Hurricane Juan (2003)

1748 MONTHLY WEATHER REVIEW VOLUME 134

Hurricane Juan (2003). Part II: Forecasting and Numerical Simulation

RON MCTAGGART-COWAN AND LANCE F. BOSART Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York

JOHN R. GYAKUM AND EYAD H. ATALLAH Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

(Manuscript received 20 August 2004, in final form 22 July 2005)

ABSTRACT

The landfall of Hurricane Juan (September 2003) in the Canadian Maritimes represents an ideal case in which to study the performance of operational forecasting of an intense, predominantly tropical feature entering the midlatitudes. A hybrid cyclone during its genesis phase, Juan underwent a tropical transition as it drifted slowly northward 1500 km from the east coast of the United States. Shortly after reaching its peak intensity as a category-2 hurricane, the storm accelerated rapidly northward and made landfall near Halifax, Nova Scotia, Canada, with maximum sustained winds of 44 m sϪ1. Although the forecasts and warnings produced by the U.S. National Hurricane Center and the Canadian Hurricane Centre were of high quality throughout Hurricane Juan’s life cycle, guidance from numerical weather prediction models became unreliable as the storm accelerated toward the coast. The short-range, near-surface forecasts from eight operational models during the crucial prelandfall portion of Juan’s track are investigated in this study. Despite continued improvements to operational numerical forecasting systems, it is shown that those systems not employing advanced tropical vortex initialization techniques were unable to provide forecasters with credible near-surface guidance in this case. A pair of regional forecasts, one successful and one from the failed model set, are compared in detail. Spurious asymmetries in the initial vortex of the deficient model are shown to hamper structural predictions and to cause nonnegligible track perturbations from the trajectory implied by the well-described deep-layer mean flow. The Canadian Mesoscale Compressible Community model is rerun with an improved representation of the hurricane’s vortex in the initial state. The hindcast produced following the tropical cyclone initialization contains reduced track, structure, and intensity errors compared with those generated by the model in real time. The enhanced initial intensity produces a direct improvement in the forecast storm strength throughout the period, and the symmetriza- tion of the vortex eliminates the interactions that plague the operational system. The southeastward relo- cation of the implanted vortex to Juan’s observed location eliminates a significant northwestward track bias under the influence of a broad area of southerly steering flow. The study concludes that the initialization of Hurricane Juan’s structure and position adds value to numerical guidance even as the storm accelerates poleward at a latitude where the implantation of a quasi-symmetric vortex may not be generally valid.

1. Introduction tional Hurricane Center (NHC) and the Canadian Hur- ricane Centre (CHC) over the storm’s life cycle were of The landfall of Hurricane Juan (September 2003), an very high quality, many numerical models showed lim- unusual event at midlatitudes because of the storm’s ited short-range predictive skill as the hurricane neared tropical structure and intensity, is an excellent case in the Canadian Maritimes. which to examine the performance of numerical fore- Tropical cyclones (TCs) traveling into the midlati- cast guidance in extreme conditions. While the opera- tudes generally weaken under the combined influences tional forecasts and warnings produced by the U.S. Na- of cooler sea surface temperatures (SSTs) and in- creased westerly shear. Approximately 25% of these decaying tropical vortices undergo interactions with Corresponding author address: Dr. R. McTaggart-Cowan, Uni- versity at Albany, State University of New York, DEAS-ES351, synoptic-scale features that lead to the reintensification 1400 Washington Avenue, Albany, NY 12222. of the storm as an extratropical system (e.g., DiMego E-mail: [email protected] and Bosart 1982; Klein et al. 2000; Hart and Evans

© 2006 American Meteorological Society

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2001). These extratropical transitions (ETs) are com- regional models, the problem of vortex initialization mon in the Canadian Maritime provinces, where an will increase in importance as resolution increases and average of 2–3 occur each year (Joe et al. 1995). Less more accuracy is expected in both track and intensity frequently, however, the TC’s structure and intensity forecasts from global models. may persist despite its passage over cooler SSTs as a Regional modeling systems, previously able only to result of either boundary layer decoupling (Browning reproduce coarse TC characteristics, are now being de- et al. 1998) or sufficient translation speed. In the latter veloped with the ability to accurately model the finer- case, the TC likely enters stage 1 of the Klein et al. scale banding and eyewall structures of these storms (2000) transition process (Ritchie and Elsberry 2001) in (Liu et al. 1997; Bender 1997; Rogers et al. 2003). The which distinct asymmetries appear in its cloud and pre- vortex initialization problem noted above is equally cipitation structures; however, the storm may still pen- valid in the limited-area modeling framework. Because etrate northward a considerable distance without losing these systems provide only short-range guidance, there many of its tropical characteristics. Since the accelera- is little time for the model to spin up a realistic vortex tion of the vortex is dependent on a strengthened steer- from weak globally derived initializations devoid of ing flow, the TC itself must be able to withstand the hurricane-like features. Especially in cases where accu- effects of the implied increase in shear during this stage rate near-storm observations are available, the value of of its life cycle (Jones 1995). With a return period of 3 realistic mesoscale vortex initialization should not be yr (McTaggart-Cowan et al. 2006, hereafter Part I), underestimated. cases of such storms are not frequent in the North At- Discussions during the Fifth International Workshop lantic, but can have significant socioeconomic impacts on Tropical Cyclones (IWTC; WMO 2003), held in specifically because they make landfall in regions not Cairns, Australia, in 2002, repeatedly emphasized the usually influenced by hurricane activity. Given the need for further studies of near-storm data assimilation complexity and relative infrequency of these events, it and vortex initialization techniques. The tempting sim- is not surprising that Hurricane Juan challenged nu- plification that an initial state with a well-located storm merical models as it accelerated northward toward the of the observed intensity will consistently yield forecast coast of Nova Scotia. improvement has been repeatedly shown to be invalid. Ongoing improvements to global operational predic- The National Centers for Environmental Prediction tion systems have greatly enhanced their ability to pro- (NCEP) abandoned regular vortex initialization in the vide skillful guidance for hurricane track and intensity Global Forecast System (GFS) on the grounds that the forecasting (e.g., Sheets 1990; Goerss and Jeffries 1994; implanted storms had a detrimental influence on the Heming and Radford 1998; Williford et al. 1998). As tropical environment. This modification of the back- the resolution of global models edges toward the me- ground flow was found to lead to poor track forecasts soscale, the importance of accurate physical parameter- based on incorrect steering winds. Similar concerns ization schemes and reliable initial conditions increases. prompted researchers at the European Centre for Me- The use of physical packages developed for regional dium-Range Weather Forecasts to design their system models addresses the former of these concerns; the lat- to run without vortex specification, and forecasters at ter however, presents a significant challenge for both the IWTC to explicitly request track information from observing and data assimilation systems. Model initial- all operational models regardless of initialization. ization is particularly problematic for TC structures for As hurricanes move into the midlatitudes and begin two primary reasons. The first is the high degree of ET, the validity of traditional vortex initialization tech- uncertainty in the initial state of the hurricane vortex niques becomes questionable, as noted by the IWTC (structure, intensity, and location). The second is di- Working Group on Extratropical Transition. The am- rectly related to the formulation and the resolution of plification of asymmetric structures in hurricanes accel- the model, a combination of which determines the ex- erating poleward and recurving under the influence of tent to which an intense tropical vortex can be balanced baroclinic westerlies leads to the development of com- and maintained in the model atmosphere. Even in well- plex structures that are difficult, if not impossible, to formulated, high-resolution models tuned for hurricane represent in an implanted vortex. However, the favor- simulation, numerical diffusion leads to vortex decay able midlatitude environment into which Hurricane over time (Robert 1993; Xue 2000). This problem is Juan traveled allowed the storm to maintain its tropical compounded by many others in operational systems structure well into the midlatitudes (Part I). The success constructed to produce valid short- and medium-range of vortex initialization in this case may therefore set an predictions at all points on the globe. Especially for upper bound on the applicability of idealized vortex short-range guidance and the initialization of nested insertion.

