Characteristics of the Asymmetric Flow of Tropical Cyclone Shanshan (2006) During Its Turning and Intensification Period

In this paper, characteristics of the asymmetric flow of Tropical Cyclone (TC) Shanshan (2006) during its turning and intensification period over the oceanic area east of Taiwan are investigated, based on the simulation results from the nonhydrostatic mesoscale model WRF (Weather Research and Forecasting). It is found that the symmetric flow strengthened as the TC intensified, whereas the amplitude of the asymmetric flow of wavenumber 2 increased more significantly, which was strong enough to be comparable with or even exceed that of wavenumber 1, becoming the main part of the asymmetric flow sometimes. The asymmetric waves rotated around the TC center mainly counterclockwise. The closer to the center, the faster the asymmetric waves rotated. Moreover, the asymmetric flow rotated rapidly (slowly) during the slow (rapid) intensification of the TC, and the radial wavenumber showed an increase during the TC intensification. Furthermore, because of the superposition of intensified symmetric flow with the positive perturbation of the asymmetric flow, the maximum wind speed of TC Shanshan became larger. During the merger of the double eyewalls of Shanshan, the symmetric flow showed less increase in strength and the intensification of maximum wind speed was mainly related to the energy accumulation caused by the phase change of the asymmetric waves. The energy accumulation was realized when the asymmetric waves altered the strength and distribution of the inner and outer maximum wind cores, leading to the combination of the inner and outer eyewalls and eventually resulting in the intensification of the TC.