VOLUME 15 WEATHER AND FORECASTING DECEMBER 2000

Dynamical Track Forecast Errors. Part I: Tropical Region Error Sources

LESTER E. CARR III AND RUSSELL L. ELSBERRY Naval Postgraduate School, Monterey, California

(Manuscript received 28 January 2000, in ®nal form 16 June 2000)

ABSTRACT All highly erroneous (Ͼ300 n mi or 555 km at 72 h) Navy Operational Global Atmospheric Prediction System (NOGAPS) and U.S. Navy version of the Geophysical Fluid Dynamics Laboratory model (GFDN) tropical cyclone track forecasts in the western North Paci®c during 1997 are examined. Responsible error mechanisms are described by conceptual models that are all related to known tropical cyclone motion processes that are being misrepresented in the dynamical models. Error mechanisms that predominantly occur while the tropical cyclone is still in the Tropics are described in this paper, and those errors that are more related to midlatitude circulations are addressed in a companion paper. Of the 69 NOGAPS large-error cases, 39 were attributed to excessive direct cyclone interaction (E-DCI), 12 cases of excessive ridge modi®cation by the tropical cyclone (E-RMT), and 10 cases of excessive reverse trough formation (E-RTF). Of the 50 GFDN large-error cases, 31 were E-DCI, and only two E-RMT and two E-RTF cases were found, but 9 cases involving a single cyclone were attributed to excessive tropical cyclone size (E-TCS). Characteristics and symptoms in the forecast tracks and model ®elds that accompany these frequently occurring error mechanisms are documented and illustrative case studies are presented. When a sudden deviation from previous track guidance or a track outlier from the other dynamical model guidance appears, the forecaster should diagnose whether this is an error, or is indicative of a real track change. If the conceptual models of large-error mechanisms proposed from this retrospective study can be applied in real time, track forecasting will be improved.

1. Introduction track forecast guidance for the forecaster has been achieved since 1994. First, the Geophysical Fluid Dy- The long-range objective of the systematic and in- namics Laboratory (GFDL) model was demonstrated to tegrated approach to tropical cyclone (TC) track fore- provide superior guidance over the other statistical and casting (hereafter the systematic approach) of Carr and empirical techniques (Kurihara et al. 1995). That re- Elsberry (1994) is to assist the forecaster achieve sig- ni®cant improvements in of®cial track forecasts. When gional model was subsequently modi®ed to use the ini- the systematic approach was developed, the TC fore- tial conditions and lateral boundary conditions from the casters relied primarily on statistical and empirical track Navy Operational Global Atmospheric Prediction Sys- guidance (Elsberry 1995). Although dynamical model tem (NOGAPS) for provision of track forecast guidance guidance was available, nearly all of the models had in the western North Paci®c, and is referred to as the systematic errors (e.g., a marked poleward bias for low- GFDN model. Both the NOGAPS and the U.K. Met. latitude TCs moving westward). In the original system- Of®ce (UKMO) global models were signi®cantly im- atic approach concept, the plan had been to apply sta- proved in October 1994 by the introduction of improved tistical corrections for different synoptic patterns to cor- TC synthetic observations (Goerss and Jeffries 1994; rect for systematic errors in the dynamical model guid- Heming et al. 1995). Various improvements were intro- ance. A reduction in the systematic errors of the duced to the Meteorological Agency Global Spec- dynamical models used by the forecaster at that time tral Model (JGSM) and Model (JTYM) prior would presumably have led to a reduction in the annual to the 1997 typhoon season. Thus, three global (NO- average track errors. GAPS, UKMO, and JGSM) and two regional (GFDN A major gain in the accuracy of the dynamical TC and JTYM) tracks are typically available for western North Paci®c TCs at the synoptic (0000 and 1200 UTC) and off-synoptic (0600 and 1800 UTC) times, respec- tively. Corresponding author address: L. E. Carr III, Department of Me- One recent improvement in the dynamical model teorology, Code MR/Cr, 589 Dyer Rd., Room 254, Monterey, CA 93943-5114. guidance has been the reduction in the systematic errors. E-mail: [email protected] Although Elsberry et al. (1999) have shown it is possible

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caster detect when the dynamical guidance is likely to be erroneous and thus should be rejected during prepa- ration of the warning. Elsberry and Carr (2000) have examined the track forecast errors as a function of the spread (maximum distance to consensus centroid) among these ®ve dynamical models. Their ®ve-member consen- sus approach is an extension of the Goerss (2000) three- global-model or two-regional-model consensus technique at the synoptic and off-synoptic times, respectively. Goerss demonstrated that his consensus forecasts were either the best or the second-best guidance in about 70% of the forecasts. As might be expected based on expe- rience with ensemble prediction systems, an average of ®ve independent dynamical models with only small sys- tematic errors provides an improvement over the three- member or two-member consensus. Although Elsberry and Carr (2000) documented that a small spread (Ͻ300 FIG. 1. Frequency of occurrence of 72-h track errors for the NO- n mi, or 555 km) along the ®ve model tracks often im- GAPS (solid) and GFDN (open) forecasts of western North Paci®c plied a small consensus forecast error, in a sizeable frac- TCs during 1997. tion of the small spread cases the consensus error ex- ceeded 300 n mi. Another important result was that a to apply a statistical adjustment to improve the NO- large spread among the ®ve model tracks did not nec- GAPS tracks at 12±36 h, no statistically signi®cant im- essarily imply a large consensus track error, because the provement was achieved beyond 36 h. With the reduc- errors of two (or more) of the models may be compen- tion in systematic errors, old rules about the perfor- sating. Elsberry and Carr (2000) did demonstrate that a mance of the models as a function of initial latitude or large spread implies that at least one of the dynamical track orientation are not as valid. As this research has models will have an error larger than that spread. They found [see examples in Elsberry and Carr (2000) and propose a selective consensus approach in which the Carr and Elsberry (2000)], the same dynamical model model guidance suspected to have a 72-h error greater that was good in one case (e.g., recurvature) can be the than 300 n mi is ®rst eliminated prior to calculating the worst in another essentially identical case. Thus, the average of the remaining four model tracks. They dem- original systematic approach concept of applying sta- onstrate that simply omitting the worst of the ®ve dy- tistical adjustments to the dynamical model tracks need- namical model tracks would indeed improve the selective ed to be changed. consensus over the nonselective consensus. Although the dynamical models typically have skill This paper describes the characteristics and symptoms relative to a climatology and persistence forecast, the in the forecast tracks and model ®elds that a forecaster dynamical models occasionally have large errors. For might use to detect likely cases of large (300 n mi at 72 example, the distribution of 72-h NOGAPS and GFDN h) dynamical TC track errors. Conceptual models and track forecast errors during the 1997 western North Pa- case studies are presented in this paper for large-error ci®c season are shown in Fig. 1. Notice these two model scenarios that are most common when the TC is still in error distributions are skewed toward the larger errors. the Tropics, that is, equatorward of the subtropical ridge. Even though the largest NOGAPS error (1226 n mi or In a companion paper (Carr and Elsberry 2000), those 2268 km) is signi®cantly larger than for the GFDN mod- conceptual models that apply more frequently when the el (931 n mi or 1722 km), the percentages of 72-h fore- TC is poleward of the subtropical ridge will be described. cast errors exceeding 300 n mi (555 km) are very similar As demonstrated by Elsberry and Carr (2000), a suc- (33.1% for NOGAPS and 34.5% for GFDN). cessful application of these conceptual models by the Thus, the new systematic approach focus is the re- forecaster would reduce the TC track forecast errors. duction in the number of of®cial track forecasts with large errors. Although not numerous during most sea- 2. Methodology sons, these forecast ``busts'' provide such poor guidance to the customer that con®dence is degraded. If these The approach has been to identify and analyze cases large errors could be eliminated, the warnings would be during 1997 of large (300 n mi or 555 km at 72 h) track more consistent in time. Then the areas warned would errors by the NOGAPS or GFDN models (Fig. 1). Of be reduced so that customers in adjacent areas would the ®ve dynamical models mentioned above, only for not unnecessarily make preparations, and those custom- these two models were the analyses and forecast ®elds ers in the warned areas could more con®dently make available to search for explanations of the large errors. the appropriate preparations. Only the tracks of the other three models were available The basic motivation for this work is to help the fore- (®elds for JGSM and the UKMO model have recently

