NUMERICAL SIMULATION STUDY OF THE INNER-CORE STRUCTURES AND THE MECHANISM FOR INSHORE STRENGTHENING OF SOUTH CHINA SEA TYPHOON VONGFONG (0214) DURING LANDFALL

An explicit simulation with a fine mesh at intervals of 6 km is used to explore the inner-corestructures of Vongfong (0214). The dynamic mechanism for the inshore strengthening of Vongfong isexamined. It is found as follows. (1) The radius of maximum wind of the axisymmetric structures of thetyphoon decreased with height during its mature stage. When Vongfong was inshore, the strongestlow-layer inflow located in front of it and the outflow was to the rear of it, which was just reversed fromthe Atlantic hurricanes and other Pacific typhoons. (2) The dynamic and thermodynamic fields were highlyasymmetric in structure. Convection was stronger in the northwest quadrant of the typhoon than in thesoutheast; the strongest convective cloud bands were consistent with the maximum wind region. During itsstrengthening stage, it was cold west of and warm east of the eye in the lower layer but warm in the westand cold in the east of the mid-upper layer. During its mature stage, a warm-core structure was evident inthe lower and middle-upper layer. (3) The interactions between a mid-latitude cold low in themiddle-upper troposphere and the typhoon were responsible for the latter to strengthen inshore. Firstly, theouter circulation of the cold low entered the typhoon from the middle troposphere when an outer coldairflow from the cold low flowed into the northwest quadrant of the typhoon so that geopotentially instableenergy increased and convection developed. Secondly, the downdraft in the cold low was just thecorresponding branch of the secondary circulation of the typhoon system; when the cold low weakenedwhile moving south, the typhoon strengthened inshore. Due to the CISK mechanism, these two phenomenamight be realized.