暴雨中不稳定的动力学诊断方法

Method of dynamical diagnosis of instability in rainstorm

为了将动力理论研究的结果应用于实际暴雨过程的诊断,给出了一种对实际暴雨中不稳定进行动力学诊断的方法。根据暴雨的环境场来诊断暴雨中不稳定的最大增长率、最优增长尺度以及暴雨雨带与盛行风向的夹角。利用无粘非绝热线性化的Boussinesq方程组,采用标准模方法将小扰动的不稳定问题转化为复系数常微分方程组的特征值问题。在分别提取了背景流场、层结稳定度和凝结加热垂直分布函数的基础上,对该问题进行了数值求解,并用该方法对实际暴雨个例做了诊断。结果表明,在暴雨发生之前,盛行气流的动力不稳定起了触发积云对流的作用,而在暴雨盛期,对流凝结加热反馈则对不稳定的迅速增大起了关键作用。暴雨盛期不稳定增长率最大,且属于二维平面波的不稳定。实际的观测结果与动力学诊断相一致,表明诊断方法是切实可行的。

A method of dynamical diagnosis of the instability in rainstorm was studied. It was helpful to apply the results of the dynamical theoretical research to the diagnosis of actual rainstorms. The keystone of this method was the application of the background field to diagnosing the most instable growing ratio, the most propitious growing scale and the angle between the rain bond and the basic flow. The nonviscous, adiabatic linear Boussinesq equations were introduced and the instability problem of disturbance was transformed into an eigenvalue problem of complex variable-coefficient ordinary differential equations by using the normal mode approach. By extracting the background flow field, stratification stability and the perpendicular function of condensation heating, the numerical solution of the problem can be obtained. Then a case of rainstorm was diagnosed by using the above method, and the results show that dynamical instability may trigger the cumulus convection before the rain fall, and convective condensation heating feedback plays a key role in the rapid increase of instability at the highest of rainstorm. Dynamical instability actually exists in the rainstorm process, and achieves its maximum value near the highest of rainstorm. There is an angle between the spreading direction of the disturbance and the basic flow, and it is the two dimension plane wave instability. The results of dynamical diagnosis correspond to the observational data, so the above method is reasonable and feasible, and is helpful to the theoretical research and forecast of rainstorms.