Unauthenticated | Downloaded 09/28/21 01:18 AM UTC DECEMBER 2000 CARR AND ELSBERRY 643 become available and appear to have similar character- In the following sections, conceptual models of the istics when large errors occurred; these evaluations will mechanisms leading to the large track errors will be be published separately). presented and described. An indication of the frequency An overall total of 108 (99) cases of large errors in of occurrence will be given based on the 1997 typhoon the tropical and midlatitude regions by the NOGAPS season. One or more case studies will be presented to (GFDN) model were examined. Analyses and predic- illustrate the sequence of events. More case studies and tions of the winds at 500 mb and the sea level pressures supporting materials are given by Carr and Elsberry were the primary source materials to study these large- (1999). These case studies are in the form of 12-panel error cases. When the TC intensity was only tropical ®gures that depict the predicted tracks and the NOGAPS storm or tropical depression, the wind ®elds at 700 or and GFDN forecasts (either winds or sea level pres- 850 mb would be examined. If vertical wind shear ef- sures). For each mechanism, a summary is provided of fects were suspected (see Carr and Elsberry 2000), the the track and ®eld characteristics during periods in 200-mb winds would be examined. which the dynamical model had large track errors. Since Since the meteorological knowledge base (see recently this is a retrospective study of cases for which it is revised synoptic pattern/region de®nitions in the appen- known that the model had a large error, it is a separate dix) has been shown to describe the actual TC motion, issue whether these conceptual models can be applied the approach was to search for evidence that the dynam- in real time to detect those track predictions that are ical model was not properly predicting these motion- likely to be in error by more than 300 n mi (555 km) related effects. That is, if it is known that a particular at 72 h. That issue will be addressed in future work. environment structure or synoptic pattern±region tran- sition was occurring in nature, an explanation for a large track error may be sought in an incorrect prediction of 3. Binary cyclone interactions that environment, an improper transition, or incorrect tim- All three of the modes of binary cyclone interaction ing of that transition. An important point is that all of (i.e., direct, semidirect, and indirect) de®ned by Carr et the error mechanisms to be described below are related al. (1997) were identi®ed as causes for highly degraded directly to an incorrect prediction by the model of phys- NOGAPS and/or GFDN track forecasts. Because erro- ical processes known to be important in TC motion. For neous DCI was responsible for roughly one-third of all convenience, the physical error mechanism will be la- highly erroneous NOGAPS and GFDN track forecasts beled with acronyms (summarized in Table 3 below) such in the western North Paci®c during 1997, this error as direct cyclone interaction (DCI), with a pre®x of E mechanism will be thoroughly discussed and illustrated. (excessive) or I (insuf®cient) to indicate its nature. Three NOGAPS and one GFDN forecasts degraded by Those 69 (50) cases in which the large NOGAPS erroneous semidirect cyclone interaction involved only (GFDN) track errors predominately occurred in the one TC. Since this semidirect case was rather unrep- Tropics are discussed in this paper. One group of trop- resentative in that the TC was poleward of the subtrop- ical-related errors involving two binary TC interactions ical ridge, it is not described here. The four GFDN will be described in section 3. Another frequent error forecasts degraded by indirect cyclone interaction (ICI) in the Tropics is an improper TC structure in the model. also involved only one TC. However, this ICI case is For example, ®ve (nine) large-error cases for the NO- important because it involves a very evident under- GAPS (GFDN) model were attributed directly to an forecast of TC size that is apparently a unique capability improper TC size (labeled TCS) in the initial conditions. of GFDN owing to the high resolution of the innermost An improper prediction of the TC structure can also grid. Underrepresentation of TC size by GFDN was also contribute indirectly to a large track error via the beta- determined to be responsible for the one instance of effect propagation (Elsberry 1995; Carr and Elsberry insuf®cient ridge modi®cation by a TC (discussed later 1997), which will be discussed in section 4. A total of in section 4), which suggests that underrepresentation 37 (38) NOGAPS (GFDN) large-error cases predomi- of TC size is a recurring (albeit infrequent) trait of nantly attributable to midlatitude circulation in¯uences GFDN forecasts that the forecaster should be aware of, is described in Carr and Elsberry (2000). In many of and thus warrants discussion in this report. those cases, an improper prediction of the environment of the TC was considered to be the likely cause of the erroneous track forecasts. a. Direct cyclone interaction No unambiguous error mechanism could be found for 1) DESCRIPTION 2 of the 108 NOGAPS large-error cases and for 2 of the 99 GFDN large-error cases. As the largest of these An excessive DCI (E-DCI) error occurs when the TC four errors was only 433 n mi (801 km), a discernable circulation is forecast to directly interact with an adjacent physical explanation could be provided for virtually all cyclonic circulation such that the predicted interaction is of the large-error cases when the ®elds were available. either false or is signi®cantly more vigorous than in re- Unfortunately, no ®elds were available for nine of the ality. The concept of DCI is analogous to the direct TC GFDN cases, so no error mechanism could be identi®ed. interaction described by Carr et al. (1997), except that

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ening of the model TC as it merges with the overly strong adjacent cyclone. Reasons for E-DCI with another real cyclonic cir- culation include (i) too large a horizontal extent and associated outer wind strength of the TC and/or the other cyclone in the initial analysis, or during the forecast (Fig. 2); (ii) mislocation of the TC and/or the other cyclone in the initial analysis, or during the forecast such that the separation of the two cyclones is smaller than in reality; and (iii) overly deep penetration of an upper-level midlatitude circulation into the lower tro- posphere where it can affect the steering of the TC. It is also possible for numerical models to forecast E-DCI between the TC and a ®ctitious cyclone. This scenario was found to be relatively rare in this 1997 sample of NOGAPS and GFDN forecasts. Another situation that has been occasionally observed in the NOGAPS model, but not in the GFDN model, is a self-interaction caused by a signi®cant difference be- tween the TC location in the ®rst-guess ®eld and the synthetic TC cyclone inserted during the data assimilation cycle. As a result, the NOGAPS initial analysis contains two noncollocated representations of the same TC that will then tend to rotate around each other during the early stages of the model integration. This phenomenon may occur when (i) the warning position, about which the synthetic observations are centered, is signi®cantly offset from the initial position in the 12-h NOGAPS forecast that serves as the ®rst guess for the analysis, or (ii) a FIG. 2. Conceptual model of DCI in which a TC circulation interacts signi®cant relocation of the analysis position or short- with another cyclone (C) to cause a counterclockwise (Northern Hemi- term forecast occurs between two successive synoptic sphere) rotation of the axis between the cyclone centers (heavy dashed times. These situations are less likely to occur in the line) and a possible merger of the two cyclones in which the combined GFDN because the TC vortex is effectively removed by circulation becomes larger with time [(c) and (d)]. The TC may also be the smaller of the two cyclones, or the model may be applied to two ®ltering the global model analysis before a model-com- TCs of similar sizes in which the tracks of both TCs will be affected. patible vortex is inserted at the warning position.

