2008年台风“风神”强迫次级环流的诊断分析

利用WRF（weather research and forecasting）模式模拟资料对2008年06号台风“风神”进行诊断分析,采用准地转PV-ω方程对台风外围中尺度对流系统较强的6月20日10时（世界时）的资料进行分析.通过PV-ω方程诊断了潜热、摩擦及干动力过程对台风次级环流的作用,结果显示潜热强迫产生的次级环流最强,摩擦强迫主要集中在边界层,而干动力过程则在台风中心附近产生影响.加入摩擦、潜热得到的准平衡流场能够描述70％左右的台风环流.环境垂直切变在台风中心附近强迫产生横向次级环流的垂直切变与环境垂直切变相反,次级环流会使得台风一侧的上升气流减弱而另一侧上升气流增强,从而使得台风不对称增强.同时,发现垂直切变可能在其最大垂直切变方向右侧激发台风外围中尺度系统.通过构造理想的准平衡的台风及叠加在其上的中尺度系统环流,选择不同的切变和环境平均气流,发现增大切变会使得强迫次级环流增强,而增大环境平均气流不一定能够使得强迫次级环流增大,反而可能使得强迫次级环流减弱.通过诊断发现由切变强迫次级环流造成的中尺度对流系统上方扰动可能是中尺度对流系统持续存在的原因.

Diagnostic analysis of the forced secondary circulations by typhoon Fengshen in 2008

The WRF （weather research and forecasting model） is used to simulate the typhoon Fengshen in 2008.PV-ω inversion diagnostic is applied to quantitatively diagnose the simulated data of typhoon Fengshen at 1000 UTC on 20 June.In this study,the contributions of latent heating,friction and dry dynamical processes to the forced secondary circulations （FSCs） of a typhoon vortex are analyzed respectively.The results show that the latent heating FSC accounts most for the forced secondary circulations.The friction FSC is mostly constricted in the boundary layer （PBL） and dry dynamical processes mainly influence the central area of the typhoon.It is shown that the inverted quasi-balanced flow with friction and latent heating can depict 70％ of the secondary circulations.In contrast,the dry dynamical forcing,which is determined by vertical shear and system-relative flow,can account for as much as 40％ of vertical motion in typhoon core area.The FSC reduces the typhoon vertical motion in upshear direction and intensifies the vertical motion in downshear direction,thus the asymmetry of typhoon comes into being.It is found that a mesoscale convective system in the typhoon outer core region develops on the right side of down shear direction.An ideal quasi-balanced typhoon superposed by the mesoscale convective system is constructed to examine the influence of various vertical shear and system relative flow.The result shows that the shear FSC is almost linearly related to the magnitude of environmental vertical shear,while increasing the magnitude of the system-relative flow may possibly weaken the FSC.The disturbance aloft on the mesoscale convective system,which is caused by the shear FSC,may be the reason for the persistence of the mesoscale convective system.