A Climatology of ECMWF Ensemble Hurricane Track Forecast Variability

Ensemble forecasts of Hurricane Sandy and Tropical Storm Debby (2012) exhibited a wide range of possible storm tracks, which in the case of Debby included a 180° difference in the direction of motion at 48 h lead time. This kind of variability in track guidance presents a significant challenge for forecasters; therefore, it is of interest to determine how often similar track forecasts occur, and how the leading direction of position variability corresponds to the along and across-track directions. This project derives a global climatology of tropical cyclone track variability within the 51 member European Center for Medium-Range Forecasting (ECMWF) ensemble from 2008-2012 contained in the THORPEX Interactive Grand Global Ensemble (TIGGE) dataset. For each 12 h lead time, eigenvectors of ensemble member position variability are computed to determine the direction of maximum track variability. The standard deviation of position uncertainty in the direction of maximum variability is compared with the standard deviation in the normal direction. Lead times where the position variability is larger in one direction relative to the other are characterized by elliptical position variability.

The results are stratified by basin, motion, intensity, and total position variance to ascertain under what conditions forecasts with elliptical position variability are most likely to occur. Preliminary results show that elliptical position variability is more likely to occur in the West Pacific, with the Atlantic basin the least frequent. Both the Atlantic and West Pacific basins favor variability in the across-track direction in the deep tropics, while the forecasts in the extra-tropics favor along-track variability. The talk will conclude by evaluating the source of large track differences associated with Tropical Storm Debby.