2) FREQUENCY AND CHARACTERISTICS the adjacent cyclone in the analysis or forecast is not In the 1997 sample of NOGAPS and GFDN track necessarily a TC. If real, the other cyclone may be (i) a forecasts, 18 periods (based on model initialization named TC or a remnant circulation, (ii) a tropical dis- time) were identi®ed when E-DCI occurred sometime turbance that does not develop into a named TC, or (iii) during the model integration (Table 1). The 18 periods an upper-tropospheric circulation of midlatitude origin. of model-predicted E-DCI involved only 14 TCs, since Notice that the conceptual model of DCI (Fig. 2) two periods of model-predicted E-DCI occurred during involves an apparent rotation of the two cyclones and the existence of Marie (TC number 05W), three during a possible merger into one circulation that is usually Winnie (14W), and two during Amber (18W). The num- larger in size than the analyzed TC. The TC in Fig. 2 ber of model integrations affected by E-DCI ranged is depicted as the larger, and thus more dominant, cir- from as few as 1 to as many as 11 (Table 1, last row) culation into which the second cyclone merges. In this consecutive model runs, that is, over a period of more case, the TC track forecast by the model may be only than 3 days. With the exception of TC 05W (Table 1, moderately affected by the interaction, since such an row 2), the TC environment structure (see Fig. A1 for interaction depends on the strength of the other cyclone. de®nitions) during the period of E-DCI was classi®ed It is also possible that the second cyclone will be an- as being standard/tropical easterlies (S/TE), standard/ alyzed and forecast in the model to be the dominant poleward ¯ow, poleward/poleward ¯ow (P/PF), or a circulation into which the TC tends to merge. In this transitional state between two of these pattern/region case, the TC track forecast by the model will often ex- combinations (Table 1, column 3). In two of these cases hibit a de®nite cyclonic loop, and the forecast track may in which the pattern/region classi®cations were P/PF, be prematurely terminated owing to excessive weak- the TC was at a relatively low latitude. Thus, the E-DCI

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TABLE 1. Cases of model-predicted E-DCI in the western North Paci®c during 1997. Nature of Location Synoptic environment Intensity during second of second Models TC no. Starting times of affected model runsa of TC interactionb cyclonec cycloned affectede 04W 0000 UTC 7 May±1200 UTC 8 May S/TE 25 → 45 PTC WSW N, G 05W 0000 UTC 26 May±1200 UTC 27 May S/EW → P/PF 25 PTC WNW G, N 05W 0000 UTC 29 May±1200 UTC 29 May P/PF → S/PF 45 ULC NW G, N 06W 1200 UTC 27 May±0000 UTC 29 May S/TE → P/PF 30 → 45 FTC SW* N, G 07W 1200 UTC 10 Jun±0000 UTC 11 Jun S/TE → S/PF 140 → 130 Pre-08W SW N 08W 0600 UTC 16 Jun S/PF 40 ULC NW G 09W 1800 UTC 23 Jun S/TE → P/PF 35 PTC W G 10W 0000 UTC 20 Jun±0000 UTC 22 Jun S/TE → P/PF 70 → 100 ULC NNE G, (N) 11W 0000 UTC 27 Jul±0000 UTC 30 Jul P/PF 30 → 45 Pre-12W SW* N, G 12W 1200 UTC 30 Jul±1200 UTC 1 Aug S/TE 30 → 50 PTC E N, G 14W 0000 UTC 8 Aug±0000 UTC 10 Aug S/TE 25 → 65 PTC W N, (G) 14W 0600 UTC 9 Aug±0600 UTC 10 Aug S/TE 35 → 75 PTC E G 14W 0000 UTC 16 Aug±0000 UTC 18 Aug S/TE 85 → 75 Pre-17W WSW N, G 18W 0600 UTC 22 Aug S/TE-ICIE 60 PTC W G 18W 0000 UTC 26 Aug±0600 UTC 29 Aug S/TE-DCI 85 → 105 20W W N, (G) 02C 0000 UTC 13 Sep±0000 UTC 14 Sep S/TE → S/PF 105 → 95 FTC S N 24W 1200 UTC 22 Sep±0000 UTC 25 Sep S/TE → P/PF 30 → 55 PTC WSW* N, G 05C 0000 UTC 7 Dec±0000 UTC 10 Dec S/TE 45 → 50 PTC SW N, G

a The date and times (UTC) indicated in column 2 give the starting times of the ®rst and last model run affected noticeably by E-DCI. For example, in the case of Nestor (07W) two model runs were affected by E-DCI and had starting times of 1200 UTC 10 Jun and 0000 UTC 11 Jun 1997, respectively. If more than one model was affected by E-DCI, the period for the model that was affected the longest (most model runs) is given. b Intensities in column 4 are in kt and correspond to the values at the beginning and end of the DCI period shown in column 2. c PTC ϭ probable tropical circulation, or FTC ϭ false tropical circulation. A probable (false) assignment is made if the existence of the disturbance is (is not) supported by discernible convection in satellite imagery. ULC ϭ upper-level circulation, which is usually a cutoff low originating from the midlatitudes; ##W ϭ a JTWC-designated TC that is involved in the E-DCI; pre ϭ a precursor to a JTWC-designated TC that is involved in the E-DCI. d The set of letters is the compass direction from the TC involved in the DCI when the interaction begins. The presence of an asterisk indicates that a signi®cant error in the position of the TC (relative to the ®nal best track position) toward the location of the second cyclone appeared to cause or in¯uence the E-DCI. e N ϭ NOGAPS; G ϭ GFDN. Parentheses indicates that degradation to the forecast track was visibly evident, but did not result in a 72- h FTE greater than 300 n mi. phenomenon almost always occurred in this sample In 13 of 15 cases that involved a second tropical when the TC was moving westward in the vicinity of circulation (including designated, probable, or false en- the monsoon trough, or recently had turned poleward, tries in Table 1, column 5), that cyclone was located in but was still in proximity to the tropical easterlies. the west or south quadrant of the affected TC (Table 1, In 12 (67%) of the cases, the TC was a moderate column 6) as in Fig. 2a. In the two other cases, the tropical storm (Ͻ50 kt; 25 m sϪ1) or depression when opposite orientation occurred with the second tropical the E-DCI period commenced (Table 1, column 4), circulation to the east. If the second circulation is ini- which is expected because a numerical model misrep- tially to the west (east) and E-DCI occurs, the model resentation of a nearby cyclone would have a greater forecast track will usually have a poleward (equator- impact on the model-predicted track of a weak TC. The ward) bias. Not surprisingly, E-DCI involving an upper- second cyclone was a tropical cyclonic circulation (ei- level circulation originating in the midlatitudes began ther a TC, TC precursor, or nondeveloping disturbance) with the second circulation in the northern semicircle in 13 of the cases, and a midlatitude upper-level cyclone of the affected TC. In the three E-DCI cases with an in three cases (Table 1, column 5). When the second asterisk in column 6, a signi®cant mispositioning of the cyclone in the model was a nondeveloping disturbance TC toward the E-DCI source apparently caused, or at (11 times), a circulation was evident in the satellite im- least contributed to, the E-DCI situation. agery (e.g., weakly organized convection) at some time Notice that only during the second period of E-DCI during the model integration for all cases except Marie affecting Amber (18W) was actual DCI occurring. Thus, (06W) and Oliwa (02C). All three E-DCI cases involv- E-DCI in this one case was an exaggeration of an actual ing a midlatitude cyclone had clear satellite indications interaction of the TC with another cyclone (20W, Cass). of the other cyclone (or trough), particularly in the water In the other 17 cases, the E-DCI was deemed to be false. vapor channel. Thus, for the time period studied, the If the 1997 season is representative, model-predicted NOGAPS and GFDN models were much more likely E-DCI occurs considerably more often than real mutual to overdevelop a real second cyclone than to falsely DCI. This difference has very important rami®cations for develop a possibly nonexistent cyclone (two times). TC forecasting. Speci®cally, given that the forecaster can

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FIG. 3. (a) Track of TC Nestor (07W) and forecast tracks by selected models (see inset), with the global models beginning at 0000 UTC 11 Jun 1997 and the regional models at 0600 UTC 11 Jun. The 24- (plus), 48- (cross), and 72-h (asterisk) positions are shown for the global models, and the 18-, 42-, and 66-h positions for the regional models. (b)±(d) The verifying NOGAPS sea level pressures; (e)±(h) the 0-, 24-, 48-, and 72-h NOGAPS forecasts; and (i)±(l) in real time discern the occurrence of DCI in a dynamical 3) CASE STUDIES model, the probability is high that the predicted DCI is false, or at least excessive. Therefore, the forecaster is The primary purpose of the case studies is to illustrate justi®ed in either ignoring or giving low weight to the the important aspects of the E-DCI error mechanism forecast track of that dynamical model when formulating and list clues that the forecaster can use to detect and the of®cial track forecast. The case studies that follow account for expected degradations to dynamical model will show that E-DCI is comparatively easy to identify track forecasts. The ®rst case of Typhoon (TY) Nestor if the forecaster knows what clues to look for and has (07W) illustrates how E-DCI is manifest in sea level access to the proper model ®elds and ®eld-display tools. pressure forecast ®elds when the TC is much more in-

