Tropical cyclone track forecasting and small scale wind velocity field in the Eastern Caribbean. Case study : Dean (2007). Elizabeth Hicks(1), Constantin Pontikis(2) Ida Mitrani Arenal(3), Daniel Martínez Castro(3), Israel Borrajero Montejo(3)
(1) Université des Antilles et de la Guyane, UFR Sciences, 97159, Pointe-à-Pitre cedex, Guadeloupe (FWI). Phone : +590590483099, Fax : (2) ICERMAN, 23, Village Viva, 97190, 97190, Le Gosier. Phone : + 590590 909937, Fax : (3)Instituto de Meteorología (INSMET) , Apdo. 17032 ; Casablanca, Regla, C.P.11700, La Habana -17, CUBA . Phone: (537) 881 34 11 Fax: (537) 866.80.10.
Corresponding author : [email protected]
1. Introduction
During the hurricane season (July-November), the islands of the Caribbean archipelago are regularly concerned by tropical cyclones passing in their vicinity. In the case of impact on given stakes, the resulting damages on property and life depend partially upon the accuracy of the meteorological forecasts at island scales. Indeed, for a given island, the cyclonic crisis management and the mitigation decisions to be taken depend upon the accuracy of the “a priori” evaluation of the potentially expected damages. The latter are strongly related to the cyclonic passage effects (wind velocity, precipitation, etc). Such evaluations, implying the use of high resolution mesoscale models, could be very useful for decision makers and for crisis management. They are relatively scarce in the Caribbean. In an attempt to contribute to the hurricane trajectory and potential damage forecast efforts in the Caribbean, the Regional Authority of Guadeloupe, the "Université des Antilles et de la Guyane" and the "Instituto de Meteorologia (Cuba)" have developed a collaborative project (PREVIOS, 2009, http://www.previos.fr) financed by the European Community. The aim of this access free project was to create a framework consisting of meteorological mesoscale models, statistical means and vulnerability models for the prediction of hurricane paths, hurricane intensities and potential damages at the scale of the small Caribbean islands. The ensemble should constitute a research tool for improving the above mentioned forecast lacks. The work presented here concerns the use of the mesoscale model ARPS (Xue et al., 2000; 2001; 2003), in the above mentioned framework context, in order to obtain the cyclonic wind field at island scale. The methodology is illustrated on the example of hurricane Dean (2007). The example presents the results of a one way nesting numerical experiment allowing the prediction of both, the hurricane trajectory at a 5 km grid and the estimation of the horizontal wind velocities at an 800 m scale over the island of Martinique (French West Indies). Further, based on historical data, expressions relating the expected main residence damages in a given location to the predicted wind velocity have been developed for French Caribbean type of homes.
2. Model, numerical experiment and meteorological data description
The ARPS model is a three-dimensional, non hydrostatic, compressible model developed at the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma. It is formulated in generalized terrain-following coordinates. The latest available version of this model (arps5.2.12) was used in this work. Additional subroutines were implemented in order to extract the hurricane trajectory, following the minimum pressure value, and the field of the 3rd power of the horizontal wind velocity component that represents a characteristic index of the hurricane destruction potential. Dean (2007), a Capo Verde type tropical cyclone, was the strongest tropical cyclone of the Atlantic 2007 hurricane season. It formed on August 13, and followed a West-Northwest path from the Eastern Atlantic Ocean, passing through the channel separating Martinique from Saint Lucia in the Caribbean archipelago on August 17 as a category 1 hurricane, affecting Martinique (Franklin, 2008), rapidly strengthening on the same day and becoming a category 2 hurricane and finally reaching category 5 on 18 August, 24h later. The numerical simulation consisted of a one way nesting experiment starting on the 16 August 2007 (18:00Z) for a 12h forecast. In this experiment, the large domain extended over a (1000km x 500 km) surface, centered at 13.20° N and 58.00°W, with a grid spacing of 5km while the small domain extended over a (80km x 80km) surface with a grid spacing of 0.8 km. The latter was centered at 14.65° N and 61.00° W, thus containing entirely the island of Martinique. For both domains, a 35 vertical level structure with an altitude varying grid spacing was chosen. For turbulence, the 1.5 TKE formulation associated to the isotropic turbulence hypothesis for the large domain and to the anisotropic turbulence hypothesis for the small domain were selected. The Kain- Fritsch cumulus parameterization was used for the cumulus rain production. The hurricane best track (BT) provided by the NHC and the observations of pressure and horizontal wind velocity made by the regional Météo-France meteorological station in Martinique were used respectively in order to validate the model simulation results.
