非对称型强飓风中的准平衡流特征分析

The diagnoses of quasi-balanced flows in asymmetric intense hurricane

本文在论述飓风发生发展生命史过程中平衡、准平衡和非平衡态动力学特征的基础上,应用PV-ω方法,对具有非轴对称和长时间强度维持特征的飓风Bonnie(1998)进行了反演诊断分析,结果表明:基于非线性平衡模式的平衡流能够描述飓风水平涡旋场的基本特征,而加入准平衡ω方程得到的准平衡流能反映边界层入流、高层出流、眼墙区的剧烈倾斜上升运动和眼心区域的下沉运动.准平衡流描述了具有较长生命史组织化过程的强对流系统,而与其相联系的辐散运动与涡度同量级,证明了飓风准平衡流场具有涡散运动共存的特征,但在边界层顶的入流急流区和高层出流区仍存在高度非平衡态的超梯度流.利用反演的准平衡流场分析发现,当由环境风场低层到高层存在顺切变时,飓风内中尺度对流带移动方向的左侧,有利于强对流单体的发展和新对流单体的形成,右侧则相反,同时强气旋式旋转流场的作用,使得对流单体形成后随基本气流传播至对流减弱区,造成了飓风非对称结构的形成和维持.

Based on the discussion of the balanced, quasi-balanced and non-balanced dynamic characteristics in hurricanes life cycle, the potential vorticity （PV）-ω inversion system is applied to analyze the hurricane Bonnie（1998）, which exhibited the features of asymmetry and steady intensity. The results show that the basic vortex can be well described by the balanced flows derived through the nonlinear balanced model, whereas by adding the quasi-balanced ω equation, the quasi-balanced flows can reflect the structure of convergent inflow in planetary boundary layer （PBL）, divergent outflow in upper troposphere, strong slantwise updraft in eyewall and subsidence motion in hurricane eye. Because the quasi-balanced flows can describe the organized severe convective systems with long life span and the correlative convergence motion have the same magnitude with vortex motion, the quasi-balanced flows have the co-existence feature of divergence and vorticity motions. It is found, however, that the inflow area in PBL and outflow area in upper-level contain significant unbalanced supergradient flows. Under＇the quasi-balanced downshear environments, the moving direction left of mesoscale severe convective bands has the favorable condition for the genesis and the development of convective cells, and the opposite of the right side. So the severe convective systems in eyewall and spiral rain bands weaken and die out as soon as propagating to moving direction right with the basic cyclonic rotational flows, which causes the formation and maintenance of asymmetric structure in eyewall and spiral rain bands.