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FIG.3.(Continued) are the 0-, 18-, 42-, and 66-h GFDN forecasts. Shading starts for a pressure of 1008 mb and the increment is 4 mb. For the analyses, the asterisk is the JTWC position and the number is the intensity in kt. For the forecasts, the asterisk is the forecast position and an intensity is given only for the GFDN forecasts.

tense than the second cyclone. The second case of TY (i) Typhoon Nestor (07W) Winnie (14W) illustrates with 500-mb wind ®elds the For this TC, the NOGAPS track forecasts were af- appearance of E-DCI with two other tropical circula- fected by E-DCI for two model runs initiated at 1200 tions that affect a relatively weak TC. This case also UTC 10 June and 0000 UTC 11 June 1997 (Table 1, shows that E-DCI may affect both NOGAPS and GFDN row 5), although only the second degraded forecast re- and, by inference, the accuracy of the other numerical sulted in an 72-h error exceeding 300 n mi. A com- model track forecasts available to the Joint Typhoon parison of the NOGAPS and GFDN sea level pressure Warning Center (JTWC), . forecasts initiated at 0000 and 0600 UTC 11 June, re-

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FIG. 4. (a)±(l) As in Fig. 3 except for 500-mb wind NOGAPS and GFDN forecasts for Winnie initiated at 1200 and 1800 UTC 8 Aug 1997, respectively. Isotach shading starts at 20 kt and the increment is 20 kt. spectively, is presented in Fig. 3. The NOGAPS track 48 h and rotates cyclonically by 72 h (Figs. 3g,h). The is slow and to the left of the track of Nestor (Fig. 3a), increase in horizontal extent of the sea level pressure with a 72-h error of 541 n mi (1001 km). By contrast, ®eld around the TC is another clue that the E-DCI con- the GFDN forecast track is very close to the actual track ceptual model (Fig. 2) may apply. Although a low pres- of Nestor with only a slight slow bias, and the JGSM sure extension is evident to the south of Nestor in the track has a moderately fast and right bias. Interaction GFDN 42-h forecast (Fig. 3k), a separate, albeit weak, in the NOGAPS forecast is with a circulation well to low pressure area to the south of Nestor is predicted at the south. Notice the lobe of sub-1004-mb pressures to 66 h (Fig. 3l). Also, the cyclonic rotation of the lobe the southwest of Nestor becomes more pronounced at of sub-1004-mb pressure evident in the NOGAPS fore-

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FIG.4.(Continued) cast is absent in the GFDN forecast. Comparison of the relative to the GFDN track (Fig. 3a) is explainable in NOGAPS and GFDN forecast ®elds with the verifying terms of E-DCI that occurs in the NOGAPS, but not NOGAPS analyses (Figs. 3b±d) con®rms that both NO- the GFDN, forecast. GAPS and GFDN are overdeveloping the second cy- clone to the south. In the NOGAPS forecast, excessive (ii) (14W) interaction occurs because both cyclones have larger horizontal extents than in reality. Presumably, the more Three separate periods of model-predicted E-DCI compact representation of Nestor in the high-resolution were identi®ed for TY Winnie (Table 1). The ®rst period GFDN model was suf®cient to preclude E-DCI, even affected model integrations initiated at 0000 UTC 8 Au- though the second cyclone is forecast to be too strong. gust±0000 UTC 10 August 1997. A comparison of the Thus, the westward shift of the NOGAPS track forecast NOGAPS and GFDN 500-mb wind ®elds and track fore-

Unauthenticated | Downloaded 09/28/21 01:18 AM UTC 650 WEATHER AND FORECASTING VOLUME 15 casts initiated at 1200 and 1800 UTC 8 August, re- vestigate further these symptoms in the track forecasts. spectively, is presented in Figs. 4a±l. Since Winnie is In the streamline ®elds, the adjacent closed cyclone± moving west-northwest in the relatively uniform ¯ow trough rotates around and often merges with the TC of a S/TE pattern±region (see Fig. A1 for de®nition), circulation. Streamlines at 500 mb (or below for weaker the E-DCI occurring in the NOGAPS and GFDN fore- storms) are the most affected if the adjacent cyclone is casts is clearly manifest as cyclonically curved tracks tropical in nature, and 500 mb and above are most af- (Fig. 4a). Notice that the directions to the NOGAPS and fected levels if the adjacent cyclone is a midlatitude or GFDN 24-h forecast positions depart dramatically from tropical upper-tropospheric trough (TUTT) cell. In the the recent motion vector of Winnie. The combination sea level pressure predictions, a closed low or a trough of a poor agreement between the short-term forecast and rotates around and often merges with the TC circulation recent TC motion, and the clear cyclonic curvatures to with an increase in the horizontal extent of characteristic the 72-h position, are important clues to the forecaster isobars. that the forecast ®elds should be examined for indica- A key result for the forecaster is that whereas DCI tions of E-DCI. Even though the inference is made in seems to occur relatively frequently in dynamical mod- the absence of forecast ®elds, the agreement of the short- els (e.g., 18 periods, involving 14 TCs that degraded 39 term UKMO (denoted EGRR in Fig. 4a) track with Win- NOGAPS forecasts and 31 GFDN forecasts in 1997), nie's recent motion, and the lack of signi®cant cyclonic vigorous track-altering direct interactions between real curvature suggests that E-DCI is not occurring to a sig- TCs and other cyclones appear to be rare. Therefore, ni®cant degree in the UKMO model. the forecaster will normally be justi®ed in treating any In the NOGAPS and GFDN analysis ®elds (Figs. 4e model-predicted DCI as E-DCI, and either rejecting or and 4i), an extensive cyclone appears at about 15Њ±17Њ giving low weight to that track forecast. It is emphasized lat to the west-southwest of Winnie. Despite a separation that many of these frequent cases of E-DCI are asso- distance that well exceeds the limit for real DCI, the ciated with a number of large track errors in Fig. 1. two circulations rotate and merge into one very large circulation in the NOGAPS forecast ®elds (Figs. 4f±h). Although the corresponding GFDN forecast ®elds (Figs. b. Indirect cyclone interaction 4j±l) have the extensive western cyclone dissipating as 1) DESCRIPTION it merges with Winnie, a third, smaller cyclone forms at 18 h (Fig. 4j) between Winnie and the larger western The conceptual model for indirect cyclone interaction cyclone, and this cyclone clearly interacts and merges on an eastern TC (ICIE) is shown in Fig. 5a. The ICIE with Winnie (Figs. 4k,l). model is analogous to the indirect binary TC interaction In the verifying NOGAPS analyses (Figs. 4b±d), the conceptual model developed by Carr et al. (1997), ex- western cyclone±trough does not appear to interact sig- cept the western cyclone may be any large cyclonic ni®cantly with Winnie, but rather remains in roughly circulation (such as a large TC, monsoon gyre, deep the same location and dissipates as Winnie passes to the midlatitude trough or cutoff low) that tends to generate north and deepens. Although the Winnie circulation in a strong peripheral anticyclone to the southeast (North- the NOGAPS analysis is large at 1200 UTC 11 August ern Hemisphere). In a barotropic, beta-plane model, this (Fig. 4d), it is still smaller than the size of the cyclone anticyclone is a result of Rossby wave dispersion of the formed by the merger of Winnie and the other cyclone(s) large western cyclone (cf. Carr and Elsberry 1997, their in the corresponding NOGAPS 72-h and GFDN 66-h Fig. 12c). However, baroclinic in¯uences may have a forecast ®elds (Figs. 4h and 4l, respectively). signi®cant role in the development of this anticyclone in the atmosphere. Excessive ICIE (E-ICIE) occurs when the dynamical model predicts that this peripheral 4) SUMMARY OF E-DCI anticyclone generated by the western cyclone will force Carr and Elsberry (1999) describe additional cases of the eastern TC to take a more equatorward track than E-DCI listed in Table 1 and document that this error in reality. This situation tends to occur when the anti- mechanism typically begins and terminates rapidly. cyclone is predicted to be too strong and/or the eastern Thus, an additional symptom is the (lack of) time con- TC is predicted to be too small. Only the GFDN model sistency in the dynamical model track forecasts. The was affected by this phenomenon in 1997. Conversely, indications of E-DCI in the model tracks include (i) insuf®cient ICIE occurs when the dynamical model pre- cyclonic curvature or even looping as the model TC dicts that the track of the eastern TC will be less equa- interacts with an adjacent cyclone; (ii) sudden changes torward than in reality either because the eastern TC is in the temporal progression of the 72-h forecast posi- too large or the peripheral anticyclone of the western tions; (iii) de¯ection, and even acceleration, toward the cyclone is too weak. This phenomenon was not re- adjacent cyclone; and (iv) translational deceleration of sponsible for degrading track forecasts by either NO- the TC motion if it had previously been moving away GAPS or GFDN in 1997, but is a physical possibility. from the adjacent cyclone. It is emphasized that the The conceptual model for ICI on a western TC (ICIW) dynamical model ®elds must then be available to in- is shown in Fig. 5b and is analogous to the indirect

