The Spread of Tropical Storm Tracks in Three Versions of NCEP’s Global Ensemble Model: Focus on Hurricane Edouard (2014)

Abstract

AbstractSince 2012, the National Centers for Environmental Prediction?s Global Ensemble Forecast System (GEFS) has undergone two major upgrades. Version 11 was introduced in December 2015, with a new dynamic scheme, improved physics, increased horizontal and vertical resolution, and a more accurate initialization method. Prior to implementation, retrospective model runs over four years were made, covering multiple hurricane seasons. The second major upgrade was implemented in May 2016, when the data assimilation system for the deterministic Global Forecast System (GFS) was upgraded. Because the GEFS initialization is taken from the deterministic GFS, this upgrade had a direct impact on the GEFS. Unlike the previous upgrade, the model was rerun for only a few tropical cyclones. Hurricane Edouard (2014) was the storm for which the most retrospective runs (4) were made for the new data assimilation system. In this paper, the impact of the GEFS upgrades is examined using seasonal data for the 2014?17 hurricane seasons, and detailed data from the four model runs made for Hurricane Edouard. Both upgrades reduced the spread between ensemble member tracks. The first upgrade reduced the spread but did not reduce the likelihood that the actual track would be included in the family of member tracks. The second upgrade both reduced the spread further and reduced the chance that the real storm track would be within the envelope of member tracks.