3. Numerical experiment results and discussion The study of Dean's trajectory started at 18:00 (Z) on the 16 August 2007 and finished at 06:00 (Z) on the 17 August 2007. This trajectory was determined by using the large domain ARPS outputs. According to the NHC BT, the corresponding initial and final positions of Dean were respectively (13.80°N, 55.50° W) and (14.30°N, 59.80 W). Dean's initial position, as determined by using the minimum pressure value of the GFS analysis was (14.20°N,54.88°W), while Dean's initial position, as determined by using the minimum pressure value of the ARPS' pressure fields, was (14.14°, 55.08°W). The small latitude-longitude differences may be attributed to resolution uncertainties related to the chosen grid spacing. Dean's initial and final positions, as extracted from the ARPS calculated pressure fields, are shown in Figs. 1(a,b). Table 1 presents Dean’s 3-hourly ARPS and BT positions for the studied time-laps. The agreement between these two series of values is fairly good, thus attesting for the acceptable predictive skill of ARPS for short time-scales. Note however that the ARPS determined track is slightly shifted towards the North in comparison to the BT. For the same time-laps, the meteorological situation in the neighborhood of Martinique was extracted from the ARPS-outputs of the small domain experiment. As an example, Figs. 2(a,b) present the horizontal wind velocity for selected locations in Martinique respectively on the 16/08 at 18:00 (Z) and on the 17/08 at 05:00 (Z). The wind velocities reported on Fig. 2b, characteristic of the minimum values of a category 1 hurricane, reveal that the model has fairly well represented the evolution of the cyclonic perturbation potential for the considered time-laps. This evolution, is illustrated in Table 2 that presents the calculated horizontal wind velocity on the 17 August (06:00 Z) for a 12h simulation and the maximum wind velocity developed during the numerical
(a)
(b)
Figures 1 (a, b): Dean's initial and final positions as determined from the large domain experiment.
Table 1: Dean’s positions (ARPS and Best Track) Time (Z) lat-ARPS lat-BT (Lat) lon-ARPS lon-BT (Lon) 16/08-18h00 14.14 13.80 0.34 -55.08 -55.50 0.42 16/08-21h00 14.14 14.00 0.14 -56.56 -56.50 -0.06 17/08-00h00 14.19 14.00 0.19 -57.86 -57.80 -0.06 17/08-03h00 14.19 14.10 0.09 -58.93 -58.70 -0.23 17/08-06h00 14.23 14.30 -0.07 -59.99 -59.80 -0.19
(a)
(b)
Figures 2(a,b) : ARPS horizontal wind velocity outputs for selected locations in Martinique (FSD : Fonds St Denis, LAT : La Trinité, LAM : Le Lamentin, LED : Le Diamant, LEV : Le Vauclin) on 16/08/2007 at 18:00 (Z) (a) and on 17/08/2007 at 05h00 (Z) (b). experiment time-laps, compared respectively to the corresponding observed values at 5 selected places for which meteorological wind observations were available. The differences between the ARPS wind velocity values and the observed values may be partly attributed to terrain differences related to the specific location of the wind velocity measuring devices. Further, for most of the selected places, the measured and calculated maximum horizontal wind velocities occur at 06:00 Z. Note, that the knowledge of the wind vulnerability of stakes allows, even with the above mentioned differences, the estimation of the potential damages due to the hurricane impact.
Table 2 : Comparison between observed and calculated horizontal velocities (maximum value between 18:00 (Z) on 16/08/2007 and 06h00 (Z) on 17/08/2007, and value at 00:06 (Z) on 17/08/2007).