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ed by E-ICIE arising from Typhoon Ivan (27W) to the west, and only one GFDN forecast for Typhoon Ivan (27W) was degraded by I-ICIW associated with Ty- phoon Joan (28W) to the east. The size of the sample is presently too small to warrant comments about fre- quency and characteristics. Such an analysis must be deferred until additional years of NOGAPS and GFDN track forecasts can be evaluated.

3) CASE STUDY The (28W) case illustrates E-ICIE oc- curring in the GFDN model that appears to result from the TC initial condition speci®cation that is applied to only one of two TCs. The Typhoon Ivan (27W) case of I-ICIW in the GFDN model is analogous in that the initial condition speci®cation is applied to Ivan but not Joan (see Carr and Elsberry 1999 for a discussion of the Ivan case). An example of the ®ve dynamical model forecast tracks relative to the actual track of Joan during the degraded track period from 0000 UTC 15 October to 1200 UTC 16 October 1997 is shown in Fig. 6a. Notice that the GFDN forecast is an outlier with a track toward the west-southwest, whereas the other four model tracks are oriented toward the west-northwest. In the GFDN analysis (Fig. 6b), an anticyclone appears between Joan and Ivan to the west. The presence of this anticyclone and the slight equatorward turn of Joan at 0000 UTC FIG. 5. Conceptual model as in Fig. 2 when large forecast track errors are associated with insuf®cient (dotted arrow) or excessive 15 October (Fig. 6a) indicate that weak ICIE occurred (dashed arrow) indirect cyclone interaction on (a) an eastern TC during 15 October. (ICIE) or (b) a western TC (ICIW). Notice in the 48- and 72-h GFDN forecast ®elds (Figs. 6c,d) that the anticyclone between Ivan and Joan re- binary TC interaction conceptual model of Carr et al. mains connected with the subtropical anticyclone to the (1997). In the case of ICIW, the eastern cyclone may north of Joan and, thus, continually subjects Joan to be any cyclonic circulation (such as a TC or TUTT equatorward steering. That is, ICIE continues to occur cyclone) that can act to erode or preclude the devel- in the GFDN model throughout the forecast, and the opment of a signi®cant peripheral anticyclone associated left bias of the GFDN forecast track (Fig. 6a) con®rms with the western TC. Insuf®cient ICIW (I-ICIW) occurs the predicted indirect TC interaction is too strong. It is when the model predicts that the western TC will track hypothesized that the circulation of the western TC Ivan more poleward than in reality because the eastern cy- is poorly represented in the GFDN model integration in clone does not suf®ciently inhibit (or preclude) the de- which TC Joan is the target. Because the special GFDN velopment of a signi®cant peripheral anticyclone as- initial condition speci®cation is not applied to Ivan, its sociated with the western TC. Only the GFDN model circulation (from the NOGAPS analysis) is too broad. was affected by this phenomenon in 1997. Conversely, Consequently, the Rossby wave dispersion from this excessive ICIW occurs when the model predicts that the broad representation of Ivan is too vigorous, which western TC will track less poleward than in reality be- builds the peripheral anticyclone to the east that is con- cause the eastern cyclone is too effective in weakening tributing to the equatorward de¯ection of Joan. The ver- or precluding the development of the peripheral anti- ifying analyses at 48 and 72 h (not shown) do not have cyclone of the western TC. This phenomenon was not a pronounced peripheral anticyclone to the east of Ivan. responsible for degrading track forecasts by either the Since the track of Joan also becomes increasingly pole- NOGAPS or the GFDN models in 1997, but is a physical ward after 0000 UTC 16 October, the period of real possibility. ICIE from Ivan has actually concluded. This case of excessive ICIE by the GFDN model is concluded to be a special circumstance of the improper 2) FREQUENCY AND CHARACTERISTICS treatment of this binary cyclone interaction phenomena In the western North Paci®c during 1997, only three because only the target TC of the GFDN integration is GFDN forecasts for Typhoon Joan (28W) were degrad- initialized with the special initial conditions. Thus, the

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FIG. 6. (a) Tracks as in Fig. 3a except for TC Joan initiated at 1200 and 1800 UTC 15 Oct 1997. (b) GFDN analysis as in Fig. 4i except for 1800 UTC 15 Oct, corresponding to (c) 48-h and (d) 72-h GFDN forecasts as in Figs. 4k and 4l.

forecaster needs to be alert for such an equatorward a. Ridge modi®cation by the tropical cyclone track de¯ection when ICIE conditions are present, or 1) DESCRIPTION are predicted. For the western North Paci®c TC forecasts during 1997 evaluated in this study, all the instances of erro- 4. Beta effect±related processes neous RMT occurred when the TC was embedded in the Rossby wave train of a large cyclonic circulation to The error mechanisms that involve the beta effect are the west and north (Fig. 7). This western cyclonic cir- ridge modi®cation by the TC (RMT), reverse trough culation (e.g., a second large TC, a large monsoonal formation (RTF), and TC initial size (TCS). All of these disturbance or gyre, or a midlatitude cutoff low or error mechanisms are associated with the well-estab- trough that has a southwest-to-northeast orientation) lished dependence of the beta effect (both propagation generates a peripheral anticyclone to the northwest of and Rossby wave train generation) on TC size (Carr and the affected TC. If the steering ¯ow associated with this Elsberry 1997). It will be seen that all instances of er- anticyclone causes the TC to move south of west, then roneous RMT in the NOGAPS and GFDN forecasts are ICIE is actually occurring, and the TC would be in the associated with erroneous forecasts of the TC size by equatorward ¯ow (EF) region of a poleward (P) synoptic the model. Since the excessive TCS, which occurred pattern (see Fig. A1 for pattern/region de®nitions). Be- only in the case of TY Paka, actually degrades model cause energy in the Rossby wave train propagates to the forecast tracks via the excessive RMT mechanism, it southeast (in the Northern Hemisphere) from the west- will not be treated separately here. Carr and Elsberry ern cyclonic circulation to the anticyclone circulation (1999) describe in detail the sensitivity of the initial to the TC, the TC and its associated peripheral anti- TCs in the NOGAPS and GFDN models to the fore- cyclone also tend to grow in horizontal extent. As its caster-speci®ed TC characteristics. peripheral anticyclone grows, the environment structure

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occurs in a numerical model, the southeastward (North- ern Hemisphere) propagation of energy is more or less vigorous than in reality. When excessive growth of the peripheral anticyclone to the southeast of the TC is pre- dicted as in Fig. 7, the TC will have a forecast track that is poleward of the actual track. Whereas a TC that is embedded in a Rossby wave usually does turn pole- ward, the predicted turn is premature (delayed/missed) in a model in which E-RMT (I-RMT) is occurring.

