Hurricane Felix was the first major hurricane of the very active 1995 hurricane season. The system initially formed near the Cape Verde Islands reaching tropical storm strength of August 8th and steadily moved in a west-northwest during the next four days as it intensified. It reached maximum intensity on August 13th, 0000 UTC, with maximum winds of 59 m/s and minimum sea level pressure of 930 hPa. Felix turned to the north-northwest on August 13 and then almost due north, weakening 35 hPa during the next 2 days. The first forecast began on August 13th, 0000 UTC, at the time of maximum intensity, with the second forecast beginning 24h later. Both tracks were very well forecasted Fig. 1 with the storm speed also very well reproduced in both forecasts. During this period, the slow northward movement of Felix caused a very large cold wake to form to the right of the storm track Fig. 2. In the forecast starting from the Aug 14, 0Z initial time, the maximum SST cooling was about 5°C (23.2°C minimum SST) centered at about 26°N, with a maximum SST cooling at nearly 6°C (22°C) centered at 27°N 24h later.
During August 15th, the center of Hurricane Felix passed within 140 miles southwest of the island of Bermuda, also passing within 65 km of the Bermuda Testbed Mooring located at 31° 43.4'N and 64° 10.1'W (Dickey et al, 1997). Since both the track and speed were well forecasted as the storm passed this location, these measurement provided an excellent opportunity to verify the predicted SST from the coupled model integration against observed values. In the vicinity of Bermuda, Felix was a category 1 on the Saffir-Simpson scale with wind speed peaked over the Testbed Mooring at about 37 m/s. The storm was moving northwestward in a nearly straight line path at about 7 m/s. The temperature at 25 m decreased by 3.5-4.0 °C at the mooring location and well correlate with satellite SST (high-resolution AVHRR) maps (Nelson, 1997) that show a swath of cool water (by ~3.5 to 4.0°C of 400 km wide) left by Felix. These observations also well agree with the coupled model prediction shown in Fig. 2. More detailed comparisons of the model and AVHRR derived SSTs along the 31.75°N transaction Fig. 3 indicate that both the prestorm NCEP SSTs at August 14 and in the cold wake on August 17 are in a good agreement. Minimum of the model predicted SST in the cold wake is about 25.5°C and only 0.5°C less than in the observations. Both model and observations display marked asymmetry of the cold wake relative to the storm track as the position of maximum cooling is biased or displaced to the right of the storm pass. This rightward bias is primary related to the wind stress vector rotation which is effectively clockwise on the right side of tracks and often in near resonance with the inertially rotating hurricane induced surface currents. The level of asymmetry is therefore largely dependent on the time scale over which the ocean experiences the hurricane's wind stress. The later is controlled by the storm's forward speed and horizontal wind structure. The position of maximum cooling in the model simulated SST is displaced slightly more to right of the storm pass as compared to observations. Since the forward speed is predicted rather well by the coupled model, these differences may be related to the differences between the predicted and real surface winds.
The resulting change in the storm intensity is shown next Fig. 4. The model resolution could not adequately reproduce the inner core structure of Felix during its intense period, and the initial minimum sea-level pressure in the first forecast was about 17 hPa greater than observed. Similarly, an initial weakening of nearly 10 hPa occurred in the forecast starting on Aug 14, 0000 UTC, even without coupling included. After a several hour period of adjustment, in both uncoupled forecasts, the storms subsequently began to intensify during the next 24h while the observed storm rapidly weakened. This resulted again in significant over intensification. By 36h, the model predicted a minimum sea-level pressure of 937 and 950 hPa for both experiments, compared to observed values of 962 and 968 hPa, respectively. With the coupling included, the storm began to weaken much faster than observed for the first forecast, then maintaining a nearly constant intensity of 960 hPa for the remainder of the forecast. It is uncertain how much the incorrect rate of weakening during the first 12 hours will be better represented with higher resolution. Nevertheless, during the remaining day and half of the forecast, the model minimum sea-level pressure remained nearly constant, in excellent agreement with the observed storm intensity and intensity change. In the coupled experiment starting one day later, the storm tended to weaken too much, particularly after the second day.
These results again suggest that the large SST cooling that occurred
as Felix's forward speed slowed down (Fig. 2) had an important impact
on the storm weakening that occurred during this period. It again
indicates that greater resolution will be necessary to adequately
resolve the interior structure of hurricanes particularly during
their intense periods, if reliable intensity forecasts are to be made
from dynamical models such as the GFDL model.
To Tropical Cyclone-Ocean Interactions
