摘要：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 different 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 significantly different variation mechanisms; (ii) for the vertical helicity, the vertical perturbation pressure gradient force, buoyancy, the divergence-related effect, 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.