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FIG. 1. The NHC best tracks for Hurricanes Isabel and Juan. Positions are plotted in 6-h intervals with markers and intensity circles at 0000 and 1200 UTC. Insets show minimum sea level pressure (solid lines) and maximum sustained wind speed (dotted lines) for each storm.

Further motivation for this study arises from the servation is supported by the similarity of the large- questions posed by the IWTC Working Group on scale flow in the initial states of the operational models Tropical Cyclone Motion: “Are major forecast failures to be presented later in this paper. In a qualitative a natural consequence of numerical prediction, and will sense, Hurricane Juan is somewhat unique in that the always occur? Can they be eliminated?” The working dominant margin for initialization error appears to rest group goes on to suggest that a research archive of in the representation of the TC vortex. Despite recon- forecast failures be developed and that model inter- naissance flights into the storm by both United States comparisons and data exchanges be encouraged as sug- and Canadian aircraft, a broad spectrum of vortex in- gested by Nagata et al. (2001). The work presented here tensities and structures evolves in model initializations adds to the research archive a diagnostic, modeling and at later stages of the storm’s life cycle. intercomparison study of a compact, high-latitude hur- The work presented in this paper represents an ex- ricane and addresses the working group’s questions tension of the diagnostic study of Hurricane Juan un- from a case study perspective. dertaken in Part I. The current work focuses on prob- As shown in Fig. 1, Hurricane Juan’s track should lems with the representation of Hurricane Juan in op- have facilitated numerical forecasting of storm motion erational forecasts initialized at 0000 UTC 28 because of its proximity to the data-rich North Ameri- September, just 27 h before the storm’s landfall in Nova can continent at all times during the TC portion of the Scotia. A brief analysis of Juan’s life cycle is presented storm’s life cycle. The radiosonde network was fully in section 2. Section 3 summarizes global and regional capable of providing in situ observations that resolved numerical model guidance and presents a detailed the structure of the amplified large-scale flow that led analysis of two representative forecasts, one that pro- to Juan’s rapid northward acceleration toward Nova duced a credible short-range prognosis of Hurricane Scotia. Well before landfall, the synoptic-scale condi- Juan and one that did not. Analysis of a sensitivity test tions to the north and west (along-track and upshear, designed to eliminate one of the primary error modes in respectively) of the storm were well known. This ob- the underperforming model described in section 3 is

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FIG. 2. Mosaic of 850-hPa relative vorticity in Juan’s precursor easterly wave (contoured and shaded at 8 ϫ 10Ϫ6 sϪ1 intervals above zero only) at (a) 0000 UTC 11 Sep, (b) 0000 UTC 14 Sep, (c) 1200 UTC 17 Sep, (d) 0000 UTC 20 Sep, and (e) 0000 UTC 21 Sep 2003.

presented in section 4. A hindcasting modeling study southeast of Bermuda (Fig. 1) although, as shown in in which the implantation of a synthetic vortex dramati- Part I, the storm still possessed links to its extratropical cally improves a real-time research model’s perfor- triggers until 0000 UTC 26 September. [An analysis of mance for the case is presented in section 5. The paper the tropical transition (TT) of Hurricane Juan is one of concludes with a summary and discussion in section 6. the key findings presented in section 3 in Part I.] Within 12 h (Fig. 6d in Part I), the system reached tropical storm strength with a central pressure estimated at 1006 2. Description of Hurricane Juan hPa and winds of 17 m sϪ1. Tracking to the north, Juan steadily strengthened a. Life cycle synopsis from 0000 UTC 25 September to 0000 UTC 27 Septem- The precursor for Hurricane Juan came in the form ber (Fig. 6 in Part I and Fig. 3 here), reaching hurricane of a strong easterly wave whose associated depression status at 1200 UTC 26 September (Fig. 3a, labeled “J”) crossed the West African coast in mid-September 2003. as a compact system with a radius of hurricane force Although the wave (Fig. 2) was effective in producing winds (wind speeds greater than 33 m sϪ1) estimated at convection as it moved across the equatorial Atlantic, under 50 km by NHC forecasters. The storm reached its the organization of the cloud field was weak, owing in peak intensity at 1800 UTC 27 September as a cat- part to interaction with a mid-Atlantic trough (not egory-2 hurricane (wind speeds exceeding 42 m sϪ1) shown) modified by the outflow from the powerful with sustained winds estimated at 45 m sϪ1 and a cen- Hurricane Isabel (Fig. 1). The easterly wave turned tral pressure of 969 hPa. By 0000 UTC 28 September northward on the west side of the subtropical high and (Fig. 3e), the storm lay 900 km south of Halifax, Nova on 23 September began to interact with a deep shear Scotia, Canada, and was moving northward at just over line associated with the filamenting tail of a midlatitude 4msϪ1 (Fig. 4a). Juan’s intensity remained nearly con- cyclone transiting the North Atlantic (Fig. 5 in Part I). stant as it accelerated northward under a strengthening The buckling of this elongated strip of vorticity led to steering flow between a ridge–trough couplet over east- the evolution of pronounced asymmetries in the cloud ern North America. field associated with the developing system. The hybrid As shown in Part I, a high-amplitude ridge over the cyclone (containing both tropical and baroclinic com- western North Atlantic Ocean played a critical role in ponents) continued to develop on 24 September (Fig. the later phases of Juan’s life cycle by establishing a 6a in Part I), as deep convection focused vorticity near weakly sheared southerly steering flow and a thermal the core, strengthening the lower-level circulation. By environment conducive to the maintenance of the 1200 UTC 24 September, forecasters at the NHC re- tropical vortex. By 0000 UTC 29 September (Figs. 3f classified the storm as Tropical Depression 15, 600 km and 4b), Juan’s propagation speed was 15 m sϪ1 and

