Improvement of the NCEP Global Model over the Tropics: An Evaluation of Model Performance during the 1995 Hurricane Season

Abstract

An evaluation of the performance of the National Centers for Environmental Prediction Medium-Range Forecast Model was made for the large-scale tropical forecasts and hurricane track forecasts during the 1995 hurricane season. The assessment of the model was based on changes to the deep convection and planetary boundary layer parameterizations to determine their impact on some of the model deficiencies identified during previous hurricane seasons. Some of the deficiencies in the hurricane forecasts included a weakening of the storm circulation with time that seriously degraded the track forecasts. In the larger-scale forecasts, an upper-level easterly wind bias was identified in association with 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 of 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 vertical structure of the forecast storm. This is shown to contribute to a substantial improvement in the track forecasts.