520 WEATHER AND FORECASTING VOLUME 22 Dynamical Tropical Cyclone 96- and 120-h Track Forecast Errors in the Western North Pacific RYAN M. KEHOE,MARK A. BOOTHE, AND RUSSELL L. ELSBERRY Graduate School of Engineering and Applied Sciences, Naval Postgraduate School, Monterey, California (Manuscript received 15 February 2006, in final form 29 September 2006) ABSTRACT The Joint Typhoon Warning Center has been issuing 96- and 120-h track forecasts since May 2003. It uses four dynamical models that provide guidance at these forecast intervals and relies heavily on a consensus of these four models in producing the official forecast. Whereas each of the models has skill, each occa- sionally has large errors. The objective of this study is to provide a characterization of these errors in the western North Pacific during 2004 for two of the four models: the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the U.S. Navy’s version of the Geophysical Fluid Dynamics Laboratory model (GFDN). All 96- and 120-h track errors greater than 400 and 500 n mi, respectively, are examined following the approach developed recently by Carr and Elsberry. All of these large-error cases can be attributed to the models not properly representing the physical processes known to control tropical cyclone motion, which were classified in a series of conceptual models by Carr and Elsberry for either tropical- related or midlatitude-related mechanisms. For those large-error cases where an error mechanism could be established, midlatitude influences caused 83% (85%) of the NOGAPS (GFDN) errors. The most common tropical influence is an excessive direct cyclone interaction in which the tropical cyclone track is erroneously affected by an adjacent cyclone. The most common midlatitude-related errors in the NOGAPS tracks arise from an erroneous prediction of the environmental flow dominated by a ridge in the midlatitudes. Errors in the GFDN tracks are caused by both ridge-dominated and trough-dominated environmental flows in the midlatitudes. Case studies illustrating the key error mechanisms are provided. An ability to confidently identify these error mechanisms and thereby eliminate likely erroneous tracks from the consensus would improve the accuracy of 96- and 120-h track forecasts. 1. Introduction ance is likely to be erroneous and thus should be re- jected during preparation of the warning. Carr and Elsberry (2000a,b) examined all of the A change in paradigm to consensus tropical cyclone highly erroneous (Ͼ300 n mi or 555 km at 72 h) Navy track forecasting was occurring at the same time. Operational Global Atmospheric Prediction System Goerss (2000) had developed a three-global-model or (NOGAPS) and U.S. Navy version of the Geophysical two-regional-model consensus technique at the synop- Fluid Dynamics Laboratory model (GFDN) tropical tic and off-synoptic times, respectively. As part of the cyclone track forecast errors in the western North Pa- Systematic Approach Forecasting Aid (SAFA; Carr et cific during 1997. They described the responsible error al. 2001), the above models and the Japan Global Spec- mechanisms in terms of conceptual models that are re- tral Model (JGSM), the Typhoon Model (JTYM), and lated to known tropical cyclone motion processes that the Met Office (UKMO) global model were interpo- are being misrepresented in the dynamical models. The lated in time so that five tracks would be available each motivation for the Carr and Elsberry study was to help 6 h (Carr et al. 2001). Elsberry and Carr (2000) docu- the forecaster detect when the dynamical model guid- mented that a small spread (Ͻ300 n mi) among these five model 72-h tracks often implied a small consensus error, but that a large spread did not necessarily imply a large consensus track error because the errors of two Corresponding author address: R. L. Elsberry, Dept. of Meteo- rology, MR/Es, Rm. 254, 589 Dyer Rd., Monterey, CA 93943- (or more) of the models may be compensating. One of 5114. the objectives in SAFA is to examine the model guid- E-mail: [email protected] ance and eliminate the model tracks that are likely to DOI: 10.1175/WAF1002.1 Unauthenticated | Downloaded 10/10/21 09:03 PM UTC WAF1002 JUNE 2007 KEHOE ET AL. 521 have large errors to form a selective consensus of the cause the consensus to be seriously degraded. Never- remaining model tracks that should be more accurate theless, the hypothesis is that elimination of the erro- than a nonselective consensus of all five dynamical neous model track and reduction of the consensus to models. only three model tracks would still lead to a more ac- The Joint Typhoon Warning Center (JTWC), with curate forecast. Since this is a retrospective study of the assistance of the Naval Research Laboratory, in cases in which it is known that the