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FIG.3.GOES-East infrared satellite imagery at 12-h intervals for (a)–(f) 1200 UTC 26 Sep–0000 UTC 29 Sep 2003. Details concerning specific annotations are contained in the text. asymmetries in the storm’s wind field were apparent in clinic environment may also have contributed to the enhanced satellite imagery (not shown). Landfall oc- forecast challenge. curred shortly thereafter (0310 UTC 29 September; Fig. A consensus forecast of Juan’s landfall was derived Ϫ 1) with sustained wind speeds estimated at 44 m s 1 and following discussions between forecasters at the NHC Ϫ gusts exceeding 51 m s 1. The impact of Juan’s landfall and the CHC, and was very accurate for both the timing was particularly significant because the storm’s eastern and the location of the event. Again, intensity forecasts eyewall hit the heavily populated Halifax area with the proved marginally problematic as the system was ex- strongest winds of the storm, along with a storm-total pected to weaken more rapidly than it did as it tracked rainfall of 38 mm at Halifax International Airport over cooler waters off the Nova Scotia coast. However, (CYHZ). skillful high wind and wave, heavy rain, and storm- surge warnings were issued by the CHC as much as 2 b. Operational forecasts days in advance of the system, and allowed high-risk The forecasts produced for Hurricane Juan by both areas to prepare for the storm’s landfall. Dropsondes the NHC and the CHC were of consistently high quality released from a Canadian research reconnaissance air- throughout the storm’s lifetime. Forecast track errors craft shortly before landfall recorded winds of over 50 by the NHC for Juan were 36% below 10-yr averages; msϪ1 (1 km above sea level), and Doppler radar re- however, intensity prediction errors exceeded (by 12%) trievals estimated wind speeds in excess of 58 m sϪ1 at means from the last decade (more information avail- the same altitude. Similar low-level jet structures were able online at http://www.nhc.noaa.gov/2003juan. observed during the ET of Hurricane Michael (Abra- shtml). Some of this reduced skill in the intensity fore- ham et al. 2002), and have been shown by Browning et casts may have been due to the hybrid nature of the al. (1998) to be associated with the decoupling of the system throughout a large portion of its life cycle. The tropical circulation from the stabilized boundary layer system’s rapid acceleration into an increasingly baro- resulting from passage over cooler SSTs. The increased

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FIG. 4. The CHC and Maritimes Weather Center manual analyses for (a) 0000 UTC 28 Sep and (b) 0000 UTC 29 Sep. Analyses of storm track, intensity, and sea level pressure structure (solid lines, plotted at 4-hPa intervals) were produced in real time during the event. Available observations are also plotted on the analyses in standard format. roughness length over land may have contributed to storm during its genesis phase may have limited long- enhanced downward mixing of this fast-moving air, range predictability as the developing system under- leading to the stronger-than-forecasted winds on land- went TT (see section 3 in Part I). The processes in- fall and to the gusts that caused significant damage in volved in TT (Bosart and Bartlo 1991; Bracken and localized areas. The overall quality of the warning, ad- Bosart 2000; Davis and Bosart 2001, 2002, 2003, 2004) visories, and forecasts made by both the NHC and the have received relatively little attention in the literature CHC for Hurricane Juan was commendable. until recently and yet remain just as complex as those associated with ET. Further difficulties were encoun- c. Numerical guidance tered as the storm accelerated poleward prior to land- fall. These challenges will be discussed in detail in the The quality of deterministic operational numerical following section. guidance for Hurricane Juan was mediocre compared with the skill shown by the operational forecasts. Of the global models to be included in this study, only one (the 3. Operational model guidance GFS) provided track guidance of higher quality than the operational 0–72-h forecasts over the storm’s life- The present work focuses on the later stages of time, with a mean error of 61 km compared with the Juan’s life cycle as the storm approached Nova Scotia operational mean forecast track error of 71 km. With as a category-2 hurricane. We will therefore restrict the mean 51-km, 0–3-day track errors, the limited-area remainder of our analysis of operational numerical pre- Geophysical Fluid Dynamics Laboratory’s (GFDL) dictions to model runs initialized at 0000 UTC 28 Sep- Hurricane Model (GHM) had the most success in pre- tember as the system accelerated northward approxi- dicting the track of the system. The hybrid nature of the mately 900 km south of Nova Scotia. The synoptic time

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TABLE 1. Description and reference list of primary model dynamical parameters for the forecasting systems investigated in this study (c. 2003).

Number of cycles Model Type Resolution (times) Vertical coordinate Time stepping Advection GFS Spectral T254L64 (ϳ0.5°) 4 (0000, 0600, 1200, and 64 Sigma (surface– Semi-implicit Leapfrog 1800 UTC) 0.27 hPa) GEM-G Grid point 0.9° 2 (0000 and 1200 UTC) 28 Eta (surface– Implicit Semi-Lagrangian 10 hPa) NOGAPS Spectral T239L30 (ϳ0.5°) 2 (0000 and 1200 UTC) 30 Sigma Semi-implicit Leapfrog UKMet Grid point 0.83° ϫ 0.56° 2 (0000 and 1200 UTC) 38 Sigma Semi-implicit Semi-Lagrangian Eta Grid point 12 km 4 (0000, 0600, 1200, and 60 Eta Euler backward Janjic 1800 UTC) GEM-R Stretched 24 km (central 2 (0000 and 1200 UTC) 58 Eta (surface– Semi-implicit Semi-Lagrangian grid window) 10 hPa) GHM Gridpoint 0.5° and 0.17° 4 (0000, 0600, 1200, and 42 Sigma Two-step Box method nested 1800 UTC) iterative MC2 Gridpoint 36 and 12 km 1 (0000 UTC) 25 Gal-Chen Semi-implicit Semi-Lagrangian nested

closest to landfall (0000 UTC 29 September) will be track and general characteristics of an identifiable used as the verification time, thereby focusing on the tropical vortex should be represented at least in the short-range (24 h) numerical guidance of the storm’s short range. Of the global modeling systems investi- structure and motion just prior to landfall (approxi- gated in this study, two successfully produced credible mately 0300 UTC 29 September). The deterministic op- predictions of a hurricane-like vortex (NOGAPS, erational forecast models whose guidance is presented UKMet), while two lost the tropical structure in favor in this paper are a subset of those that were available to of secondary cyclogenesis over the Gulf of Maine within NHC and CHC forecasters during the event and in- 24 h of forecast time (GFS, GEM-G). It is instructive to clude: the GFS, the Eta-coordinate (Eta), and GHM note that the vortex initialization schemes in both of the models run at NCEP; the Navy Operational Global At- successful models allow for a greater range of pertur- mospheric Prediction System (NOGAPS) run at the bation from the background (analysis) state than those Fleet Numerical Meteorology and Oceanography Cen- in the models that failed to track the hurricane. ter; the global and regional versions of the Global En- The primary attributes of the global models com- vironmental Multiscale model (GEM-G, GEM-R) run pared in this study are presented in Tables 1 and 2. A by the Canadian Meteorological Centre (CMC); the detailed description of the individual characteristics of U.K. Met Office (UKMO) model (UKMet); and the each of the models is beyond the scope of this work, Mesoscale Compressible Community (MC2) model run which itself focuses more on the influence of initial vor- by McGill University in support of a Canadian ET field tex representation than on model errors per se. Table 3 experiment under way in September 2003. Surface and thus provides a description of the vortex initialization near-surface fields are used in this study as indicators of algorithms used by each of the pertinent modeling sys- deterministic forecast quality. This is both because of tems in September 2003. their impact on the population and because they pro- The poor performance of the GFS guidance from vide the most direct avenue for comparison between 0000 UTC 28 September (Figs. 5a,b) is of particular the quality of the operational forecasts/warnings and interest, given the model’s overall success at predicting the numerical guidance. Juan’s track as noted in section 2c. Investigations of the GFS forecast cycles initiated earlier in Juan’s life cycle a. Global forecasting systems show that the GFS produces reliable guidance through- The ability of global forecast models to resolve hur- out the period of the storm’s slow northward propaga- ricane-like structures has increased rapidly over the last tion (1200 UTC 24 September–0000 UTC 28 Septem- decade owing to enhancements to horizontal resolu- ber). The quality of the forecasts over this period is tion, vortex initialization, and model physics. While it is attributable to efforts made by NCEP over the last de- unreasonable to expect that the structure or intensity of cade to improve the GFS’s ability to depict tropical a strong, meso-␤-scale feature such as Hurricane Juan structures (Lord 1991, 1993; Surgi et al. 1998). How- would be accurately depicted in global models, the ever, none of the GFS forecasts (including the 0000

