一次中尺度对流低涡增强阶段的能量诊断分析

Diagnostic Analysis on the Energy of a Mesoscale Convective Vortex in Enhancing Stage

本文基于CFSR每日4个时次、水平分辨率为0.5°×0.5°全球预报场资料,美国NCEP中心每日4个时次、水平分辨率为1°×1°FNL全球再分析格点资料,以及华中地区国家基准站逐小时的加密降水资料,围绕2015年6月1日华中地区的一次中尺度对流低涡(mesoscale convective vortex,MCV)天气过程,通过WRF模拟和能量诊断的方法,重点研究了低涡增强期内的能量分布特征及其对低涡发展的影响机制。研究结果表明:此次MCV初生于湖北中部地区,低涡生成后向湖北东北部大别山地区移动且不断发展加强,MCV增强阶段的降水带分布由早期的三中心分布(分别位于宜昌、荆州、随州)演变为后期的纬向型雨带分布。降水产生的凝结潜热释放、对流有效位能的增强、低层暖湿气流的输送以及中层干冷空气的侵入等有利的环境场条件对低涡的增强起到了重要的推动作用。低涡的增强对能量演变有重要影响,具体表现为一方面MCV外围辐合气流随低涡发展而增强,引起对流层低层扰动动能的增加,另一方面MCV外围降水产生的凝结潜热,导致对流层中层扰动有效位能的增加,之后通过垂直气流作用使扰动有效位能向上输送,从而使对流层高层的扰动有效位能增加。另外,此次MCV增强阶段的能量制造项依次为:扰动有效位能向扰动动能的转换,不同高度层的基本气流黏性力作用效果,纬向平均有效位能向扰动有效位能的转换,以及来自系统外部扰动动能的输入。其中,扰动有效位能向扰动动能转换是对MCV发展增强的直接贡献项,对其空间分布特征进一步分析可知,在对流层低层和顶层,扰动有效位能向扰动动能转换,使辐合辐散气流增强;而在对流层中高层,扰动动能向扰动有效位能转换,为低涡发展成熟后的继续维持储备了必要的能量。

Based on the four-times daily CFSR global reanalysis data and the hourly observed national base station precipitation in the Central China region, we focused on one mesoscale convective vortex （MCV） weather process that occurred in Central China on 1 June 2015, and studied the characteristics of energy distribution and the mechanism that impacted the MCV during the enhancement phase by using the WRF simulation and energy diagnosis method. The results showed that the MCV was born in the central part of Hubei Province, then it moved towards Dabie Mountain in northeast of Hubei Province with enhancing in- tensity of MCV. By analyzing the precipitation distribution characteristics during the vortex enhancing stage, we found that the precipitation mainly focused on three regions at the early stage, then evolved into the distribution of zonal pattern. By analyzing meteorological conditions corresponding to the vortex en- hancing stage, we found that latent heat caused by precipitation, the warm and wet southwest flow trans- port in the lower troposphere, the cold and dry air intrusion in the mid-level and a sharp increase of CAPE value were favorable for the enhancement of MCV intensity. On one hand, the convergence flow was strengthening in MCV peripheral section while the vortex was developing, resulting in the increase of per- turbation kinetic energy. On the other hand, latent heat of condensation generated by precipitation resul- ted in the increase of perturbation available potential energy in middle level of troposphere. Then, the available potential energy was transferred upward by vertical air flow, hence it caused the enhancement of available potential energy in upper level of troposphere. With the method of calculating budget of the above two kinds of energy, we found that the energy producing items in the MCV enhancing stage were the conversion between perturbation available potential energy and perturbation kinetic energy which was trig- gered by pressure change between inside and outside of the MCV system, the effect of viscous force pro- duced by basic air flow in different levels of troposphere, the conversion between zonal mean available po- tential energy and the perturbation kinetic energy caused by latent heat release as well as the input of per- turbation kinetic energy from the outside of MCV system. However, the conversion between perturbation available potential energy and perturbation kinetic energy was the most contributable item for the develop- ment of MCV. By further analyzing its spatial distribution characteristics, we concluded that in the lower level and upper levels of troposphere, the forward conversion was propitious to strengthen the convergence and divergence flow, but in middle-level and upper-level troposphere, the reverse conversion was propi- tious to store the energy to satisfy the need of maintenance of vortex after it developed into maturity.