APRIL 2014 E L S B E R R Y E T A L . 271 Extended-Range Forecasts of Atlantic Tropical Cyclone Events during 2012 Using the ECMWF 32-Day Ensemble Predictions* RUSSELL L. ELSBERRY,HSIAO-CHUNG TSAI, AND MARY S. JORDAN Department of Meteorology, Naval Postgraduate School, Monterey, California (Manuscript received 5 September 2013, in final form 25 October 2013) ABSTRACT Previous studies have demonstrated the capability of the European Centre for Medium-Range Weather Forecasts (ECMWF) 51-member, 32-day ensemble to forecast tropical cyclone (TC) events (formation and tracks) in the western North Pacific on the extended range (5–30 days). In this study, the performance of the ECMWF ensemble in extended-range forecasting of Atlantic TCs during May–December 2012 is evaluated using similar approaches. The conclusion from this evaluation is that Atlantic TC events have lower fore- castability using the ECMWF ensemble than in the western North Pacific. Hurricanes Kirk and Leslie and Tropical Storms (TSs) Joyce and Oscar were successfully forecast in weeks 1–4 and, thus, are labeled as highly forecastable. Somewhat forecastable storms that are only forecast in three of the four weeks include Hurricanes Ernesto, Isaac, Nadine, and Sandy plus TS Florence. The limited forecastable storms that were successful in only the first two weeks include Hurricanes Gordon and Rafael plus TS Debby. The surprising result was that two hurricanes (Chris and Michael) and three TSs (Helene, Patty, and Tony) were not even forecast in week 1 before the starting time in the National Hurricane Center working best track (WBT) for these storms. As was the case in the western North Pacific, a substantial number of false alarm storms (no matches with any WBT) are predicted, with about 35% occurring in the first week. Except for the African wave–type false alarms, three other false alarm types may be easily recognized. A larger sample will be required to statistically verify the reliability of the probabilistic forecasts for the African wave–type en- semble storms. 1. Introduction Although verifications of the ensemble mean tracks might be expected to be less skillful than the tracks of the Ensemble predictions of tropical cyclone (TC) tracks ‘‘mother’’ deterministic high-resolution model because have become available from a number of numerical the ensemble model is integrated at a coarser horizontal centers for TCs that are already present in the initial resolution, some studies indicate the ensemble mean may conditions. For example, the European Centre for have marginally more skill at longer forecast intervals. Medium-Range Weather Forecasts (ECMWF) integrates Elsberry (2010) notes that the ensemble products are not the Variable Ensemble Prediction System (VarEPS) in available to the forecasters until 6 h later than the con- addition to its medium-range deterministic model (van der Grijin et al. 2005). The new generation of ECMWF sensus of deterministic models that many forecasters use products includes an extension of the tracking and strike as the primary track guidance, and this time delay must be probability maps from 5 to 10 days and probabilistic in- taken into account when comparing ensemble track er- formation on the storm intensity (Vitart et al. 2012). rors with the official track forecast errors. Rather than just generating a mean track that might be marginally better, a primary objective of the track * Supplemental information related to this paper is available at ensemble is to provide information on the uncertainty in the Journals Online website: http://dx.doi.org/10.1175/WAF-D-13- 00104.s1. the track forecast. The ECMWF constructs strike prob- abilities that the center of the storm will pass within 120 km of a point within the next 10 days. Superposing Corresponding author address: Russell L. Elsberry, Dept. of the ensemble mean track on these strike probabilities Meteorology, Naval Postgraduate School, 589 Dyer Rd., Monte- rey, CA 93943. then provides a visualization of the ensemble spread E-mail: [email protected] around the mean, and this swath is an indication of the DOI: 10.1175/WAF-D-13-00104.1 Unauthenticated | Downloaded 10/03/21 09:33 AM UTC 272 WEATHER AND FORECASTING VOLUME 29 track uncertainty. Dupont et al. (2011) have evaluated Elsberry et al. (2010, 2011) used a two-step objective calibrated probabilistic track forecasts in the South In- plus subjective approach for verifying the ensemble dian Ocean based on the ECMWF VarEPS and found storm tracks relative to the Joint Typhoon Warning skill relative to climatology in predicting track un- Center (JTWC) best tracks. Their approach was to first certainty for lead times up to 3 days. Whereas Majumdar treat the JTWC track as another storm and compare all and Finocchio (2010) had found that the ECMWF was of the applicable forecast ensemble storms at each 12-h the most skillful ensemble they