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TABLE 2. Description and reference list of primary model physical parameters for the forecasting systems investigated in this study (c. 2003).

Boundary Horizontal Vertical Model Convection Microphysics layer diffusion diffusion Radiation GFS Grell (1993) Zhao and Carr Troen and Leith (1971) Troen and Mahrt Mlawer et al. (1997) Mahrt (1986) (1986) (1997); Hou et al. (2002) GEM-G Kuo (1965) Sundqvist (1978) Benoit et al. Second-order Benoit et al. Garand and (1989) Laplacian (1989) Mailhot (1990); Fouquart and Bonnel (1980) NOGAPS Emanuel (1991) Teixeira and Louis et al. Fourth-order Louis (1979) Harshvardhan Hogan (2001) (1982) Laplacian et al. (1987) UKMet Fritsch and Wilson and Lock (2003) Staniforth et al. Lock (2003) Edwards and Chappell Ballard (1999) (2003) Slingo (1996) (1980) Eta Janjic (1994) Rogers et al. Janjic (1994) Second-order Janjic (1994) Ferrier et al. (2001) Laplacian (2003) GEM-R Kain and Fritsch Sundqvist (1978) Benoit et al. Second-order Benoit et al. Garand and (1990) (1989) Laplacian (1989) Mailhot (1990); Fouquart and Bonnel (1980) GHM Kurihara (1973) Kurihara et al. Kurihara et al. Second-order Mellor and Schwarzkopf and (1998) (1990) Laplacian Yamada Fels (1991); (1974) Lacis and Hansen (1974) MC2 Kuo (1965) Kong and Yau Benoit et al. Sixth-order Benoit et al. Garand and (1997) (1989) Laplacian (1989) Mailhot (1990); Fouquart and Bonnel (1980)

UTC 28 September initialization studied here) accu- This failure mode will be investigated in detail in sec- rately predict Juan’s track or circulation as the system tion 3b. accelerates toward Nova Scotia (Fig. 5b). Both the NOGAPS (Figs. 5e,f) and the UKMet (Figs. A similar failure mode is present in the GEM-G 5g,h) models produce reasonable guidance for Juan’sac- guidance from the 0000 UTC 28 September initializa- celeration and landfall. The use of similar vortex initial- tion (Figs. 5c,d). Both the GEM-G and the GFS mod- ization algorithms (Table 3) in the two models results in els favor a solution characterized by an east–west- comparable, high-quality initial depictions of Hurricane oriented trough centered in the Gulf of Maine. Al- Juan (Figs. 5e,g). After 24 h, both models retain closed though this feature produces enhanced onshore flow circulations south of Nova Scotia and the UKMet model along the coast of Nova Scotia, no evidence of a closed registers a track error of just 35 km, less than the model lower-level circulation exists in the 24-h forecast fields. grid spacing (J. Heming 2004, personal communication).

TABLE 3. Description and reference list of vortex initialization techniques used in the forecasting systems investigated in this study (c. 2003).

Model Vortex initialization summary Reference GFS Relocation of analyzed vortex to NHC advisory location Moorshi et al. (2001) GEM-G None — NOGAPS Vortex construction using synthetic observations Goerss and Jeffries (1994) UKMet Vortex construction using synthetic observations Heming et al. (1995) Eta Variational data assimilation of artificial winds (now modified to analysis relocation) Rogers et al. (1998) GEM-R None — GHM 60-h axisymmetric spinup in a fully coupled environment Kurihara et al. (1993) MC2 None — Hindcast Static insertion of a fully-developed vortex Ueno (1995)

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FIG. 5. Selected global prediction system forecasts initialized at 0000 UTC 28 Sep 2003. Sea level pressure (solid lines; 4-hPa intervals) and winds (barbs; m sϪ1) are shown for (left) the initial state and (right) the 24-h forecasts valid 0000 UTC 29 Sep 2003. Model fields are indicated for the GFS: (a) 0000 and (b) 2400 UTC; GEM-G: (c) 0000 and (d) 2400 UTC; NOGAPS: (e) 0000 and (f) 2400 UTC; and UKMet: (g) 0000 and (h) 2400 UTC.

