Analysis of Helicity of Rainstorms Producing Easterly Wave and Its Related Vortex over Seaboards from Fujian to Zhejiang

摘要

Using the NCEP reanalysis at 1⁰times1⁰ resolution in conjunction with satellite imagery, a study is undertaken of easterly wave - related rainstorm events, on August 3~4, 2001 in seaboards between northern Fujian and southern Zhejiang, expounding the scheme for computing helicity, and exploring the rainstorm evolution and the genesis of the Yandang mountains-triggered a mesolow (Duan and Chen, 2005) by means of helicity and Q vector divergence. Besides, MM5V2 is employed to simulate the easterly wave caused meso vortex. Results show that the development of the central intensity and location of the high-valued helicity is well indicative of the fallout area and the genesis of the meso vortex, discovering that the space(time) evolutions of helicity serve as indicators of some utility for the rainfall occurrence; it is revealed that the calculated helicity is of higher precision than MM5V2 simulations in predicting the initial position and the track of the vortex, with their combination contributing to the improvement of predicting the mesolow; also,the space volutions in the superimposition of the zone of large-value helicity gradients upon that of high-value Q vector divergence gradients can be used to accurately forecast the rainstorm happening and initial position of the meso vortex, thereby illustrating the higher ability of diagnosing the precipitation and its system in the superimposed region.