IMPACT ON HURRICANE TRACK AND INTENSITY FORECASTS OF GPS DROPWINDSONDE OBSERVATIONS FROM THE SYNOPTIC SURVEILLANCE MISSION INTO TROPICAL STORM ERIKA ON 05 SEPTEMBER, 1997

Sim D. Aberson

Hurricane Research Division
Atlantic Oceanographic and Meteorological Laboratories
National Oceanic and Atmospheric Administration

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1. Synoptic situation

Tropical Storm Erika developed from an easterly wave south of the Cape Verde Islands on 30 August,1997, and moved westward, strengthening slowly. Due to the potential threat to the Virgin Islands and Puerto Rico, a synoptic surveillance mission was tasked for nominal time 05 September 1997 0000 UTC. At that time, Erika was embedded in the westerlies to the south of the subtropical ridge, about 500 km east of Martinique. A strong trough was located along the U. S. east coast, which could impact the forecast later in the period (Fig.1).

2. General Assessment of dropwindsonde impact

A. GFDL model

Figure 2 shows the GFDL forecast tracks for Tropical Storm Erika, and Table 1 shows the errors and impact of the synoptic surveillance mission. The results are mixed, though early forecasts are generally improved, and later forecasts are degraded. The dropwindsonde data caused the forecast track to be further south, away from the best track, and slightly slower, allowing for forecast improvement, than the track without the dropwindsonde data. The upper-tropospheric data improved the forecast at all except for 24 h.

B. VICBAR

Figure 3 shows the VICBAR forecast tracks for Tropical Storm Erika, and Table 2 shows the errors and impact of the synoptic surveillance mission. The dropwindsonde data improved the forecast at all times, and the upper-tropospheric data improved the forecast at all times except 48h.

C. GSM

Figure 4 shows the GSM forecast tracks for Tropical Storm Erika, and Table 3 shows the errors and impact of the synoptic surveillance mission. The dropwindsonde data had a negative impact on the GSM forecast track. The dropwindsonde data pushed the forecast further to the south, away from the best track. The upper-tropospheric data had little impact on the forecast track.

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 12 and 84 h, and the upper-tropospheric data improved the forecast at 24, 84, and 120 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 correspond to the trough exiting the U.S. east coast. A small perturbation corresponds to the circulation of Erika, and a series of perturbations extending from east to northwest of Erika pertain to the axis of the subtropical ridge. The only one of these features which was sampled during the synoptic surveillance mission was the weak one furthest west in the series representing the subtropical ridge.

Two sets of model runs have been performed. The first, the TG run, includes the dropwindsonde data taken within and around the westernmost perturbation in the subtropical ridge (the five dropwindsondes from about 28°N 67°W to 33°N 63°W, and the dropwindsonde near 18°N 63°W), or about one-fifth of the total dropwindsondes released. The other, the NT run, includes the complement of the first set. Results are shown in Tables 1-4 and Figs. 2-4. The TG run provided better forecasts than the run including all the dropwindsonde data at all forecast times in the GSM. The TG run provided better forecasts than the run including all the dropwindsonde data at 24 h and from 72 h onward in GFDL, and from 72 to 96 h in VBAR.

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 difference is off the southeastern U. S. coast in the trough. Other large differences are in the target region described above, and just to the west of a large ensemble perturbation to the northeast of Erika. The latter difference extends southward from the dropwindsonde locations, and this southern extension helps to explain the degradation of the GSM forecast with the dropwindsonde data. However, these areas are in regions in which the ensemble perturbations are small and therefore the differences are expected to decay. Figure 7 shows that, by 24 h into the forecast, the differences between the forecasts with and without the dropwindsonde data in the two western regions have slowly amplified. The largest perturbation has moved rapidly toward the northeast on the eastern side of the trough, and does not impact the track of Erika. The easternmost difference has decayed, though the eastern edge of this difference, that part nearest the large ensemble perturbation originally to the northeast of Erika, has maintained itself and moved slowly eastward. The target perturbation has remained nearly stationary to the northeast of Hispaniola. These results seem to confirm that the largest (smallest) ensemble perturbations correspond to amplifying (decaying) modes in the model. The dropwindsondes surrounding the ensemble perturbations near the western extent of the subtropical ridge is the only large perturbation well-sampled during this synoptic surveillance mission, and is expected to have the largest positive impact on the model forecasts. The results of the TG model runs confirm this. The southern extension of the dropwindsonde data impact near Erika, despite being in areas in which the impact decays, pushed the Erika forecast to the south, ultimately degrading the forecast.

4. Conclusion

The dropwindsonde data obtained during the synoptic surveillance mission for Tropical Storm Erika at nominal time 05 September 1997 0000 UTC has provided mixed results. The MRF ensemble forecasting system suggests, and the model runs confirm, that data near the western edge of the subtropical ridge north of Puerto Rico would have the greatest positive impact on the Erika forecast. Dropwindsonde data obtained to the north of Erika was spread by the data assimilation to the south, pushing the forecasts with all dropwindsondes erroneously toward the south. Forecasts using only the data around the western edge of the subtropical ridge provide better forecasts at almost all times than those including all the dropwindsonde data.

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