IMPACT ON HURRICANE TRACK AND INTENSITY FORECASTS OF GPS DROPWINDSONDE OBSERVATIONS FROM THE SYNOPTIC SURVEILLANCE MISSION INTO TROPICAL STORM CLAUDETTE ON 14 JULY, 1997.

Sim D. Aberson

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

Click here for catalog of sonde drops.
Click here for zipped post-processed sonde data.

1. Synoptic situation

Tropical Storm Claudette developed from an extratropical low pressure system off the southeastern United States coast on 13 July, and moved northward, strengthening slowly. Due to the potential threat to the United States coastline from North Carolina to New England, a synoptic surveillance mission was tasked for nominal time 15 July 1997 0000 UTC. At that time, Claudette was located between the subtropical ridge and a deep-layer cold-core low located over northern Florida (Fig. 1). A shortwave trough moving eastward through the midwestern states eventually steered Claudette toward the east away from land, and increased the shear leading to dissipating by 36 h.

2. General Assessment of dropwindsonde impact

A. GFDL model

Figure 2 shows the GFDL forecast tracks for Tropical Storm Claudette, and Table 1 shows the errors and impact of the synoptic surveillance mission. The dropwindsonde data led to substantial improvements throughout the forecast. The upper-tropospheric data degraded the forecasts after 12 h. The model was only able to track Claudette for 36 - 48 h; Claudette dissipated 42 h into the forecast.

B. VICBAR

Figure 3 shows the VICBAR forecast tracks for Tropical Storm Claudette, and Table 2 shows the errors and impact of the synoptic surveillance mission. The dropwindsondes had a negative impact on the VICBAR forecast track, though the upper-tropospheric data had only slight impact on the forecast. The degradation can be expected in cases in which the tropical cyclone is weakening due to strong vertical shear, since the tropical storm likely is advected by the mid- or lower-layer mean flow than the deep-layer-mean of the VICBAR model.

C. GSM

Figure 4 shows the GSM forecast tracks for Tropical Storm Claudette, and Table 3 shows the errors and impact of the synoptic surveillance mission. The dropwindsondes had a positive impact on the GSM forecast track, and the upper-tropospheric data had no effect. Note that the model was only able to track Claudette for 12 h, whereas Claudette did not dissipate until 42 h into the forecast.

D. Intensity

Table 4 shows the GFDL intensity forecast errors and impact of the synoptic surveillance mission. The dropwindsondes had a positive impact only at 24 h, and the upper-tropospheric data degraded the forecast. 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 (Lorenz1963), 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 large perturbation off the United States east coast corresponds to a deep-layer trough in that vicinity. Due to logistical constraints, only the part of this feature closest to Claudette was adequately sampled during the mission. However, with rawinsonde data over New England and the Canadian maritime provinces, the region was well-sampled.

Two sets of model runs have been performed. The first, the TG run, includes only the dropwindsonde data taken within and around the trough (the eighteen dropwindsondes extending from near 38°N 67°W to 36°N 65°W, or about one-third of the entire mission), and the other, the NT run, includes the complement of the first. The great number of dropwindsonde observations is due to a large number of failures during the first mission with the new dropwindsondes. Results are shown in Tables 1-4 and Figs. 2-4. The TG run provides better track forecasts at all forecast times in the VBAR and GFDL models than the runs with either all the dropwindsonde data or none of the dropwindsonde data included. However, the TG runs provided degraded intensity forecasts in the GSM.

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 are to the east and north of Claudette, in an area of relatively small ensemble perturbation (Fig. 5), with another large difference in an area of large ensemble perturbation to the southeast of Nova Scotia. However, by 36 h (Fig. 7), all the differences have decayed except the perturbation off Nova Scotia which has sped to the east in the jet stream flow. This confirms that the largest (smallest) ensemble perturbations correspond to amplifying (decaying) modes in the model. Therefore, the dropwindsondes surrounding the trough are expected to make the largest impact on the model forecast. However, since the information from these dropwindsondes was advected quickly away from Claudette, they had little positive impact on the track forecast. The large initial condition differences around Claudette, despite decaying, seems to have pushed the forecast slightly to the north, making the forecast slightly worse than the forecast without those dropwindsonde data. The differences, however, are small.

4. Conclusion

The dropwindsonde data obtained during the synoptic surveillance mission for Tropical Storm Claudette at nominal time 15 July 1997 0000 UTC has provided mixed results. The GSM and GFDL forecasts were substantially improved, whereas the VBAR forecast was degraded, by the dropwindsonde data. The MRF ensemble forecasting system suggested that data obtained in and around the trough off the United States east coast to the northeast of Claudette would be the most important in the subsequent forecast. However, data obtained in this region propagated quickly to the east and had little impact on the forecast. Data obtained around Claudette pushed the forecast slightly to the north causing a small degradation to the forecasts.

Tables :