IMPACT ON HURRICANE TRACK AND INTENSITY FORECASTS OF GPS DROPWINDSONDE OBSERVATIONS FROM THE SYNOPTIC SURVEILLANCE MISSION INTO HURRICANE GEORGES ON 23 SEPTEMBER, 1998.

Sim D. Aberson

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1. Synoptic situation

Hurricane Georges developed from an easterly wave that moved off the coast of Africa 13 September, and developed into a tropical depression two days later. The system moved westward, strengthening rapidly into a hurricane by 17 September. Due to the potential threat to Florida, a synoptic surveillance mission was tasked for nominal time 24 September 1998 0000 UTC, with a follow-on mission the next day. At that time, Georges was moving west-northwestward and was located over Eastern Cuba, to the south the subtropical ridge axis (Figure 1). A cold low was located in the Caribbean Sea to the south of western Cuba. The weak circulation to the northeast of Bermuda was a subtropical system which eventually developed into Tropical Storm Karl.

2. General Assessment of dropwindsonde impact

A. GFDL model

Figure 2 shows the GFDL forecast tracks for Hurricane Georges, and Table 1 shows the errors and impact of the synoptic surveillance mission. The results are positive except at 120 h.

Georges made landfall twice during the forecast. The first was 39.5 h into the forecast near Key West, FL. The forecast without the dropwindsonde data forecast landfall 43 h into the forecast at Marathon, FL, 84 km away. The forecast with the dropwindsondes forecast landfall 44.5 h into the forecast at Big Coppitt Key, FL, 18 km away.

The second landfall was 107.5 h into the forecast at Biloxi, MS. The forecast without the dropwindsonde data forecast landfall 96 h into the forecast at Indian Pass, FL, 345 km away. The forecast with the dropwindsonde data forecast landfall 96 h into the forecast at Laguna Beach, FL, 279 km away.

B. VICBAR

Figure 3 shows the VICBAR forecast tracks for Hurricane Georges, and Table 2 shows the errors and impact of the synoptic surveillance mission. The dropwindsonde data improved the forecasts at all times.

Georges made landfall twice during the forecast. The first was 39.5 h into the forecast near Key West, FL. The forecast without the dropwindsonde data forecast landfall at Lower Matecumbe Beach, FL, 116 km away. The forecast with the dropwindsonde data forecast landfall at Marathon, FL, 84 km away.

The second landfall was 107.5 h into the forecast at Biloxi, MS. The forecast without the dropwindsonde data forecast landfall 102 h into the forecast at Saint George Island, FL, 402 km away. The forecast with the dropwindsonde data forecast landfall 100 h into the forecast at Saint Vincent Island, FL, 376 km away.

C. GSM

Figure 4 shows the GSM forecast tracks for Hurricane Georges, and Table 3 shows the errors and impact of the synoptic surveillance mission. The dropwindsonde data improved the GSM forecast track at all forecast times.

Georges made landfall twice during the forecast. The first was 39.5 h into the forecast near Key West, FL. The forecast without the dropwindsonde data forecast landfall at Dynamite Docks, FL, 42.5 h into the forecast 175 km away. The forecast with the dropwindsonde data forecast landfall at Tavernier, FL, 41 h into the forecast 142 km away.

The second landfall was 107.5 h into the forecast at Biloxi, MS. The forecast without the dropwindsonde data forecast landfall 85 h into the forecast at Live Oak Island, FL, 443 km away. The forecast with the dropwindsonde data forecast landfall 100 h into the forecast at Saint Vincent Island, FL, 376 km away.

D. Intensity

Table 4 shows the GFDL intensity forecast errors and impact of the synoptic surveillance mission. The dropwindsonde data had a positive impact at all times except 12, 84 and 96 h. Tuleya and Lord (1997) also showed modest improvements to GFDL model intensity forecasts in the HRD synoptic flow cases.

3. Targeting

The atmosphere has long been recognized as a chaotic system (Lorenz 1963), e.g. very small perturbations to initial conditions result in increasingly large differences in the evolution of the atmosphere with time. Since the exact state of the atmosphere can never be measured, all analyses contain errors whose magnitudes can only be estimated. An indeterminate number of initial conditions consistent with the observational data can therefore be used in numerical weather prediction, and single model runs at any synoptic time only give one possible solution to the evolution of the atmosphere. Many operational forecast centers around the world, therefore, now employ ensemble forecasting as a means of quantifying the uncertainty in the evolution of the atmospheric system. Small perturbations from a best "control" state are calculated and added to and subtracted from this control to allow for different integrations starting from theoretically equally likely initial states. These perturbations are designed so as to mimic the fastest growing modes in the model and to create the largest envelope of possibilities in the forecast. Therefore, they generally correspond to locations where large analysis errors will most impact the forecast. Those features corresponding to perturbations which most impact the tracks of tropical cyclones must be found, properly sampled, and thoroughly tested, to prove the efficacy of targeting techniques.

Figure 5 shows the variance of the size of the perturbations in the National Centers for Environmental Prediction (NCEP) global model ensemble forecasting system (Toth and Kalnay 1993). The largest perturbations are associated with Hurricane Georges, and with the trailing ridge extending from northeast to southwest across the Mona Passage. Another large perturbation is centered over Nova Scotia, and is associated with a vorticity maximum in the midlatitude westerly flow.

Figure 6 shows the difference in the 850 - 200 hPa averaged winds between the runs in which all the dropwindsonde data and none of the dropwindsonde data are included. The largest differences form a semicircle around the western side of Hurricane Georges. The maximum difference maximum is located to the north of George, and occurs between locations where dropwindsonde data were obtained, and this region also extends eastward a considerable distance away from the flight pattern. The difference maximum to the southwest of George also extends more than 500 km from the location of dropwindsonde data. The third maximum over the northwestern Caribbean Sea and southeastern Gulf of Mexico is associated with the cold low over the region, and here again the difference extends almost 500 km away from the nearest dropwindsonde location. Since some of the impact extends more than 500 km away from the location of dropwindsonde observations, and since the maximum difference is between dropwindsonde observations, the data assimilation may have spread the data from the observation locations into surrounding data-void regions.

Figure 7 shows that, by 24 h into the forecast, the difference maximum between the forecasts with and without the dropwindsonde data originally to the north of Georges in an area of large ensemble spread has held about steady. The maximum originally to the southwest of Georges, in an area of smaller ensemble spread, has and also remained at the same aplitude. The maximum further to the west associated with the cold low, also in an area of relatively small perturbation, has moved westward and also held steady. These results generally confirm that the largest (smallest) ensemble perturbations correspond to amplifying (decaying) modes in the model.

4. Conclusion

The dropwindsonde data obtained during the synoptic surveillance mission for Hurricane Georges at nominal time 24 September 1998 0000 UTC has provided mainly positive results throughout the five day forecast of Hurricane Georges in all three models examined.