“96.1”暴雪期中尺度切变线发生发展的动力诊断Ⅰ:涡度和涡度变率诊断

DYNAMIC DIAGNOSES OF THE GENESIS AND DEVELOPMENT FOR THE MESOSCALE SHEAR LINE DURING “96.1” SNOWSTORM. Ⅰ: DIAGNOSES OF VORTICITY AND VORTICITY VARIABILITY

利用引入三相云显式降水方案后改进和发展的中尺度模式 (MM4)模拟输出资料 ,对“96.1”高原暴雪切变线发生、发展的结构进行了运动学和动力学诊断。涡度场演变指出 ,高原上局地涡度中心和涡度带的生成和发展不仅与暴雪切变线的形成和发展密切相关 ,而且有预测切变线生成的先兆意义 ;涡度场、散度场、垂直速度场与相当位温场的剖面结构诊断表明 ,运动场和热力场的相互配置与耦合关系极有利于暴雪切变线发展及暴雪形成与维持 ;涡度变率诊断结果指出 ,涡度正变率中心带初生于暴雪切变线附近 ,其时空演变与切变线生成和发展相伴的正涡度中心带垂直结构及演变基本一致。在对涡度变率贡献的诸因子中 ,非线性相互作用涡度变率的相对数值最大 ;时间平均涡度变率的贡献次之 ;与强波扰气流有关的扰动涡度变率贡献最小。在总涡度变率的诸强迫项中 ,散度项贡献最大 ;水平涡度平流项次之 ;垂直涡度输送项和扭转项的贡献相反 ,基本上互相抵消。

The structure of the genesis and development of “96.1” plateau snowstorm shear line was diagnosed in kinematics and dynamics using the output data of a mesoscale model (MM4), in which a three phase cloud explicit precipitation scheme has been introduced. The evolution of vorticity field showed that the genesis and development of the local vorticity center or zone not only was directly related to a snowstorm shear line, also predicted its genesis in advance. The vertical section structures of vorticity, divergence, vertical velocity and equivalent potential temperature showed that the inter allocation and coupling relationship of kinetic and thermodynamic fields were very favorable to the development of snowstorm shear line and the genesis and persistence of the snowstorm. Diagnoses of the vorticity variability revealed that the positive vorticity variability center originated near the snowstorm shear line, its space time evolution was basically consistent with that of the positive vorticity which was associated with the genesis and development of shear line. In the factors of contributing to the vorticity variability the relative numeric of nonlinear interaction vorticity variability was the most component; the time mean vorticity variability was less important; the contribution of the perturbation vorticity variability related with strong disturbed flow was small. In the forcing terms of the total vorticity variability, the divergence term contribute most; less was the horizontal vorticity advection term; the vertical vorticity transport and the twisting term played the opposite role, and roughly offset.