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.
An improved flux-gradient relationship between momentum φm（ζ） and sensible heat φh（ζ） is obtained by the use of the observational data over an alpine meadow in the eastern Tibet Plateau, in Maqu of China during the period June to August, 2010. The empirical coefficients of Businger-Dyer type function for the cases of unstable and stable stratification are modified. Non-dimensional vertical gradients of wind and potential temperature are calculated by three fitting func- tions; that is, the log-linear, log-square, and log--cubic functions, respectively. It is found that the von Karman constant approaches 0.4025 and the Prandtl number is about 1.10 based on the measurements in near-neutral conditions, which are within reasonable range proposed in previous studies. The revised flux-gradient profile functions of -1/5 power law for momentum and - 1/3 power law for sensible heat are best fitted in unstable stratification conditions. Meanwhile, 2/5 power law, instead of linear functions, is more appropriate in stable stratification cases for momentum and sensible heat. Com- pared with results from previous studies in which traditional functions are used, the momentum and sensible heat fluxes estimated by the revised profile functions in the current study are much closer to the observations for the unstable and stable stratification conditions.
According to the different pattern of sea surface temperature anomaly(SSTA) in the previous year of La Nia events,we categorized La Nia events into two types to investigate the different characteristics of tropical cyclone(TC) activity over the western North Pacific(WNP) in TC peak season of two types La Nia events.One type is following the previous El Nio event(La Nia I);the other is following the previous neutral phase or developing La Nia event(La Nia II).Results show that TC genesis frequency in the WNP during TC peak season of La Nia I is less than normal year,whereas it has no differences from normal year during La Nia II.The main reason is attributed to the different amplitude SSTA in the East Indian Ocean(EIO) and the western Pacific Ocean(WPO).Similar to the capacitor effect,strongly positive SSTA in the EIO-WPO during La Nia I triggers an equatorial baroclinic Kelvin wave,which intensifies the easterly in the lower troposphere and weakens the East Asian summer monsoon,and thus the TC frequency decreased during La Nia I.However,the easterly anomaly shows a weak response to the SSTA in the EIO-WPO during La Nia II,and there is no significant change in the environmental pattern over the WNP;so is TC frequency.The modulation of strong EIO-WPO SSTA on large-scale circulation over the WNP reduces the environmental barotropic energy conversion into synoptic-scale disturbances during La Nia I,and also suppresses TC disturbances.The understanding of two different types of La Nia events could help improve the seasonal prediction of TC activity in the WNP during La Nia.