obs -1 ARPS -1 obs -1 ARPS -1 Locality Vmax (ms ) Vmax (ms ) V06:00(Z) (ms ) V06:00(Z) (ms ) Le Diamant 32.0 27.40 32.0 23.40 Le Vauclin 35.3 29.00 35.3 29.00 Le Lamentin 29.0 31.90 24.0 31.90 La Trinité 29.0 34.80 29.0 34.80 Fonds St Denis 32.0 39.30 15.0 39.30
4. Vulnerability of main homes to the wind velocity
Since the knowledge of the wind vulnerability of stakes combined to the model wind field outputs at island scale could provide a good estimation of the potential damages due to the hurricane impact, we have studied the vulnerability of buildings (main homes) to the wind, for the French Caribbean type of homes, using historical data. The study was based on the wind damages registered on main homes during the passing of three cyclones over the island of Guadeloupe, each of them belonging to a different category on the Saffir–Simpson scale: Cléo ((1964), category 2), Inez ((1966), category 3) and Hugo ((1989), category 4). The latter is well documented (Hamparian, 1994; Pagney and Benito-Espinal, 1991) compared to the earlier events (P. Saffache et al., 2003). The goal of this study was to develop expressions of the percentage of severely damaged homes (50% or more of the home destroyed) in a given commune as a function of the cubic wind speed, taking into account the type of home (light structure and non-temporary material house) and its date of construction. The data necessary for this study was the inventory of the home damages in each commune after the three cyclonic events as well as the governmental census of the homes in each commune, according to their type and age, prior to each cyclonic event (INSEE, Guadeloupe). In the French Caribbean, the census occurs roughly every ten years and refers to four types of homes: shack, traditional cabin, wooden house and non-temporary material house (Fig.3).
Figure 3.The four types of French Caribbean homes as referred to in the governmental census. From left to right: shack, traditional cabin, wooden house and non-temporary material house.
An example of the analysis on hurricane Hugo (1989) data is shown hereafter. Hugo passed over Guadeloupe on the night of the 16 to 17 September 1989 and the diameter of its eye was 37 km. According to the NHC, the surface winds were around 220km.h-1 and strong winds blew for 8h. Wind instrumentation on the island collapsed and no wind data was available. Figure 4 shows the percentage of strongly damaged homes in a given commune as a function of the minimum distance “d” of the commune to the center of the eye trajectory , where the blue and red data points correspond respectively to the communes situated at the right and the left of the cyclone’s path. As expected, as the distance from the cyclone path increases and the winds weaken, the damages decrease. Nevertheless, there is a great dispersion in the data points for 0 100 80 60 40 %logements(CR)sinistrés 20 0 0 10 20 30 40 50 d (km) Figure 4. Hugo (1989): Percentage of severely damaged homes in a given commune as a function of the minimum distance “d” of the commune to the center of the eye trajectory. Blue and red data points correspond respectively to communes situated at the right and the left of the cyclone’s path. Figures 5(a,b) show respectively the percentage of the damaged light structure homes (shacks, traditional cabins, wooden houses) and non temporary material houses as a function of the minimum distance of the commune to the center of the eye trajectory. For communes at the right of the cyclone path, severe damages affected 100% of the light structure homes and 27% of the non temporary material houses. The role of the age of the houses (constructions prior to 1975 and after 1982) on the damages was also studied, and no singular trend appeared. High percentages of damaged homes were observed mainly in communes with a high percentage of recent constructions. Thus, the age factor doesn’t seem to be the essential one for the estimation of the vulnerability of constructions to the wind and, up to now, it hasn’t been taken into account in our expressions. The results of the study of the home damages for the three cyclones suggest that in the case of wind speeds corresponding to the maximum value of: - a class 4 hurricane, 100% of the light structure homes and 27% of the non-temporary material houses are severely damaged, - a class 3 hurricane, 100% of the light structure homes are severely damaged, - a class 2 hurricane, 100% of the shacks and traditional cabins are severely damaged as well as 50% of the wooden houses, - a class 1 hurricane, 100% of the shacks are severely damaged. %precdetSud/Pprec %precdetN/parcprec90 Communes destr Hamparian 120 100 80 60 40 % logements% non durs très endommagés %durdetSud/Pdur 20 %durdetN/parcdur90Communes destr Hamparian 0 5 10 15 20 d (km) 30 25 20 15 10 %logementsen dur très endommagés 5 0 0 5 10 15 20 d (km) Figure 5 (a,b). Percentage of severely damaged light structured homes (a) and non temporary material homes (b) in a given commune as a function of the minimum distance “d” of the commune to the center of the eye trajectory. The damages for a class 1 hurricane were estimated from Inez (1966) data. At 30km south of Inez’ eye trajectory, where the measured winds were in the range of class 1 values, 7% of the homes were reported damaged. As the percentage of shacks in the corresponding communes was also of 7% in average, we made the simplified assumption that all the shacks were damaged. Thus, four different expressions (Previos, 2009), each of them corresponding respectively to the given wind speed range of the