dynamical model had Monterey, California, has expanded the consensus fore- a large error, it is a separate issue whether these errors casting concept so that during the 2004 typhoon season can be detected in real time. its consensus forecast (CONW) included 10 global and regional model tracks. In addition to the five models 2. Methodology mentioned above, the CONW also includes the Na- tional Centers for Environmental Prediction Global The approach has been to analyze all cases in which Forecast System (GFS), the Australian Bureau of Me- large 120-h track errors occurred in the NOGAPS and teorology’s Tropical Cyclone Local Area Prediction GFDN forecasts of western North Pacific tropical cy- System, the Weber Barotropic Model, the U.S. Air clones during 2004. Only these two models are exam- Force’s version of the fifth-generation Pennsylvania ined because a complete archive of analyses and fore- State University–National Center for Atmospheric cast fields, which is necessary for error mechanism de- Research Mesoscale Model (MM5), and the Navy termination, was not available for the GFS and UKMO Coupled Ocean/Atmospheric Mesoscale Prediction models. System (COAMPS). Each of these models has been The definition of “large” 96- and 120-h track errors improved over the years, and with the application of the as being equal to 400 and 500 n mi was based on the consensus forecasting paradigm, the JTWC has been histogram of errors during 2003 and 2004 (Kehoe 2005, able to markedly improve its 72-h forecast accuracy his Figs. 3 and 4). Whereas this definition is somewhat (Jeffries and Fukada 2002). arbitrary, it is consistent with the approach of Carr and Based on these improvements and internal tests of Elsberry (2000a,b) in selecting a value that is twice as 96- and 120-h track forecasts during the 2001 and 2002 large as a reasonable goal for the 96- and 120-h track seasons, the JTWC implemented official 96- and 120-h forecast accuracies. The error needs to be large enough forecasts beginning in May 2003. Only four models pro- that an error mechanism can be confidently established, vide guidance for these longer forecast intervals: and the elimination of Ͼ500 n mi errors at 120 h would NOGAPS, GFS, GFDN, and UKMO. During the 2004 greatly improve the seasonal error statistics and the season, the first two models provided guidance each 6 confidence of the customer in the warning system. h, but the GFDN (UKMO) tracks were only available To maximize the number of 120-h forecast verifica- for 0600 (0000) UTC and 1800 (1200) UTC. Because tions for both NOGAPS and GFDN, the best-track po- the dynamical model forecasts are not available until sitions were manually extended beyond the point that 4–5 h after the synoptic time, the track forecasts are JTWC declares the tropical cyclone to be extratropical. interpolated from the prior 6-h integration (or 12 h for Continuing the positions using mean sea level pressure the GFDN and UKMO models) to the position at the (MSLP) analyses is considered valid since the hazards warning time. As demonstrated by Kehoe (2005, his associated with the wind, precipitation, and waves ac- Fig. 2), the JTWC relies heavily on the consensus of companying an extratropical cyclone do not suddenly these four models for its 120-h forecasts since the cor- diminish at the time a storm is declared extratropical. relation coefficient between the JTWC and consensus When the MSLP center was predicted by the GFDN errors was 0.943 during 2004. model to have left the model domain, the last location The approach in this study follows that of Carr and inside the domain was used as the predicted position for Elsberry (2000a,b) with the objective being to deter- calculating the errors. This is a conservative estimate mine the 96- and 120-h track error characteristics and because the actual error would be larger than this cal- thus provide guidance to the forecaster as to likely er- culated error. Consequently, the error summaries in roneous forecasts. Jeffries and Fukada (2002) had dem- this study will not match the JTWC summaries and will onstrated that more than three track forecasts were in fact be larger. By following these procedures to maxi- highly desirable for 72-h consensus forecasting to mize verifications of 120-h forecasts, the number of achieve the canceling of random errors, which would cases with large errors increased nearly 28% (24%) for suggest that all four of the 120-h track forecasts are NOGAPS (GFDN) (Table 1). One reason that the needed. A dilemma then exists when one of the four GFDN increase was lower than for NOGAPS was be- models has a highly erroneous 120-h track that may cause 26 of the 134 large-error cases in GFDN (de- Unauthenticated | Downloaded 10/10/21 09:03 PM UTC 522 WEATHER AND FORECASTING VOLUME 22 TABLE 1.