Accuracy of early GFS and ECMWF Sandy (2012) track forecasts: Evidence for a dependence on cumulus parameterization

The extremely damaging Hurricane Sandy (2012) is noteworthy for the significant track bifurcation among several forecast models approximately 6-7days before landfall. The operational versions of the European Center for Medium-Range Weather Forecasting (ECMWF) and Global Forecast System (GFS) models exemplify this difference over their runs early in Sandy's life cycle, as the former routinely forecast a storm which would make a U.S. landfall, while the latter routinely forecast a track toward the central North Atlantic. This was also generally true of their respective ensemble members. This paper demonstrates that these differences were caused not by resolution or initial condition differences but rather due almost exclusively to choice of cumulus parameterization. Simulations performed with the Weather Research and Forecasting model using an ECMWF-like cumulus parameterization in conjunction with GFS initial conditions yield forecasts whose accuracy is similar to that of ECMWF forecasts at extended time ranges (up to 1 week before landfall). Key Points Global Sandy forecasts are dependent upon choice of cumulus parameterization Disparate tracks can be reproduced using identical initial conditions