For better understanding the variation of helicity and its governing mechanisms,based on the primary momentum equation under the local Cartesian coordinate,a set of horizontal and vertical helicity equations are derived in this study.On this basis,a storm-relative helicity budget equation is derived,the main factors that govern the variation of helicity are discussed,and the key mechanisms underlying the helicity variation are illustrated by using schematic images.Both scale analysis and real case diagnosis are used to compare the relative importance of di erent factors on the variation of helicity.For a meso-α system,it is found that:(i)horizontal helicity is much larger than vertical helicity,and they show signi cantly di erent variation mechanisms;(ii)for the vertical helicity,the vertical perturbation pressure gradient force,buoyancy,the diver-gence-related e ect,and the conversion between vertical and horizontal helicity govern its variation(whereas,the conversion is negligible for the evolution of horizontal helicity);and(iii)baroclinity is crucial for the variation of horizontal helicity,but it is only of secondary importance for the vertical helicity variation.
In order to get insight into the mechanism of summer rainstorm occurred in central Shandong Province and further improve the prediction accuracy,the FY-2E satellite TBB data and 6 h of NCEP/NCAR 1°×1° reanalysis data were used to carry out the weather dynamics diagnosis and TBB cloud image analysis of a heavy rain happened in Anqiu City on July 18,2010.The results showed that the Anqiu torrential rain was caused by common effects of the subtropical high and upper trough and ground cyclone system;TBB strong low clouds moved with cloud band along the low level jet direction,and closely associated with water vapor and energy belt,while the heavy rain occurred in the north of TBB light in low temperature zone,making the unstable energy release above the Anqiu area;the changes of the vertical spiral degrees at center could well guide the movement of the ground cyclonic center.
