Probability Ellipse for Tropical Cyclone Track Forecasts with Multiple Ensembles

The effectiveness of the probability ellipse for tropical cyclone (TC) track forecasts is investigated with multiple ensembles from the Japan Meteorological Agency (JMA), the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Centers for Environmental Prediction (NCEP), and the Met Office in the United Kingdom (UKMO). All TCs during the 3 years from 2016 to 2018 are included in the verification. We show that the multiple ensembles composed of these four global ensembles are capable of predicting the situation-dependent uncertainties of TC track forecasts appropriately in both the along-track (AT) and cross-track (CT) directions. The use of a probability circle involves the implicit assumption of an isotropic error distribution, whereas the introduction of the probability ellipse makes it possible to provide information as to which is more uncertain; the direction or speed of TC movement. Compared to the probability circle adopted operationally at JMA, the probability ellipse can potentially reduce the area by 16, 15, and 24 %, on average, at forecast times (FT) of 3, 4, and 5 days, respectively, indicating that narrowing warning areas of TC track forecasts by the probability ellipse enables us to enhance disaster prevention/mitigation measures.