Unauthenticated | Downloaded 09/23/21 10:52 AM UTC JULY 2006 M C T AGGART-COWAN ET AL. 1757 b. Regional forecasting systems this failure for the global models, such error mecha- nisms should be minimized in these regional mesoscale With higher spatial resolutions and enhanced physi- models. However, a qualitative evaluation of the errors cal parameterizations, regional limited-area models are in these mesoscale forecasting systems shows that they expected to perform better for systems whose scales are at least as large as those of their global counter- and processes are ill-resolved by global models. How- parts. ever, regional models and analyses ultimately rely on The obvious exception to this observation is the global model data for boundary conditions, and are GHM (Figs. 6e,f). This model provides the highest- therefore heavily influenced by their global counter- quality guidance of the forecast models described in parts, especially at longer lead times. Even in the short this study. Using an advance vortex initialization range, a great degree of coupling can occur if the re- scheme (Ross and Kurihara 1992; Kurihara et al. 1993; gional model is initialized using global analysis data or Table 3), the scale and structure of Hurricane Juan at an analysis cycle initiated from global grids. 0000 UTC 28 September are accurately depicted in the Forecasts from regional models for Hurricane Juan model’s initial state with a central MSLP of 975 hPa and should be expected to show a realistic hurricane-like winds in excess of 45 m sϪ1. The position and intensity vortex whose near-surface intensity and structure par- of the system are well predicted at 0000 UTC 29 Sep- allels that of the observed storm, at least in short-range tember (Fig. 6f) as the GHM emerges as the only re- forecasts. Given the estimated 450-km radius of tropi- gional model to forecast an intense closed circulation cal storm force winds (0000 UTC 28 September; NHC), after 24 h of integration. even a model with 30-km grid spacing—quite large for current regional models—should show some predictive c. MC2 and GHM comparison skill for both general structure and intensity. Of the The similarity in the failure mode of both the global regional models investigated in this study, three fail to and the regional models that produce poor short-range accurately depict the nature of the lower-level flow in (24 h) guidance for Hurricane Juan suggests that there the short range (Eta, GEM-R, and MC2), and one is a fundamental process responsible for the degraded proves capable of providing accurate guidance to fore- forecasts. In the simplest case, it could be the incorrect casters (GHM). Of particular interest in the description translation of a weak initial vortex; however, as noted of these forecasting results is the important influence of in the introduction, such a purely linear process is un- near-vortex initial conditions. likely to be the sole source of error, given the complex A summary of the primary attributes of the regional interactions involved in hurricane propagation into the models compared in this study is also presented in midlatitudes. To address the question of why a strong, Tables 1 and 2. As noted in section 3a, the focus of this quasi-symmetric initial vortex appears to be so crucial study is on the evolution of the initialized vortex in each in this case—even at a latitude where the insertion of an modeling system rather than on the particulars of the essentially symmetric circulation may be question- model itself. A description of the vortex initialization able—two regional model forecasts are analyzed in de- method employed in each of the presented modeling tail in this section. The MC2 (Figs. 6g,h) is chosen to systems is also provided in Table 3. represent the failed set of models and the GHM (Figs. The poor performance of the Eta, GEM-R, and MC2 6e,f) is taken as the baseline measure of a successful forecasting systems is evident in Fig. 6. A comparison of forecast. the failed models’ initial vortex representations (Figs. 6a,c,g) shows persistently weak systems with minimum 1) COMPARATIVE SYNOPTIC ANALYSIS mean sea level pressures (MSLP) near 1008 hPa. Given the high horizontal resolution of the regional models The near-surface MSLP and wind fields shown in Fig. (Table 1), these initializations of Hurricane Juan do not 6e clearly show that the MC2 model is initialized with compare well with the NHC issued guidance of 969 hPa an unrealistically weak vortex. The MC2 enclosing (MSLP) with sustained winds of 45 m sϪ1. The failure 10-m horizontal wind circulation at 0000 UTC 28 Sep- mode of these models is similar to that described in tember reaches a minimum of just 5 m sϪ1 in the south- section 3a for the GFS and GEM-G global models. In ern quadrant of the storm compared with an essentially particular, the favored solution involves a zonal trough isotropic value of 15 m sϪ1 encircling the initialized anchored in the Gulf of Maine that produces little more GHM vortex. The three-dimensional structure of Hur- than 10 m sϪ1 of onshore flow for the Nova Scotia ricane Juan in the MC2 initialization (Fig. 7a) shows a coast. While coarse resolution and simplified physical similar lack of intensity throughout the lower tropo- parameterizations may explain a significant portion of sphere. A weak, amorphous 573-dam, 1000–500-hPa

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FIG. 6. Same as in Fig. 5, but for the regional prediction system forecasts. Model fields are indicated for Eta: (a) 0000 and (b) 2400 UTC; GEM-R: (c) 0000 and (d) 2400 UTC; GHM: (e) 0000 and (f) 2400 UTC; and MC2: (g) 0000 and (h) 2400 UTC. Minimum MSLP contours are 984 hPa in (e) and (f).

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FIG. 7. The MSLP (light gray contours at 4-hPa intervals), 1000–500-hPa thickness (black contours at 3-dam intervals), 250-hPa wind speed (shading in m sϪ1 as indicated on the color bar), and 500-hPa winds for the (a) 0-, (b) 6-, (c) 12-, (d) 18-, and (e) 24-h fore- casts from the MC2 model. Wind barbs are plotted in m sϪ1 with short, long, and flag pennants representing 2.5, 5, and 25 m sϪ1, respectively.

thickness maximum overlies the MSLP minimum and lower troposphere (Fig. 8a) in which the thickness the 500-hPa circulation appears to be centered 200 km anomaly is collocated with the MSLP minimum. A to the northwest of the surface low. In contrast, the 1000–500-hPa thickness maximum in excess of 579 dam GHM vortex representation is strong and upright in the is positioned directly above the lower-level center im-

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FIG. 8. Forecast fields for the GHM plotted the same as in Fig. 7. (a)–(e) Minimum MSLP values are approximately 984 hPa.

plying a mean warm-core perturbation of over 4 K rela- initial states is highly similar and suggests that the mid- tive to the surrounding environment. Aside from these latitude flow analyzed in section 2a and Part I is well important differences in the representation of the tropi- depicted in both models at 0000 UTC 28 September. cal vortex, the broad background structure of the two Over the first6hofforecast time, the MC2 storm

Unauthenticated | Downloaded 09/23/21 10:52 AM UTC JULY 2006 M C T AGGART-COWAN ET AL. 1761 appears to split along a northwest–southeast axis as the tion on the mean vector. The results presented here are weak thickness ridge associated with the system breaks insensitive to changes in the details of the steering flow apart (Fig. 7b), allowing the northwestern system to calculation. assume a more equivalent barotropic structure. This In both the MC2 and GHM forecasts, Hurricane feature progresses generally northward over the re- Juan is steered generally northward with the southerly mainder of the forecast period (Figs. 7c–e) with the component of the steering flow increasing monotoni- strength of the mean lower-tropospheric warm-core cally over time. The GHM vortex is well behaved anomaly remaining constant near 0.5 K. Enhanced throughout the forecast period (Figs. 9b,d,f,h,j), closely southwesterly winds associated with the approaching following the sense of the steering flow as it accelerates trough encroach upon the weak vortex center in the toward Nova Scotia. In contrast, the initially weak and 24-h forecast (Fig. 7e) and the system dissipates rapidly fragmented MC2 system—as evinced by the compli- thereafter (not shown). cated lower-level vorticity structure of Fig. 9a—propa- The intensity of the warm-core perturbation in the gates at a sharp angle to the deep-layer steering flow at GHM increases as convection is initiated in the full all times. Over the first 12 h of integration (Figs. 9a,c,e), three-dimensional model. After6hofforecast time the storm moves almost due westward despite the 5–10 (Fig. 8b), the lower-tropospheric warm anomaly msϪ1 southerly steering flow; thereafter, it takes up a reaches a maximum of 6 K and remains at that level for northeasterly track almost 45° to the right of the deep- the remainder of the forecast period (Figs. 8c–e). The layer mean winds (Figs. 9g,i). vortex core remains upright even as a baroclinic zone It is hypothesized that the complicated structure of develops in the path of the hurricane. The weak 1000– the MC2 vortex (Fig. 9) evolves rapidly over the fore- 500-hPa thickness gradient south of Nova Scotia at 0000 cast period and leads to precessions in the storm’s track UTC 28 September (Fig. 8a) is enhanced by the north- that disobey steering flow translation. The induction of ward transport of tropical air associated with Hurricane storm motion components beyond basic steering is to Juan. A developing near-surface front is reflected in the be expected in cases where multiple vorticity maxima thickness field and may be responsible for the shift in interact over the tracking period. precipitation to the northern quadrant of the storm prior to landfall (not shown). However, the GHM vor- tex has very little time to interact with the incipient 4. Secondary vortex removal study baroclinic zone as the approaching trough enhances the This section presents the results of a diagnostic study southerly steering flow over the storm (Fig. 8e). and sensitivity test designed to challenge the hypothesis 2) STEERING AND STRUCTURE advanced in the previous section, namely, that the ex- istence of a dual-vortex structure in the initial state of Despite the similar initial (0000 UTC 28 September) the MC2 model (Fig. 9a) leads to a complicated track near-surface vortex locations in the MC2 and GHM that does not consistently conform to steering flow ar- models shown in Figs. 6e,g, the 24-h forecast fields show guments. Satellite imagery for the period (Fig. 3) does Hurricane Juan in markedly different positions with not support the existence of a secondary vortex at 0000 MSLP minimum of the MC2 storm located 250 km to UTC 28 September. The component of the forecast the west of the GHM center. The work of Renard failure mode that is directly attributable to strong asym- (1968) and others suggests that the motion of a tropical metries in the initial vorticity field is analyzed using a cyclone is well described by a deep-layer mean wind modified set of initial conditions for the MC2 model (i.e., the steering flow). A direct comparison of the from which the spurious secondary vortex is removed. steering flow vector and track of Hurricane Juan in the The results from this forecast sensitivity test suggest MC2 and GHM forecasts is shown in Fig. 9. The depth that the symmetrized MC2 follows the steering-implied of the layer-averaged winds used to compute the steer- track more closely but that the weak, misplaced initial ing flow depends on the depth of the vortex, defined as TC still has a negative impact on predictions at 0000 the 950-hPa level to the top of the tropospheric poten- UTC 29 September. tial vorticity (PV) tower where the curvature of the near-core PV profile is minimized. The depth of the a. Vortex interaction layer used to compute the steering flow changes over time for each model and is indicated in Fig. 9. Winds in The lower-level vorticity field from the MC2 model is an annular ring with an inner radius of 350 km and an shown in Fig. 10. At 0000 UTC 28 September (Fig. 10a) outer radius of 600 km are averaged on an area- there is very little circulation around Juan (labeled “J”) weighted basis to remove any influence of map projec- as indicated by the layer-mean 925–850-hPa flow. In

