INTERACTION BETWEEN HURRICANE FLORENCE (1988) AND AN UPPER-TROPOSPHERIC WESTERLY TROUGH

The Naval Research Laboratory's limited-area numerical prediction syst em, a version of Navy Operational Regional Atmospheric Prediction Syst em, was used to investigate the interaction between Hurricane Florence (1988) and its upper-tropospheric environment. The model was initiali zed with the National Meteorological Center (now the National Centers for Environmental Prediction)/Regional Analysis and Forecasting System s 2.5 degrees analysis at 0000 UTC 9 September 1988, enhanced by a set of Omega dropwindsonde data through a three-pass nested-grid objectiv e analysis. Diagnosis of the 200-mb level structure of the 12-h foreca st valid for 1200 UTC 9 September 1988 showed that the outflow layer w as highly asymmetric with an outflow jet originating at approximately 3 degrees north of the storm. In agreement with the result of an ideal ized simulation (Shi et al. 1990), there was a thermally direct, circu m-jet secondary circulation in the jet entrance region and a thermally indirect one in a reversed direction in the jet exit region. In sever al previous studies, it was postulated that an approaching westerly je t had modulated the convection and intensity variations of Florence. T n a variational numerical experiment in this study, the approaching we sterly jet was flattened out by repeatedly setting the jet-level merid ional wind component and zonal temperature perturbations to zero in th e normal mode initialization procedure. Compared with the control expe riment, the variational experiment showed that the sudden burst of Flo rence's inner core convection was highly correlated with the approachi ng upper-tropospheric westerly jet. These experiments also suggested t hat the approaching upper-tropospheric westerly jet was crucial to the intensification of Florence's inner core convection between 1000 and 1500 UTC 9 September, which occurred prior to the deepening of the min imum sea level pressure (from 997 to 987 mb) between 1200 UTC 9 Septem ber and 0000 UTC 10 September. Many earlier studies have attempted an explanation for the effect on tropical cyclones of upper-tropospheric forcings from the eddy angular momentum approach. The result of this s tudy provides an alternative but complementary mechanism of the intera ction between an upper-level westerly trough and a tropical cyclone.