tested, the VarEPS forecast time to determine if at least one point on the forecasts in the Atlantic were underdispersive (i.e., en- ensemble storm track matched within a separation dis- semble spread underestimated the track uncertainty). tance «(t) the JTWC position at exactly that 12-h time. Yamaguchi et al. (2012) also found the ECMWF en- Second, a subjective assessment was made to assign semble was the best individual ensemble in their multi- a quality metric (excellent, above average, good, below center grand ensemble. average, and poor) of the match of the ensemble storm A new ‘‘genesis’’ prediction from ECMWF that was to the JTWC track. provided in 2012 is the strike probability for the occur- Tsai et al. (2013) have developed an objective track rence of a TC-like vortex with a maximum wind speed of analog verification technique to select all ensemble 2 at least 8 m s 1 passing within 300 km of a given location storm tracks predicted by the ECMWF 32-day ensemble during some period in the medium-range forecast that match the overall JTWC tracks. For ensemble (Vitart et al. 2012). A similar display of the TC activity storms within specified time and space differences of the within 300 km is available from the ECMWF 32-day JTWC track that are potential analogs, four metrics of ensemble integrated twice a week from initial conditions shortest distance, average distance of the matched at 0000 UTC on Mondays and Thursdays. points, distance at formation time, and distance at end- Vitart (2009) had demonstrated that an ECMWF ing time are calculated. An objective quality measure 15-member ensemble hindcast could reproduce the that assesses the overall track similarity between the main characteristics of the observed distribution of TCs, potential analogs and the JTWC storm is calculated in although the TC activity in the model was higher than terms of membership functions for the four track met- observed. Vitart (2009) had attributed the skill of these rics. Weighting factors multiplying these membership activity forecasts to the ability of the ECMWF ensemble functions were adjusted to approximately match the model to predict the Madden–Julian oscillation (MJO). subjectively determined quality measures for the Vitart et al. (2010) also demonstrated that after cali- ECMWF ensemble storm forecasts during the 2009 bration the ECMWF ensemble could predict the TC ac- season (Elsberry et al. 2011). tivity in the Southern Hemisphere up to 3 weeks with skill Objective verifications for the 2009 and 2010 seasons comparable to the statistical models. Belanger et al. (2010) were then summarized by Tsai et al. (2013) in terms of examined the predictability of North Atlantic tropical cy- hits, misses, false alarms, and correct negatives that no clones using the 32-day ECMWF VarEPS during the 2008 TC would be present in the western North Pacific. The and 2009 seasons. Predictability to 15–21 days was in- most important result was that the ECMWF ensemble dicated in the main development region for Atlantic TCs. was able to predict out to 4 weeks nearly all of the TCs in Belanger et al. (2012) have also studied the predictability both seasons and with only a small number of misses that of TC genesis and regional outlooks of TC activity for the generally were short-lived tropical depressions. Good Arabian Sea and the Bay of Bengal. They found similar performance in terms of correct negatives was achieved, levels of predictability as in the North Atlantic. especially during the 2010 season, which had the least Elsberry et al. (2010) evaluated the predictability of TC activity in recent times. western North Pacific TC formations and tracks out to False alarms were defined by Tsai et al. (2013) to be all 4 weeks during the 2008 season using the ECMWF ensemble storms that could not be matched with JTWC 32-day ensemble forecasts. Rather than a strike proba- storms within the specified thresholds. Evaluations of bility approach, they combined similar ensemble member the characteristics of the false alarms indicate seasonal vortex tracks and used a weighted mean vector motion and geographic biases and that about 50% of the false technique (Elsberry et al. 2008) to form ensemble storm alarms during the first week originate from the initial tracks. Elsberry et al. (2011) examined the more typical conditions in the model. After a minimum of false and active 2009 season using the same approach and alarms originating in the week-2 forecasts, which was found improvements in predicting the formations and attributed to the decrease in horizontal resolution in the tracks of typhoons and the improvement also applied model that occurs at day 10, a steady and nearly uniform to most of the tropical depressions on time scales of increase in false alarms originating in the week-3 and -4 5–30 days. forecasts was documented. 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