Physics-Oriented Evaluation of the GEFS Reforecast

The Global Ensemble Forecasting System (GEFS) Reforecast Version-2 is evaluated using physics-oriented diagnostic tools with a special focus on tropical cyclones. The GEFS captures the seasonality of TC genesis (TCG) reasonably well over the Atlantic. The performance of the model varies over different basins, and quantitative errors are found in the spatial distribution of TCGs. Analyzing the genesis potential index (GPI) shows that the biases in the monsoon trough and subtropical high induce the TC biases in the western North Pacific, while the southward displacement of the ITCZ leads to errors in TCG over the eastern North Pacific. The over-prediction of TCGs near the West African coast is associated with the hyperactive tropical easterly waves over African continent. The diabatic heating (Q1) field in the GEFS forecasts suggests a much deeper and stronger convection than in the ERA-Interim, which can be attributed to the deficiency of model cumulus scheme. The further diagnosis of the precipitation and moisture fields indicates that deep convection is initiated too early in terms of the column water vapor accumulation. It is suggested that an improved cumulus parameterization may help reduce the mean errors of the model forecast and improve tropical cyclone prediction skill on the regional scale.