An observational study of environmental influences on the intensity changes of Typhoons Flo (1990) and Gene (1990)

The European Centre for Medium-Range Weather Forecasts Tropical Ocean-Globa l Atmosphere advanced analysis was used to study the mechanisms that affect the intensity of Typhoons Flo (1990) and Gene (1990). The outflow structur e, eddy momentum flux convergence, and the mean vertical wind shear were ex amined. The evolution of potential vorticity (PV) in the outflow layer show ed low PV areas on top of both Typhoons Flo and Gene, and the low PV areas expanded as the typhoons intensified. The outflow pattern of the two typhoo ns was influenced by the upper-tropospheric environmental systems. The uppe r-level environmental features were shown to play a crucial role in the int ensification of the two typhoons. The tropical upper-tropospheric trough cell east of Flo provided the outflo w channel for the typhoon. The enhanced outflow, the upper-level eddy flux convergence (EFC), the low vertical wind shear, and the warm sea surface te mperature provided all favorable conditions for the development of Ro. On t he other hand, the intensification of Gene was associated with its interact ion with an upper-level midlatitude trough. The approach of the trough prod uced upper-level EFC of angular momentum outside 10 degrees lat radius, and the EFC shifted inward with time. As the EFC shifted into the vicinity of the storm core, Gene started to intensify steadily until the midlatitude tr ough passed over. The intensifying processes of the above cases indicate the importance of th e upper-tropospheric systems to the intensity change of typhoons. The influ ence of upper-level environmental systems on the tropical cyclones is promi nent in the low inertial stability outflow layer. However, results from the piecewise PV inversion of the upper-level environmental PV anomalies showe d little evidence that the intensification of both typhoons were directly a ssociated with the superposition of PV anomalies.