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FIG. 9. Lower-level relative vorticity (925–850-hPa mean plotted with gray con- Ϫ Ϫ tours at 1 ϫ 10 4 s 1 intervals), steering vector over the storm center (heavy black arrow with magnitudes as indicated in the lower-right corner of each panel), and storm track (black line plotted for the full 24-h period in all panels) for the (a), (b) 0-, (c), (d) 6-, (e), (f) 12-, (g), (h) 18-, and (i), (j) 24-h forecasts of the MC2 and GHM models, respectively. The depth of the layer used to compute the steering vector is indicated and is computed as described in the text. OLUME 134 JULY 2006 M C T AGGART-COWAN ET AL. 1763

FIG. 10. Lower-level relative vorticity (925–850-hPa mean plotted at 0.25 ϫ 10Ϫ4 sϪ1 intervals above 1 ϫ 10Ϫ4 sϪ1 in black contours), mean precipitation rates for the previous 3 h (shaded as indicated on the grayscale bar), and mean 925–850- hPa winds for (a) 0-, (b) 6-, (c) 12-, (d) 18-, and (e) 24-h forecasts from the MC2 model. Wind barbs are plotted in msϪ1 with short, long, and flag pennants representing 2.5, 5, and 25 m sϪ1, respectively. Hurricane Juan is indicated with a J and the secondary vorticity maximum is labeled S. Maximum vorticity values vary between 3 ϫ 10Ϫ4 sϪ1 and 7 ϫ 10Ϫ4 sϪ1 for the MC2 and 12 ϫ 10Ϫ4 sϪ1 and 16 ϫ 10Ϫ4 sϪ1 for the GHM.

fact, the MSLP field shown in Fig. 6a is centered on the the secondary vorticity maximum disappears over the secondary (labeled “S”) vortex rather than on Juan it- first6hofforecast time (Fig. 10b) as the S vortex is self. This leads to the apparent northwestward vortex “strained out” from an elliptical structure to a thin strip tilt noted in section 3c(1). The weak circulation around (Fig. 10c). Simultaneously, the vortex strip wraps cy-

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the shearing of the S feature as it wraps around the primary circulation and the merger of the cores never occurs (Fig. 10). Important variables during binary vortex interaction include the relative magnitudes of the vortices, their sizes, and their separation distance. As shown in Fig. 11, the initial separation distance is approximately ␦ ϭ 450 km. The J and S features have maximum relative vor- ␨ ϭ ϫ Ϫ4 Ϫ1 ␨ ϭ ϫ Ϫ4 Ϫ1 ticities of J 3 10 s and S 2 10 s , and ϭ their equivalent radii are estimated at RJ 100 km and ϭ RS 50 km, respectively. Figure 3 of Prieto et al. (2003) summarizes interaction regimes based on initial param- eters of the vortices whose “1” and “2” subscripts cor- respond to the J and S subscripts used here. We note FIG. 11. Evolution of the separation distance between the pri- that Prieto et al. (2003) define ⌬ as the separation be- mary (J) and secondary (S) vortices as shown in Fig. 10 over the tween the vortex edges: 24-h forecast period. Upper and lower bounds correspond to the findings of Ritchie and Holland (1993) for critical TC vortex core R R ␨ ␨ 2 ϵ S ϭ 2 ϵ S Ӎ merger separations. 0.5, ␨ ␨ 0.6 R1 RJ 1 J and clonically around Juan’s vorticity maximum and inten- ⌬ ␦ Ϫ ͑ ϩ ͒ sifies as precipitation (shaded in Fig. 10) develops along RJ RS ϵ ϭ 3. the shear line. These wrapping and precipitation en- R1 RJ hancement processes continue over the latter half of This places the interaction in the elastic (EI) regime the simulation (Figs. 10d,e), resulting in the compli- where both features retain their identities as they rotate cated 0000 UTC 29 September structure noted in sec- around each other. However, after6hofintegration, tion 3c(2). ⌬/R begins to decrease dramatically (Fig. 11) and the The vortex interaction process described here has 1 interaction mechanism changes to “partial straining been investigated in detail through idealized simula- out” (PSO) as ⌬/R decreases to 2.5 (␦ ϭ 400 km). The tions and observational studies by Dritschel and Waugh 1 PSO regime is characterized by the cyclonic wrapping (1992), Ritchie and Holland (1993), Prieto et al. (2003), of a partially filamented secondary feature about the and others. These studies find a set of interaction re- primary vortex as illustrated in Fig. 2 of Prieto et al. gimes identifiable by the structures and tracks of the (2003). This idealized result obtained on an f plane with two vorticity components. The classification method no physical processes, agrees remarkably well with the suggested by Dritschel and Waugh (1992) and adopted structural evolution of the binary vortices in the failed by Prieto et al. (2003) will be used as a baseline for this MC2 forecast. investigation of Hurricane Juan’s structure in the MC2 In addition to structural evolution, binary interaction model. Of additional utility is the result of Ritchie and induces perturbations in the track of the individual el- Holland (1993) that suggests that a critical separation ements. Prieto et al. (2003) show that the tracks of the distance for the TC-like vortex core merger exists be- interacting vorticity centers do not bear a simple rela- tween 150 and 300 km. That study also notes that the tionship to the ␤ gyres solely responsible for TC motion straining out process can preclude the merger of vorti- in the absence of a mean flow. By extension, this sug- ces by relegating the weaker member to the outer gests that the departures from the implied steering flow bands of the stronger. Although the two vorticity track noted in the MC2 forecast likely result from the maxima in Fig. 10 do not represent independent TCs, interaction of the initialized vortical elements. they are individual vortical features that occupy a local environment characteristic of a tropical system. b. Vortex removal (MC2-VR) The separation distance between the J and S vortices in Fig. 10 is shown in Fig. 11. Initially over 400 km, the To investigate whether it is indeed a binary vortex distance between the features decreases rapidly be- interaction that erroneously complicates the track of tween the 6–12-h forecast times to below 200 km. In the storm in the MC2 forecast, a removal of the sec- agreement with the conclusion of Ritchie and Holland ondary (S) feature in Fig. 10a is accomplished using the (1993), the relative weakness of the S vortex results in piecewise PV inversion framework of Davis and Eman-