2) FREQUENCY AND CHARACTERISTICS Erroneous RMT in NOGAPS and/or GFDN resulted in degraded 72-h track forecasts for seven cases during 1997 (Table 2, column 1). Although the environmental structure of the TC when the erroneous RMT occurred in the model was always in the standard tropical east- erlies pattern±region (Table 2, column 3), in one case (18W) ICIE was beginning to in¯uence the track of the TC. The track forecasts of the model(s) were degraded by E-RMT in every case except the GFDN forecast for Bing (19W) that was initiated at 0600 UTC 30 August, which involved I-RMT (Table 2, column 4). Whereas the cyclonic circulation to the west (Fig. 7) was a des- ignated TC in ®ve of the seven cases, it was a false tropical circulation in the case of Nestor (07W) (Table 2, column 5). Since the Rossby wave train is usually triggered by an overly large cyclonic circulation to the west, it is not surprising that this cyclone is frequently a named TC. In each of these cases, the cyclone that generates the wave train is located to the west-northwest at distances ranging from roughly 15Њ to 30Њ lat (Table FIG. 7. (a) Excessive RMT as in Fig. 5 in¯uencing the forecast 2, column 6). track of a TC embedded in the Rossby wave train of a western cyclone The case of Ivan (27W) was somewhat different in (C) to the northwest (Northern Hemisphere). (b) If the Rossby wave dispersion leads to an overly large peripheral anticyclone trailing the that the western cyclone was a midlatitude trough that TC, the TC forecast track error at the later time will be poleward caused the subtropical ridge axis to have a west-south- (arrow). west to east-northeast slope as in Fig. 7. Although the Ivan case was unique in the 1997 dataset, the authors have identi®ed instances in other years and other basins of the TC often undergoes a transition from the S/TE of wave trains that seem to be initiated by midlatitude or P/EF (if ICIE has been causing equatorward motion) troughs that have the proper tilt. Another case was that to the P/PF pattern/region (see Fig. A1 for de®nitions). of Typhoon Rex during 1998 as described in Carr and When excessive RMT (E-RMT) or insuf®cient RMT Elsberry (1999, see their appendix B).

TABLE 2. Cases of erroneous model-predicted RMT in the western North Paci®c during 1997. See Table 1 for explanatory footnotes.

Distance (Њlat) Starting times of affected Synoptic environment Identity of second and bearing (Њ)to Models TC no. model runsa of affected TC Degree of RMT cyclonec second cyclone affectede 07W 0000±0600 UTC 6 Jun S/TE Excessive FTC 15.0Њ at 283Њ G, N 12W 1200 UTC 1 Aug±1200 S/TE Excessive 13W 18.4Њ at 278Њ N, (G) UTC 2 Aug 18W 1200 UTC 21 Aug S/TE-ICIE Excessive 17W 19.4Њ at 286Њ N 19W 0000±1200 UTC 28 Aug S/TE Excessive 18W (20W) 32.7Њ at 294Њ N, (G) 19W 0600 UTC 30 Aug S/TE → P/PF Insuf®cient 18W (20W) 23.9Њ at 301Њ G 21W 1200 UTC 13 Sep±1200 S/TE Excessive 02C 30.8Њ at 288Њ N, G UTC 14 Sep 27W 1200 UTC 18 Oct±1200 S/TE Excessive Trough N/A N UTC 16 Sep

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FIG. 8. (a)±(l) As in Fig. 4 except for 500-mb NOGAPS and GFDN wind forecasts for Tina initiated at 0000 and 0600 UTC 2 Aug 1997, respectively.

Notice in Table 2 (column 7) that during the six cases 3) CASE STUDY of E-RMT, the GFDN forecasts were more likely to be either moderately degraded (i.e., Ͻ300 n mi, denoted The case of Typhoon Tina (12W) provides a typical by parentheses), or not degraded at all. This predomi- illustration of E-RMT for a TC embedded in the wave nance in NOGAPS errors may occur because the west- train of another TC. A comparison of the 500-mb wind ern cyclone and/or the eastern TC are more likely to be ®elds and TC tracks from the NOGAPS and the GFDN too large due to the relatively coarse resolution of NO- forecasts initiated at 0000 UTC 2 August and 0600 UTC GAPS, which may then excite excessively strong Ross- 2 August 1997, respectively, is provided in Fig. 8. In by wave trains. the initial ®elds of both models (Figs. 8e and 8i), the

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FIG.8.(Continued) circulations of Tina (eastern TC) and Victor (western cates that a transition from an S/TE to a P/PF pattern± TC) are roughly 20Њ long apart and oriented east-south- region (see Fig. A1 for de®nitions) is in progress for east to west-northwest with anticyclonic ¯ow between Tina in accordance with the RMT transitional mecha- the TCs, which is consistent with the conceptual model nism. in Fig. 7. In the verifying NOGAPS analyses over the During the NOGAPS integration (Figs. 8f±h), the cir- next three days (Figs. 8b±d), Victor makes landfall but culation of Victor dissipates rapidly and the peripheral the peripheral anticyclone of Victor is still present to anticyclone of Victor that is initially to the northwest the northwest of Tina, and a peripheral anticyclone also of Tina has also dissipated slowly. Notice the excessive develops to the southeast of Tina. The shift of the isotach amplitude of the peripheral anticyclone to the southeast maximum from north to northeast of Tina (Fig. 8d) and of Tina, and that the isotach maximum is to the east- the increasingly poleward track of Tina (Fig. 8a) indi- southeast of Tina in the 72-h NOGAPS forecast (Fig.

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FIG. 9. (a) NOGAPS 72-h and (b) GFDN 66-h forecasts of sea level pressures as in Figs. 3h and 3l except for Tina initiated at 0000 and 0600 UTC 2 Aug 1997.

8h). These features are consistent with excessive energy maximum from the northern to the eastern side of the propagation in the Rossby wave train conceptual model TC. Another key indicator that E-RMT is occurring in (Fig. 7) and represent a more rapid transition from S/TE the NOGAPS forecast, and that the S/TE±P/PF pattern/ to P/PF than occurred in the analysis sequence (Figs. region transition is premature, is signi®cant growth of 8b±d). As a result, the NOGAPS forecast track is sig- the size of the TC in the sea level pressure forecasts, ni®cantly east of the verifying best track (Fig. 8a). By particularly when such growth does not also occur in the contrast, the GFDN forecast ®elds (Figs. 8j±l) are more GFDN forecast. similar to the verifying analyses, and the GFDN track forecast does not have as much of a poleward bias. Another symptom of a Rossby wave train±related b. Reverse trough formation E-RMT is an excessive growth in the horizontal extent 1) DESCRIPTION of the eastern TC in the NOGAPS 72-h sea level pres- sure ®elds (Fig. 9a) compared to the corresponding RTF as in Fig. 10 occurs when the eastern TC is at GFDN forecast (Fig. 9b). Both the NOGAPS- and approximately the same latitude as the western TC such GFDN-predicted circulations (Figs. 8g and 8k) for the that the peripheral anticyclones (each of which is a man- western TC (Victor) do not dissipate as rapidly as in ifestation of a Rossby wave train as in section 4a) of the verifying analyses (Fig. 8c), which suggests that the the two TCs can constructively superpose to produce overly large circulation of Victor may have generated one large anticyclone. When this occurs, both TCs tend a too strong wave train in the NOGAPS and GFDN to recurve simultaneously, or near simultaneously. forecast ®elds. It is hypothesized that this excessively When a numerical model predicts excessive RTF strong wave train in the NOGAPS prediction propagates (E-RTF), the RTF processes either occur prematurely energy too fast to the southeast, which causes an overly or falsely in the model and the predicted TC track is rapid growth in the horizontal extent of Tina, and also poleward compared to reality. When a model predicts in the peripheral anticyclone of Tina to the southeast. insuf®cient RTF,the Rossby wave dispersion that occurs Whereas the NOGAPS model is presumably affected in nature takes place too slowly in the model or not at more because of its relatively coarse horizontal reso- all, so that the poleward turn of the TC is predicted too lution, the GFDN model is hypothesized to be affected late or not at all. less because its higher horizontal resolution maintains a smaller TC that is less susceptible to stimulation by 2) FREQUENCY AND CHARACTERISTICS the overly strong Rossby wave train from Victor. In summary, a key result for the forecaster is that the During 1997, E-RTF was responsible for degraded wave train E-RMT phenomenon frequently occurs in forecasts of three TCs: Rosie (10W), Ivan (27W), and NOGAPS forecasts and causes a signi®cant degradation Joan (28W). In the Rosie case, E-RTF occurred only in in the track forecast. By contrast, GFDN is usually not the NOGAPS forecast in association with overdevel- affected or only moderately affected. These differences opment of the TC and a monsoonal disturbance to the in the NOGAPS and GFDN forecast tracks will be an west, when in actuality a reverse trough did not develop. important clue that a problem may be occurring. The In the cases of Ivan and Joan, a reverse trough involving Rossby wave train effect that leads to rapid ampli®cation the TCs did develop, but E-RTF in the NOGAPS fore- of the peripheral anticyclone to the southeast of the (east- cast resulted in a premature recurvature of both TCs. In ern) TC will be indicated by the rotation of the isotach the GFDN forecast with the inner grid centered on Joan,

