opal.fli (10138824 bytes)
opal.avi (9976642 bytes)
Opal initially moved slowly, 220 km during the first 36 h period from Oct 2nd 0000 to Oct 3rd 1200 UTC. Due to the slow storm motion, both of the forecasts with the ocean coupling produced a large SST cooling in the region just northwest of the Yucatan. The forecast starting at 1200 UTC produced a maximum cooling of 4.5-5.0°C (Fig. 2) This exceptionally strong cooling is consistent with the very slow movement of Opal and shallow prestorm mixed layer depths of about 20-30 m. The area of maximum cooling around 22°N, 91°W was exposed to continues hurricane forcing over more than three days, from September 30 to October 3. Moving much faster over the open Gulf of Mexico during the next 2 days, the model storm generated cooling of 2.5 to 3°C over a 150 to 250 km wide area to the right of the track. The amplitude and spatial extend of the SST anomalies in the model are consistent with the satellite AVHRR observations (P. Black, NOAA/AOML Hurricane Research Division, pers. commun. 1997).
Fig.3 indicates the dramatic effect the cold wake had on the accumulated evaporation during the passage of the storm. Throughout the period up to landfall, the accumulated evaporation was significantly reduced compared to the uncoupled forecast. During the first 36h, as the storm slowly moved north, without the coupling the slow movement produced very large values of accumulated evaporation (3.8 cm maximum) on both sides of the track. Since the region of maxi- mum evaporation was located very close to the region of largest SST cooling in the coupled experiment, the evaporation was reduced by 65% in this region, with the region of largest accu- mulated evaporation (1.7 cm) now located to the left of the track.
The resulting change in storm intensity also indicate some of the important effects the ocean interaction might have had on Opal during its passage across the Gulf Fig4 . During the first day and a half, as Opal slowly drifted north and was impacted by the low SSTs, the minimum sea level pressure deepened only about 15 hPa. In contrast, without the effect of the ocean coupling, the model storm began to rapidly intensify with the predicted sea-level pressure in both forecasts dropping to 927 hPa and 931 hPa, respectively by Oct. 3, 12z (58 hPa and 41 hPa deepening), compared to the observed pressure of 970 hPa. With the coupling included, the storm's intensity was much better reproduced and the forecasted minimum sea-level pressure was 960 hPa and 973 hPa, respectively.
On October 3rd as Opal began to accelerate to the north it rapidly began to strengthen. In the second forecast, in which the storm speed was well forecasted, intensification correctly began after Oct 3, 12z, although at a slightly smaller rate. In the first forecast, the model storm moved too quickly, which likely caused the storm intensity to be larger than observed. The previous idealized studies and observational results also confirmed that the SST decrease is strongly correlated to the storms translational speed. Up to the time the model storm crossed the coast, the storm deepening rate was well represented compared to the uncoupled model for the first forecast. Obviously after landfall the model storm rapidly decayed while the observed storm continued to rapidly deepen until actual landfall occurred. Nevertheless, the rapid deepening of Opal during Oct. 4th would still not have likely been reproduced by the hurricane model due to the insufficient model resolution.
opal-wind.avi
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