Unauthenticated | Downloaded 09/23/21 10:52 AM UTC JULY 2006 M C T AGGART-COWAN ET AL. 1765 uel (1991). This PV modification and removal method- (Tables 1 and 2). The initial state is the only component ology has been used extensively in previous studies of the system to be modified for the hindcast, which (Huo et al. 1998; Milbrandt and Yau 2001; McTaggart- would otherwise have generated fields identical to Cowan et al. 2001) as a method for creating dynamically those shown in Figs. 6g,h (reproduced for direct com- consistent perturbed states for sensitivity tests. The sec- parison as Figs. 13a,b). The modified initial conditions ondary maximum is removed in the area indicated by (0000 UTC 28 September, shown in Fig. 13c) are modi- the box in Fig. 10a. All of the PV anomaly relative to fied to include a synthetic hurricane-like vortex whose the zonal mean is removed from the selected areas at all intensity and position are defined in accordance with levels. The resulting lower-level vorticity field is shown NHC advisories. in Fig. 12a. Following this modification, the MC2 model Storm radius, maximum wind speed, and central is rerun (MC2-VR) in a configuration otherwise iden- pressure estimates are used to create the surface struc- tical to the operational model of September 2003 ture of the storm (Fujita 1952). In this algorithm, gra- (Tables 1 and 2). dient wind balance is used to calculate the pressure The strength and orientation of the steering flow re- profile along a series of radials from the hurricane cen- mains essentially unchanged between the MC2 forecast ter. The three-dimensional mass field is derived (Fig. 9) and the MC2-VR test (Fig. 12). However, the through an empirical formula suggested by Ueno vortex in the MC2-VR integration moves generally (1995) following the work of Frank (1977), and winds along the steering vector throughout the simulation are obtained using the gradient balance equation. The without the meridional jumps noted in the unmodified unmodified initial state is used to calculate asymmetries forecast. Initial departures from steering may result arising from environmental forcings, and the full syn- from the spinup of physical processes in the model. thetic vortex is inserted into a background field from Although a secondary vorticity maximum appears which the original (analysis) storm has been removed south of 35°N after6hofintegration (Fig. 12b), it does by repeated filter passes. Ueno (1995) has shown that a not appear to influence the structure or track of the similar vortex initialization technique increases the skill primary vortex. After 24 h of integration (Fig. 12e), of TC track predictions in the Japanese Global Spectral Juan lies near the mouth of the Bay of Fundy, Nova Model. Notwithstanding the static nature of the implan- Scotia, over 300 km northwest of its observed location. tation, as opposed to the dynamic system used in the In this sense, the removal of the secondary vortex does GHM, the resulting storm is remarkably realistic and is not improve the forecast guidance; however, the ab- in good balance in the model atmosphere. The latter is sence of significant track perturbations indicates than concluded based on the lack of spurious gravity wave an accurate forecast may be possible given a corrected activity and pressure oscillations at early times in the initial vortex location. hindcast. The 24-h hindcast near-surface fields shown in Fig. 13d clearly demonstrate the increased predictive skill of 5. Hindcasting study the properly initialized system. Both the track and the intensity of the hurricane are accurately represented in This section documents a modeling study undertaken the model, resulting in a storm with a central pressure in an effort to improve track and intensity forecasts in of 982 hPa and maximum winds of 28 m sϪ1 located the short-range numerical prediction of Hurricane Juan within 65 km of Juan’s best-track position at 0000 UTC in the MC2 modeling system. The removal of the spu- 29 September. The dramatic improvements in the vor- riously initialized secondary vortex in the previous sec- tex’s representation throughout the 24 h leading up to tion greatly reduces the structural error of the forecast the system’s landfall owe their existence entirely to the vorticity fields and eliminates the complicated interac- improved character and position of the tropical feature tion-induced track oscillations; however, the quality of in the initial state. the guidance is still low since the storm track is poorly predicted. This section shows that a strengthened and b. Analysis of hindcast improvements repositioned initial vortex corrects the bias in the track The representation of Hurricane Juan in the hindcast forecast for Hurricane Juan. demonstrates improvements in intensity, structure, and track over the operational MC2 storm. The intensity a. Hindcast initialization improvement is directly attributable to the enhanced A hindcasting study of the case is performed using vortex intensity in the initial state and shows clearly the MC2 system with model setup and boundary con- that a limited-area model—even one not specifically ditions identical to those of the real-time forecast tuned for hurricane simulation—is fully capable of re-

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FIG. 12. Lower-level relative vorticity (gray contours at 1 ϫ 10Ϫ4 sϪ1 intervals), steering vec- tor over the storm center (heavy black arrow with magnitudes as indicated in the lower-right corner of each panel), and storm track (black line plotted for the full 24-h period in all panels) for the (a) 0-, (b) 6-, (c) 12-, (d) 18-, and (e) 24-h forecasts of the MC2-VR forecast sensitivity test. The depth of the layer used to compute the steer- ing vector is indicated and is computed as de- scribed in section 3c(2). Maximum vorticity val- ues vary between (a) 2 ϫ 10Ϫ4 sϪ1 and (e) 5 ϫ 10Ϫ4 sϪ1. This figure compares directly with Fig. 9.

taining a strong tropical vortex over at least the short- the initialization procedure. Although a small region of range forecast period. The improved structure of the enhanced lower-level vorticity remains to the northwest hindcast storm is primarily a result of the de facto re- of Juan’s relocated center (Fig. 14a), it is quickly en- moval of asymmetries and the secondary vortex during veloped by the strong circulation of the implanted vor-

