Abstract
Abstract Several tropical cyclone forecasting centers issue uncertainty information with regard to their official track forecasts, generally using the climatological distribution of position error. However, such methods are not able to convey information that depends on the situation. The purpose of the present study is to assess the skill of the Ensemble Prediction System (EPS) from the European Centre for Medium-Range Weather Forecasts (ECMWF) at measuring the uncertainty of up to 3-day track forecasts issued by the Regional Specialized Meteorological Centre (RSMC) La Réunion in the southwestern Indian Ocean. The dispersion of cyclone positions in the EPS is extracted and translated at the RSMC forecast position. The verification relies on existing methods for probabilistic forecasts that are presently adapted to a cyclone-position metric. First, the probability distribution of forecast positions is compared to the climatological distribution using Brier scores. The probabilistic forecasts have better scores than the climatology, particularly after applying a simple calibration scheme. Second, uncertainty circles are built by fixing the probability at 75%. Their skill at detecting small and large error values is assessed. The circles have some skill for large errors up to the 3-day forecast (and maybe after); but the detection of small radii is skillful only up to 2-day forecasts. The applied methodology may be used to assess and to compare the skill of different probabilistic forecasting systems of cyclone position.
Highlights
Tropical cyclone (TC) track forecasts have been steadily improving for several decades (Avila et al 2006), some uncertainty still remains
The present results prove that the spread of the Ensemble Prediction System (EPS) is skillful at detecting small and large errors in the Regional Specialized Meteorological Centre (RSMC) forecast
The main conclusions of this work are that d the Brier scores associated with the calibrated probability distribution given by the EPS are better than the ones from a climatological forecast, and calibration is useful and sometimes necessary to obtain valuable scores; d the uncertainty circles, defined by the radius of a calibrated probability 75%, are well correlated with the direct position error (DPE) until 60 h; d the conditional distributions of DPE to the radii of uncertainty circles demonstrate that the method is skillful at discriminating between large and small forecast errors; and d the size of the predicted radius (PR) is able to indicate a small or a large DPE at least until 72 h, but this skill becomes small from 48 h and beyond for the smallest values of DPE
Summary
Tropical cyclone (TC) track forecasts have been steadily improving for several decades (Avila et al 2006), some uncertainty still remains. Most RSMCs (Miami, Florida; Tokyo, Japan; and Hawaii) and Tropical Cyclone Warning Centers currently display uncertainty ‘‘cones’’ around their official track forecasts, using a climatological method based on their area of responsibility. A fixed climatological error is not a sufficient measure of the uncertainty of a TC track since such tracks are expected to be sensitive to internal and external factors. Tropical cyclone motion is driven by the evolution of its intensity, its structure, and its environment; and by the complex interactions between these factors. It is expected that these complex interactions result in a high dependency of the TC track predictability on the cyclone features and on its environment. Forecasting a situation-dependent uncertainty should be of significant added value
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