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south of the western cyclone by 48 h, in association with the development of a large equatorial buffer eddy. As a result, the NOGAPS model predicts a strongly poleward track for Rosie, and the cyclone to the west remains roughly quasi-stationary between the eastward steering of the equatorial eddy and the westward steer- ing from the subtropical anticyclone to the north. This poleward track of Rosie with an isotach maximum to the east (Fig. 11h) in the NOGAPS forecast is consistent with the development of a peripheral anticyclone as in the RTF conceptual model (Fig. 10). By 42 h in the GFDN integration (Fig. 11k), isotach maxima have de- veloped south of both Rosie and the disturbance to the west. However, the equatorial buffer is not as extensive, and the two isotach maxima are not connected, as in the NOGAPS 48-h forecast (Fig. 11g). Nevertheless, the strength of the eastward steering associated with the equatorial eddy to the south of the western cyclone kept the eddy over the (Fig. 11l), rather than drifting to the west (Fig. 11d). That is, a reverse-oriented (southwest to northeast) monsoon trough did not occur in nature as in the NOGAPS forecast (Fig. 11h), or to a smaller extent in the GFDN forecast (Fig. 11l). This E-RTF involving Rosie and a probable tropical circu- FIG. 10. Conceptual model for an RTF in which two initially east± lation resulted in a signi®cantly degraded (error Ͼ 300 west-oriented TCs become aligned more southwest to northeast in a reverse-oriented monsoon trough with an extensive anticyclone also n mi) NOGAPS track forecast, and a moderately de- oriented southwest to northeast so that both TCs change to a more graded (error Ͻ 300 n mi) GFDN track forecast as in poleward track. Fig. 11a. In the corresponding sea level pressure ®elds (not shown), the size of Rosie in the NOGAPS forecasts E-RTF also occurred and degraded the tracks of Joan becomes considerably larger than in the verifying anal- (and also Ivan) in the model. However, E-RTF did not yses, but the size of Rosie in the GFDN forecasts is occur in the GFDN forecast with the inner grid centered slightly smaller than in the verifying analyses. In both on Ivan and thus did not degrade the track predictions the NOGAPS and GFDN forecasts, the horizontal extent of either TC in the model. As described in section 3b(3), of the disturbance to the west of Rosie is too large. The the Ivan±Joan case again emphasizes that the GFDN overly large sizes of Rosie and the western cyclone in integrations for two simultaneously existing TCs can the NOGAPS forecast contribute to excessive Rossby predict very different evolutions of the two TCs, which wave dispersion, and the growth of peripheral anticy- complicates the forecaster's evaluation process. clones that merge and cause signi®cant E-RTF. Because the size of Rosie in the GFDN forecast remained suf- ®ciently small, a signi®cant E-RTF event (i.e., error Ͼ 3) CASE STUDY 300 n mi) did not occur, despite the overdevelopment A comparison of the 500-mb wind ®elds and TC of the western cyclone. tracks for Rosie (10W) from the NOGAPS and GFDN In summary, the impact of E-RTF on the dynamical forecasts initiated at 0000 and 0600 UTC 19 July 1997, track forecasts is analogous to the E-RMT described in respectively, is shown in Figs. 11a±l. The NOGAPS section 4a in that the number of degraded tracks was track forecast has a large poleward and eastward bias signi®cantly greater for the NOGAPS model than for relative to the best track and is an outlier compared to the GFDN model. Similar numbers of NOGAPS tracks the forecast tracks of the other four numerical models were degraded by E-RMT (12) and E-RTF (10), despite (Fig. 11a). Notice the weak cyclone to the west of Rosie a smaller number of TCs in the case of E-RTF. In ad- in the verifying NOGAPS analyses (Figs. 11b±d) as well dition, the poleward track bias during the E-RTF event as in both the NOGAPS and GFDN forecasts (Figs. in the NOGAPS model is qualitatively similar to that 11f±h, and 11i±l, respectively). Satellite infrared images during an E-RMT event (e.g., cf. Figs. 8a and 11a). Just for 19±22 July (not shown) verify an area of poorly as in the E-RMT cases, the key indicators of E-RTF in organized convection initially over the Philippine Is- the predicted ®elds are (i) rapid ampli®cation in the lands moves west during the period. streamline ®elds of the peripheral anticyclone to the In the NOGAPS forecasts, an extensive isotach max- southeast of the eastern TC (and western cyclone or imum develops to the south of Rosie by 24 h, and then TC), with a rotation of the isotach maximum from the

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FIG. 11. (a)±(l) As in Fig. 4 except for 500-mb NOGAPS and GFDN wind forecasts for Rosie initiated at 0000 and 0600 UTC 19 Jul 1997, respectively. northern to the eastern quadrant, and (ii) signi®cant 5. Summary growth in the sea level pressure ®elds of the eastern TC and the western cyclone or TC, which will be most Dynamical model TC track predictions have been evident in the NOGAPS ®elds relative to the GFDN considerably improved in the past ®ve years. Systematic ®elds when E-RTF is occurring only in the NOGAPS errors have been reduced so that statistical adjustment model. As in the case of E-RMT, the key result is that techniques (e.g., Elsberry et al. 1999) to remove such E-RTF causes a premature transition from the standard/ errors tend to be effective only during the ®rst 12±24 tropical easterlies to poleward/poleward ¯ow environ- h. The problem is that the dynamical models occasion- mental structure (see Fig. A1) and thus a poleward bias ally have large errors (Ͼ300 n mi or 555 km at 72 h) in the predicted track. that introduce uncertainty. With three global and two

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FIG. 11. (Continued) regional models producing track guidance for the west- demonstrate that ®rst eliminating the erroneous track ern North Paci®c TCs, the spread among this guidance before calculating a ``selective'' consensus mean of the may be small or large. After translation of the two re- remaining tracks will generally reduce the forecast error, gional model tracks or the three global model tracks as it is important to give the forecaster tools to recognize necessary to provide a ®ve-member consensus, Elsberry an erroneous dynamical track prediction. and Carr (2000) ®nd that a small spread about this con- This search for the large error in the dynamical model sensus usually means the consensus mean will have tracks is the motivation for this retrospective exami- small (Ͻ300 n mi) 72-h errors. Whereas a large spread nation of all cases of NOGAPS and GFDN track errors among the ®ve tracks may also have a small consensus exceeding 300 n mi at 72 h during the 1997 typhoon mean error, such a large spread may also indicate the season. In this paper, those errors that predominantly model guidance is erroneous. Because Elsberry and Carr occur while the TC is still in the Tropics are described.

