons. Some of the deficiencies in the hurrica ne forecasts included a weakening of the storm circulation with time t hat seriously degraded the track Forecasts. In the larger-scale foreca sts, an upper-level easterly wind bias was identified in association:v ith the failure of the model to maintain the midoceanic upper-tropical upper-tropospheric trough. An overall modest improvement is shown in the large-scale upper-level tropical winds from root-mean-square error calculations. Within a diagnostic framework, an improved simulation o f the midoceanic tropical trough has contributed to a better forecast of the upper-level westerly flow. In the hurricane forecasts, enhanced diabatic heating in the model vortex has significantly improved the v ertical structure of the forecast storm. This is shown to contribute t o a substantial improvement in the track forecasts.