Numerical investigation of the interaction between Hurricane Florence (1988) and its upper-level environment

Omega-Dropwindsonde (ODW) data were used to enhance the National Meteorological Center/Regional Analysis and Forecast System (NMC/RAFS) 2.5{dollar}spcirc{dollar} analysis at 0000 UTC 9 September 1988 with a nested three-pass Barnes objective analysis scheme. Results revealed that the minimum sea surface pressure (SLP) was analyzed to be 994 mb, improved from 1004 mb in the NMC/RAFS 2.5{dollar}spcirc{dollar} analysis. The Naval Research Laboratory limited-area tropical cyclone model was employed to simulate Hurricane Florence with the analysis enhanced by the ODW data. The Special Sensor Microwave/Imager (SSM/I) retrieved rain rates at 0000 UTC and 1200 UTC on 9 September 1988 were assimilated into the model during the model integration. The simulation results demonstrated that the analysis enhanced by the ODW data drastically improved the simulation in both intensity and storm track. The 24 hour storm track was improved by about 75%, while the forecast error of the intensity was reduced by about 10 mb.; Diagnosis of the 200 mb level structure of the simulated Hurricane Florence at 12 hour showed that the outflow layer was highly asymmetric, especially beyond three degrees of radius, with an outflow jet originating at approximately three degrees north of the storm center. The outflow jet was elongated with a length of roughly 2500 km and a width of 500 km judging from the isotach of 30 ms{dollar}sp{lcub}-1{rcub}{dollar}. There was a thermally-direct, circum-jet secondary circulation in the entrance region and a thermally indirect one in a reversed direction in the exit region. Additional numerical experiment with a weakened upper-level westerly trough and jet suggested that the approaching westerly trough in the upper layer was responsible for the strengthening of the outflow jet of Florence and partially responsible for the intensification of the inner-core convection. Results suggested that the weaker outflow jet coincided with a weaker circum-jet secondary circulation at 12 hour and a weaker storm at 24 hour.