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 Unauthenticated | Downloaded 09/23/21 10:52 AM UTC MWR3143 JULY 2006 M C T AGGART-COWAN ET AL. 1749 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;

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