fran.fli (4001840 bytes)
fran.avi(11738984 bytes)
Two forecasts involved two cases of Hurricane Fran starting at 0000 UTC, 1 and 2 September, 1966. During the 4 day period studied, Hurricane Fran moved in a west-north-west direction in the western Atlantic, north of the Caribbean. As shown in Fig.1 , four days prior to the start of the first integration, Hurricane Edouard , which was an intense major hurricane, moved in nearly an identical path to Fran, producing a significant cold wake ( AVHRR image of SSTs ). During the first 2 ½ day period of this study, Fran remained at nearly constant intensity with the minimum sea-level pressure only varying between 976 and 981 hPa. It is likely that the cold wake of Edouard was one of the primary reasons why during the Fran did not intensify during this period. However, the NCEP SST analysis, with its coarse resolution, did not show any sign of Edouard's wake and indicated SSTs of 28 to 29°C in this region. Consequently, the operational GFDL forecast, without coupling incorrectly forecasted rapid intensification. with the model storm deepening 40 hPa during the 2 ½ day period.
To investigate the impact of Edouard's wake on the intensity of Hurricane Fran, two sets of coupled and non-coupled experiments were run for the forecast starting at the 0000 UTC, 1 September initial time. In the first set, Edouard's cold wake was generated during the ocean initialization, by imposing the hurricane wind forcing from Edouard starting 5 days before the start of Fran's forecast. In the second set, the cold wake of Edouard was not imposed and the coupled forecast began with the SSTs obtained from the NCEP analysis, modified only by Fran's wake. The SST field at 72h for both coupled runs is shown in Fig. 2 indicating the significant strong cooling produced by Edouard in the coupled model. At the start of the forecast with the wake imposed, the SST near the storm center of Fran averaged about 2.5°C lower than the original NCEP analysis, with a maximum SST decrease of about 3°C in the region of the wake well northwest of the storm. We were unable to verify the SST cooling in the vicinity of the Fran passage, but the ocean response simulations were in a good agreement with measurements at the mooring located at 40.5 N, 70.5 W, about 110 km south of Cape Cod. The mooring was deployed during the Coastal Mixing and Optics project (07/08/96 - 09/26/96) (T. Dickey, private communication). The eye of Hurricane Edouard passed within roughly 80 km of the mooring on September 1, 1996 when the storm was already weakening. The measurements indicate a temperature decrease from about 16°C to about 14°C at the depth of 10 m which is consistent with the model SST prediction shown in Fig. 2. The model SST predictions were also verified against AVHRR derived SST (N. Nelson Bermuda Biological Station for Research, personal communication, 1997). Fig. 3 shows two sections of the SST distributions along 31°N and 32°N comparing the model simulations and observations. In both cases a marked consistency is evident. The model not only demonstrated a good skill in predicting the maximum SST cooling but more importantly, the spatial extend of the cold wake as well.
The forecasted time series of minimum sea-level pressure for both sets
of coupled and uncoupled experiments is shown in Fig. 4. In both experiments without inclusion of the wake, the storm
began to intensify from the beginning, although at a reduced rate in
the coupled case. By 36h, the minimum sea-level pressure had fallen to
953 and 960 hPa respectively compared to the observed value of 976 hPa.
With inclusion of the wake, the intensity was considerably closer to
the observed value, even without inclusion of the ocean coupling, with
a minimum sea-level pressure of 967 and 973 hPa respectively. This
indicates that the effect of Edouard's cold wake on Fran's intensity
was more important than the effect of the ocean coupling although
inclusion of both effects was necessary to correctly forecast Fran's
intensity. Beyond 36h, the coupled fore- cast even with inclusion of
Edouard's wake began to deepen. This was about one day before the
actual intensification began. as the forecasted track was well south of
the actual track (Fig. 2), causing the model storm to move away from
the wake too soon.
To Tropical Cyclone-Ocean Interactions