first four hurricane classes on the Saffir –Simpson scale, have been proposed (Table 3). Table 3. Percentage of damaged homes (at least 50% destroyed) in a given commune as a function of wind speed. The parameters h,c,b,d represent respectively the percentage of shacks, cabins, wooden houses and non temporary material houses in a given commune. % of homes with severe Wind speed parameters damages 3 33 m.s-1 -1 -1 3 49.3m.s -1 -1 3 58.2 ms Such expressions need to be improved with the data of new cyclone cases in the French Caribbean. Dean (2007) provides the occasion to verify the home damage expectancy with class 1 wind velocities and to suggest improvements of the corresponding expression. The ARPS maximum wind outputs were consistent with class 1 wind values for two communes, Fonds Saint Denis and La Trinité. The 2006 Martinique census data (INSEE Martinique) was used and the calculated percentages of homes severely damaged in Fonds St Denis and La Trinité were respectively 0.37% ( 2 homes) and 0.043% (3 homes). In reality, the town-hall of Fonds St Denis and La Trinité received respectively 57 claims, 21 of which corresponded to homes severely damaged or destroyed (corresponding to 6% of the homes in the commune) and 298 claims, 17 of which corresponded to homes severely damaged or destroyed (corresponding to 0.3% of the homes in the commune). The observation values are 6 to 10 times higher than the calculated ones. A partial reason for these differences comes from the fact that the altitude of a commune such as Fonds Saint Denis ranges from 130m to 1160m and that there was surely a wide range of wind speeds over the commune, many values exceeding the calculated value at the geographical coordinates of the town hall. Also, a better understanding of these differences would require more investigations such as the analysis of the damages by type of homes in the selected communes. Note that in other communes, such as Le Lamentin, with maximum calculated and observed wind speeds below class 1 minimum value (32 m.s-1), homes sustained important damages. For example, 3.5% of the homes in Le Lamentin were severely damaged, thus suggesting the necessity of an additional empirical expression for tropical depression wind speeds. The number of communes studied in the case of Dean is too small to be able to suggest at this stage any improvement of the proposed expressions. Nevertheless, the introduction in these expressions of a coefficient for communes in a given area, coefficient that is characteristic of the wind accelerations due to the topography of the area, seems necessary. Further, such empirical expressions require a regular up-dating of the distribution of homes, according to the four different type of homes, in each commune and a sufficient number of cyclonic events, the construction standards improving. Note that the above methodology may be used to study the impact of hurricanes on other stakes. 4. Conclusion The mesoscale model ARPS was used in order to determine the wind field over the island of Martinique (French West Indies) during the passage of hurricane Dean (2007). The results of this numerical experiment represent correctly (though with some shift) the trajectory followed by Dean in the time laps between 16 August (18h00 Z) and 17 August (06h00 Z). The observed shift could originate from the errors associated to the initial and boundary conditions extracted from the corresponding GFS outputs. Further tests seem necessary in order to investigate the influence of these conditions on the results of the small scale (800 m grid spacing) numerical experiment. The wind velocities extracted from the numerical experiment outputs are in fair agreement with the observed wind velocities. The differences may be partly attributed to the specific terrain characteristics of the meteorological stations where wind velocities were measured and partly to the impact, at small scales, of the errors associated to the GFS outputs used for the extraction of the initial and boundary conditions for the numerical experiment. Further, a study of the vulnerability of main homes to the wind velocity showed that, even at the present stage of this work, the small scale experiment results confirm that the use of mesoscale models may be useful for decision makers during the cyclonic crisis management. Acknowledgments. The funds for this study were obtained from the Interreg IIIB program (Région Guadeloupe-European Commission) and the authors wish to thank the regional and European authorities. They also wish to thank Météo-France (Service de Climatologie du Lamentin, Martinique) for the meteorological data used in this study and the town halls of La Trinité and Fonds Saint Denis (Martinique) for the data relative to the Dean wind damages on main homes. References Franklin, J. L., 2008 : Hurricane Dean 13-23 August 2007”. Tropical Cyclone Report, National Hurricane Center, http://www.nhc.gov. Previos, 2009 : Prévision des trajectoires, de l'évolution du potentiel dynamique et de l'impact des ouragans à l'échelle des îles de la Caraïbe, European Community Report, INTERREG IIIB Espace Caraïbe, pp. 302. Xue M., K. K. Droegemeier, V. Wong, A. Shapiro, K. Brewster, F. Carr, D. Weber, Y. Liu, D. Wang, 2000 : The Advanced Regional Prediction System (ARPS)- A multi-scale nonhydrostatic atmospheric simulation and prediction tool. Part I: Model dynamics and verification, M. Xue, K.K. Droegemeier, V. Wong. Xue M., K. K. Droegemeier, V. Wong, A. Shapiro, K. Brewster, F. Carr, D. 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