一次东风波及其诱生低涡的螺旋度特征分析

Characteristics of Helicity Variation during a Low Vortex Event Caused by Easterly Wave

利用美国国家环境预报中心NCEP 1°×1°再分析资料，详细阐述了2001年8月3～4日浙南闽北的东风波暴雨过程的螺旋度的计算方法，并根据螺旋度（Helicity）和Q矢量（Q vector divergence）分析了过程中的暴雨演变以及雁荡山脉诱生中尺度低涡发生发展的原因。同时，利用中尺度有限区域模式MM5V2对该东风波诱生中尺度低涡进行模拟。结果表明：螺旋度大值中心强度和位置的演变较好地反映了暴雨落区和中尺度低涡的诱生、移动，螺旋度的时空演变对暴雨发生有一定的预示意义，螺旋度计算较中尺度模式诱生低涡的初生位置、路径预报准确率高，两者集成可以提高诱生低涡的预报准确率。同时，螺旋度梯度大值区和Q矢量散度梯度大值区叠加区的时空演变对暴雨发生、诱生低涡的初生位置预示准确，说明该叠加区对降水和系统的诊断能力强，具有很好的指示意义。

This paper showed mainly the process of the heavy shower occurring in the southern seacoast of Zhejiang Province caused by the easterly wave from 3 to 4 August 2001. Based on the meteorological satellite pictures, the authors analyzed the evolution of the stronger convective cloud, and further discussed the weather background in which the synoptic systems cause the heavy shower in the seacoast of Fujian and Zhejiang provinces to develop. The analysis of thermodynamic and dynamic conditions showed that after the low cyclonic vortex emerged, it evolved and moved from the northland of Fujian Province to the southern seacoast of Zhejiang Province from 3 to 4 August 2001. Using NCEP （National Centers for Environmental Prediction） 1°× 1° reanalysis data and meteorological satellite data, the authors discussed the low cyclonic vortex and the most heavy shower caused by the easterly wave. Helicity, Q vector and their effects on the all process were investigated. The heavy shower was successfully simulated by MM5V2. The helicity and Q vector divergence were calculated using the NCEP 1°× 1°reanalysis data. The results show that there exits close relationship between helicity and the heavy shower, which presents that the higher helicity may be one of the mechanisms for rain storm development.High-level helicity together with low-level helicity can trigger a heavy shower. In this process, high-level helicity ts as important as low-level helicity. Helicity and used MM5 model together can offer more accurate low cyclonic vortex forecasting and rain storm forecasting. It is obvious that the overlapped region of the evolvement of center intensity and position of Q vector divergence maximum and the evolvement of center intensity and position of helicity maximum can forecast low cyclonic vortex and rain storm better. Therefore, the overlapped region is a good index of the weather forecast. Using Q vector divergence, helicity and MM5 together can offer a new method to forecast rain storm. The results also show that the evolvement of center intensity and position of Q vector divergence maximum and the evolvement of center intensity and position of helicity maximum can well reflect the heavy shower and the emerging of cyclonic vortex, and the forecasting is more accurate than that of MM5 model. Therefore, the combination of Q vector divergence, helicity and MM5 model can offer more accurate weather forecasting in the similar weather process. On the other hand, a substantial review of the developments on the theories of helicity, Q vector and their main application results is a complete and systematical conduct, along with a prospective study in this field.