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TABLE 3. Meanings and frequencies of the causes of large NOGAPS GFDN model to having excessively large cyclonic cir- and GFDN forecast track errors during 1997 that occurred predom- culations that generate Rossby wave trains. The TC ini- inantly in tropical regions. If two numbers are listed, the ®rst (second) is the number of times the phenomenon occurred excessively (in- tial size error mechanism in Table 3 is related to a similar suf®ciently) in the model. physical mechanism. Although this error mechanism is the secondmost frequent for the GFDN model, these Causes of NOGAPS or GFDN 72-h forecast No. of forecasts track errors greater than 300 n mi errors were all for the long-lasting, small TY Paka. Once NO recognized, the consistent poleward bias can be adjust- Phenomenon name Acronym GAPS GFDN ed. Direct cyclone interaction DCI 39±0 31±0 Only a few examples could be presented here because Semidirect cyclone interaction SCI of space limitations. The interested reader is referred to SCI on western TC SCIW 3±0 1±0 SCI on eastern TC SCIE 0±0 0±0 the technical report by Carr and Elsberry (1999) for Indirect cyclone interaction ICI more exhaustive descriptions of these error mechanisms. ICI on eastern TC ICIE 0±0 3±0 They indicate that in some cases the onset and termi- ICI on western TC ICIW 0±0 0±1 nation of an error mechanism may be rapid. Thus, one Ridge modi®cation by TC RMT 12±0 2±1 symptom of a possible error is a sudden deviation from Reverse trough formation RTF 10±0 2±0 Tropical cyclone initial size TCS 5±0 9±0 the previous track guidance, which can be easily de- All causes 69 50 tected by examining a sequence of past track forecasts relative to the actual track. When a signi®cant deviation occurs, it should alert the forecaster to search for the In the companion paper, Carr and Elsberry (2000), those cause. Such a deviation is especially noteworthy if it is large error situations that occur as the TC moves into also a track ``outlier'' from the other guidance, which the midlatitudes will be described. is why it is important to have these ®ve skillful dynam- Perhaps the most important conclusion from this ical models for each time. A track change prediction or search for the cause of large errors is that they nearly an outlier from a cluster of other tracks may not indicate always result when the dynamical model improperly an errorÐit may be indicative of a real track change. forecasts a real physical mechanism determining the TC In either case, the forecaster should then diagnose the motion. In the most common error of excessive direct cause. cyclone. interaction, the TC track is degraded because The availability of the (digital) analysis and forecast of an erroneous prediction of the TC size or the adjacent ®elds from the model is critical to the error diagnosis. cyclone size, or of the separation distance. Because this Application of the conceptual models presented here and TC±cyclone interaction is incorrectly forecast by the the symptoms to detect the potential error require either dynamical model, the TC track forecast will be adverse- the streamlines/isotachs or the sea level pressures. Only ly affected. A similar statement can be made for the the NOGAPS and GFDN ®elds were available for this large error cases associated with the physical mecha- study. More recently, the UKMO ®elds have become nisms of indirect cyclone interaction, ridge modi®cation available, and preliminary analyses indicate these con- by a TC, and reverse trough formation. Thus, the ®rst ceptual models apply to those ®elds. An expert system step in the forecast process is to understand from TC (Peak et al. 1999) is under development that will display motion principles what physical mechanisms are pres- these ®elds and guide the forecaster through the appli- ently governing the motion. Then the reasons for the cation of these large-error conceptual models. If these track changes predicted by the dynamical model can be conceptual models can be applied in real time, it is evaluated as to whether they are likely to be occurring anticipated that the number of forecast ``busts'' can be in nature or are being improperly handled. reduced, more time-consistent warnings can be issued, A summary of the predominantly tropical region and a reduction in the areas warned may be possible. causes of the NOGAPS or GFDN 72-h errors greater than 300 n mi (555 km) is given in Table 3. Of the 69 Acknowledgments. This research has been sponsored NOGAPS cases, 39 (57%) of these large errors were from the beginning by the Of®ce of Naval Research attributed to E-DCI. Similarly, 31 of the 50 GFDN cases Marine Meteorology Program, and more recently by the of large errors in predominantly the tropical region were Space Warfare Systems Command. Mark Boothe pro- attributed to E-DCI. Since DCI is actually rare, detection vided a thorough review of the manuscript, which was of this physical mechanism in these model forecasts will expertly prepared by Mrs. Penny Jones. nearly always be erroneous. Whereas three cases of the semidirect cyclone interaction were detected in NO- GAPS forecasts of a single TC, no cases of ICI were APPENDIX detected in these NOGAPS forecasts. Only one case of SCI and four cases related to ICI were found in the The original synoptic pattern/region de®nitions in the GFDN forecasts. The RMT and RTF error mechanisms systematic approach meteorological knowledge base for primarily occur in NOGAPS with 12 and 10 cases, re- the western North Paci®c (Carr and Elsberry 1994) have spectively, because that model is more prone than the been revised as shown in Fig. A1. The streamlines in

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the maximum isotach area changes as the TC moves from region to region.

REFERENCES Carr, L. E., III, and R. L. Elsberry, 1994: Systematic and integrated approach to tropical cyclone track forecasting. Part I. Approach overview and description of meteorological basis. NPS Tech. Rep. NPS-MR-94-002, 273 pp. [Available from Naval Post- graduate School, Monterey, CA 93943-5114.] , and , 1997: Models of tropical cyclone wind distribution and beta-effect propagation for application to tropical cyclone track forecasting. Mon. Wea. Rev., 125, 3190±3209. , and , 1999: Systematic and integrated approach to tropical cyclone track forecasting. Part III. Traits knowledge base for JTWC track forecast models in the western North Paci®c. NPS Tech. Rep. NPS-MR-99-002, 227 pp. [Available from Dr. L. E. Carr III, Naval Postgraduate School, Monterey, CA 93943- 5114.] , and , 2000: Dynamical tropical cyclone track forecast er- rors. Part II: Midlatitude circulation in¯uences. Wea. Forecast- ing, 15, 662±681. , M. A. Boothe, and R. L. Elsberry, 1997: Observational evidence for alternate modes of track-altering binary tropical cyclone sce- narios. Mon. Wea. Rev., 125, 2094±2111. Elsberry, R. L., 1995: Tropical cyclone motion. Global Perspectives on Tropical Cyclones, R. L. Elsberry, Ed., WMO/TD-No. 693, World Meteorological Organization, 106±197. , and L. E. Carr III, 2000: Consensus of dynamical tropical cyclone track forecastsÐErrors versus spread. Mon. Wea. Rev., 128, 4131±4138. , M. A. Boothe, G. A. Ulses, and P. A. Harr, 1999: Statistical postprocessing of NOGAPS tropical cyclone track forecasts. Mon. Wea. Rev., 127, 1912±1919. Goerss, J., 2000: Tropical cyclone track forecasts using an ensemble FIG. A1. Synoptic pattern and region conceptual models in the of dynamical models. Mon. Wea. Rev., 128, 1187±1193. systematic approach meteorological knowledge base for western , and R. Jeffries, 1994: Assimilation of synthetic observations North Paci®c TCs relative to adjacent anticyclones (A), monsoon into the Navy Operational Global Atmospheric Prediction Sys- gyre (G), or trough (MT) or buffer (B) circulations. Key to region tem. Wea. Forecasting, 9, 557±576. abbreviations: EW ϭ equatorial westerlies; TE ϭ tropical easterlies; Heming, J., A. M. Radford, and J. C.-L. Chan, 1995: A new scheme PF ϭ poleward ¯ow; MW ϭ midlatitude westerlies. for the initialisation of tropical cyclones in the UK Meteorolog- ical Of®ce global model. Meteor. App., 2, 171±184. Kurihara, Y., M. A. Bender, R. E. Tuleya, and R. J. Ross, 1995: Improvements in the GFDL hurricane prediction system. Mon. these conceptual models represent the layer-averaged Wea. Rev., 123, 2791±2801. environmental steering or equivalent steering level (usu- Peak, J. E., L. E. Carr III, and R. L. Elsberry, 1999: Systematic ally 500 mb) after removal of the TC circulation. The approach to tropical cyclone track forecasting: Development of a tropical cyclone track forecasting expert system. Preprints, 23d arrows indicate the characteristic TC motion within each Conf. on Hurricanes and Tropical Meteorology, Dallas, TX, synoptic region, and the shaded elliptical areas indicate Amer. Meteor. Soc., 1057±1060.

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