The 2015 Upgrade of NCEP's Global Ensemble Forecast System (GEFS)

NCEP's Global Ensemble Forecast System (GEFS) is going to have a major upgrade in April/May 2015. This article briefly describes the changes in the model resolution, NWP model, ensemble generation techniques and the product data sets. Product performance improvement will be demonstrated with verification statistics and example of severe weather events.

The new GEFS is based on the latest version (v12) Global Forecast System (GFS) model, with Semi-Lagrangian dynamic scheme and numerous enhancements in model physics. The horizontal resolution is increased from about 55km to about 33km for the first 192 hours (8 days) of model integration, and from about 70km to about 55km between 192 hours to 384 hours of model integration. It also increases the vertical resolution from 42 levels to 64 levels throughout the model integration.

The ensemble initialization method is modified by replacing the Bred Vector with Ensemble Transform and Rescaling (BV-ETR) scheme with Ensemble Karman Filter (EnKF) scheme. The 6-hour forecasts of the 80 EnKF ensemble members of the Hybrid Data Assimilation system, from the previous cycle, are used to initialize the ensemble perturbations. This change unifies NCEP's global ensemble systems in data assimilation and forecast and will lead to reduction in computational resources. The stochastic Total Tendency Perturbation scheme (STTP) is also modified by turning off the perturbations in surface pressure and improving the rescaling algorithm.

Extensive retrospective forecasts are generated for the last two years and the 2012 hurricane season. Verification of these forecast showed that GEFS model output is significantly improved in both general forecast guidance and severe weather event prognosis with the 2015 system upgrade.