Diagnosis of Track Forecast Errors for Hurricane Rita (2005) Using a High-Resolution Regional Reforecast Ensemble
Ensemble reforecasts are a long time series of ensemble forecasts from a stable data assimilation and forecast system. NOAA ESRL has recently generated a retrospective ensemble reforecast dataset using the 2012 version of the NCEP GFS ensemble (GEFS) forecast system. The reforecast dataset is comprised of an 11-member ensemble run once daily at 0000 UTC from 1985 to the present. The full resolution GEFS reforecast dataset was archived in mass storage to permit use as initial and lateral boundary conditions for high-resolution retrospective simulations with regional models such as ARW. The Hurricane Rita track forecasts initialized at 0000 UTC 22 September 2005 from the GEFS reforecast dataset were characterized by the same left-of-track error as the operational numerical model guidance. We will use the GEFS reforecast members as initial and boundary conditions to generate an 11-member high-resolution explicit ARW ensemble forecast that will be used to diagnose the Hurricane Rita track forecast errors. We will also examine the Rita track forecast errors in the context of the climatological track forecast errors for all TCs over the central and western Gulf of Mexico during the period of the GEFS reforecast dataset.
The results show that from the climatological perspective, 72–144 h track forecasts from the GEFS reforecast ensemble have a distinct left-of-track error over the central and western Gulf of Mexico. The left-of-track error occurs in conjunction with positive 500 hPa height errors north of the TC on the synoptic-scale that drives anomalous easterly flow – indicating that the subtropical ridge over the southeast U.S. is too strong in the GEFS reforecasts. The ARW regional reforecast for Rita revealed that the track forecasts were sensitive to the phase speed of midlatitude transients, where a more progressive pattern contributed to a more northward track and landfall near the Texas-Louisiana coastline. Conversely, a less progressive pattern and stronger subtropical ridge over the southeast U.S. contributed to a more westward track and landfall near Houston, Texas. Additionally, the vertical depth of the vortex also contributed to track errors, where the ensemble members with a deeper vortex moved on a more northward track compared to ensemble members with a shallower vortex.