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FIG. 13. MC2 forecast and hindcast fields initialized at 0000 UTC 28 Sep 2003. Plotting is the same as in Fig. 5. Model fields are indicated for the MC2 forecast: (a) 0000 and (b) 2400 UTC; hindcast: (c) 0000 and (d) 2400 UTC. Minimum MSLP values are (c) 968 and (d) 984 hPa. This figure compares directly with Figs. 5 and 6. tex (Fig. 14b). As demonstrated in section 4, the re- resentation of a weak, low-resolution vortex modified moval of the secondary vortex is important in produc- by the data assimilation cycle in the initializing analysis. ing an accurate prediction of Juan’s structure at 0000 The repositioning of the vortex in the broad area of UTC 29 September (Fig. 14e). This symmetrization of southerly steering flow that characterizes the rear- the initial state was also shown to eliminate interaction- quarter wavelength of the ridge environment results in induced perturbations about the steering flow–implied a direct forecast improvement based solely on transla- trajectory. The hindcast storm holds closely to the track tional arguments. The initialization of fully mature prescribed by the steering flow throughout the 24-h moisture fields in the hindcast appears to reduce the forecast period (Fig. 14). The dramatic improvement in initial departures from steering noted in the analysis of the track of the hindcast storm over the MC2-VR sen- the MC2-VR test by reducing the model spinup time. sitivity test is attributable primarily to the southeast- ward relocation of the initial vortex. 6. Summary and discussion The repositioning of the vortex in the initial state leads directly to an improvement of the track of the Hurricane Juan’s landfall near Halifax, Nova Scotia, hindcast system. The storm center is moved by 5° south early on 29 September 2003 represents a rare event of and 5° east in accordance with NHC advisories (cf. Figs. a moderate (category 2) hurricane having significant 9a and 14a). This places the implanted vortex slightly impact at higher latitudes as a predominantly tropical east of the geographic centroid of the combined local feature. Although the lower-level precursors for the vorticity maxima of the MC2 forecast initialization (Fig. hurricane had their origins over Africa, baroclinicity 10a) and suggests that the individual vortex cores de- east of North America was responsible for the devel- scribed in section 4 may be a poor high-resolution rep- opment of a troposphere-deep disturbance. Following a

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FIG. 14. Lower-level relative vorticity, steering vector, and storm track plotted as for Fig. 12. Maximum vorticity values vary between 11 ϫ 10Ϫ4 sϪ1 and 15 ϫ 10Ϫ4 sϪ1. This figure compares directly with Figs. 9 and 12.

TT between 24 and 26 September, the storm began to strengthened by a highly amplified ridge over the west- move slowly northward on the west side of the Ber- ern North Atlantic Ocean and a trough approaching muda high, and intensified to a category-2 hurricane on from the west, Hurricane Juan made landfall near Hali- 27 September. Accelerating in southerly steering flow fax, Nova Scotia, at 0310 UTC 29 September. The

Unauthenticated | Downloaded 09/23/21 10:52 AM UTC JULY 2006 M C T AGGART-COWAN ET AL. 1769 storm produced 48 m sϪ1 sustained near-surface winds the vortices are found to undergo a PSO mode of in- and intense rains in and around the heavily populated teraction wherein the weaker vortex is stretched into a Halifax area. quasi-linear feature and wrapped cyclonically around Operational forecasts by both the NHC and the CHC the stronger circulation center. An MC2-based sensitiv- for hurricane-force winds, heavy rains, high waves, and ity test is undertaken in which the spurious secondary storm surge were of very high quality throughout Hur- vorticity center is removed. This test yields a greatly ricane Juan’s life cycle. Numerical guidance for the improved forecast of the storm’s structure after 24 h of storm was moderately accurate while Juan drifted integration and track guidance in which interaction- northward between 25 and 27 September; however, induced perturbations from basic steering are elimi- short-range (24 h) guidance prior to the hurricane’s nated. However, the low intensity and the misplace- landfall displayed a marked qualitative decline in pre- ment of the vortex in the initializing analysis remain dictive skill. issues in the prognostic fields. Enhancements to global prediction systems over the A vortex initialization algorithm is employed to re- last decade, including increases in resolution and im- position and strengthen the analyzed vortex. An MC2 provements to physical parameterizations, have greatly hindcast is produced using only data and information increased the ability of global models to predict both that was available in real time over the forecast period. the track and the intensity of TCs. Of the global models The intensity of Hurricane Juan is well represented in presented in this study, two (GFS, GEM-G) failed to the hindcast, showing that even a medium-resolution produce useful guidance for the track and lower-level (12 km) mesoscale model is capable of sustaining a circulation of the hurricane prior to landfall, and two strong, compact tropical feature at high latitudes given (NOGAPS, UKMet) produced qualitatively useful an appropriate initial state. The repositioning of the forecasts within the confines of global operational limi- vortex corrects the westward-biased track error and tations. Notable differences between the near-storm combines with the increased intensity to yield remark- initial states of the global models arose because of the able predictive improvements over the operational varying degrees of sophistication of operational vortex MC2 forecast. initialization schemes. Both successful global forecasts Realistic representations of the initial vortex struc- were produced by systems employing TC initialization ture and position are found to be of primary impor- techniques more advanced than those of the systems tance in the production of accurate numerical guidance that failed to accurately predict the hurricane’s track or for Hurricane Juan. To a lesser extent, the intensity of intensity. the storm plays a role in determining the quality of the Higher-resolution regional models are expected to numerical forecast. These results suggest that the use of predict meso-␤ features, such as the compact Hurricane vortex initialization algorithms may be just as impor- Juan, far better than their global counterparts. How- tant for higher-resolution limited-area models as for ever, of the operational regional models presented in their global counterparts, especially when initialization this paper, three (Eta, GEM-R, and MC2) failed to from coarse-analysis grids may facilitate the develop- provide reliable short-range near-surface guidance for ment of unrealistic vortex structures in mesoscale as- Juan’s landfall, while one (GHM) showed considerable similation and modeling systems. Furthermore, the util- predictive skill. Although the GHM is tuned specifi- ity of initialization schemes that implant quasi- cally for the prediction of hurricane-like features, it is symmetric systems may be extensible, and highly shown that the primary factor inducing the regional valuable, to tropical vortices entering the midlatitudes model forecast failures was the poor representation of under appropriate weak-flow regimes. the initial vortex at 0000 UTC 28 September, rather than limitations within the remaining models’ formula- Acknowledgments. The authors thank the many re- tions. To elucidate the key components of a successful searchers who contributed substantially to this work. In forecast, a representative member of the failed regional particular, Dr. Julian Heming of the UKMO, Dr. Rob- model set (the MC2) and the model with the highest ert Tuleya of GFDL, Dr. James Goerss of the U.S. level of predictive skill (the GHM) are analyzed and Navy, and Dr. Geoff DiMego of NCEP for the provi- contrasted. Three areas of concern are identified and sion of many of the forecast datasets presented in this addressed over the remainder of the study: structure, paper and for many discussions concerning operational track, and intensity. tropical vortex initialization. Also, the authors thank The initial structure of the MC2 vortex is shown to be Jim Abraham and Peter Bowyer of the MSC for their highly asymmetric, containing two independent vortical work in collecting, analyzing, and distributing the Ca- features. Through comparison with idealized studies, nadian data for Hurricane Juan. Three anonymous re-

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