The Application of Shearing Wind Helicity and Thermal Wind Helicity in Northeastern Cold Vortex Rainfall Event
切变风螺旋度和热成风螺旋度在东北冷涡暴雨中的应用

The shearing wind helicity and thermal wind helicity are introduced and applied to a northeastern cold vortex rainfall event. In theory, shearing wind helicity is defined as the dot product between vertical wind shear and absolute vorticity vector, which represents the twisting effect of nonuniform distribution of wind vector in the vertical direction on vorticity tube. It includes two terms, torsion and the divergence of vertical vorticity. By use of geostrophic relation and thermal wind relation, the shearing wind helicity is simplified as thermal wind helicity. Its intensity and sign are determined by the collocation between the upward air stream and warm-moist air. Compared with shearing wind helicity, thermal wind helicity is calculated at a single level, which avoids vertical differential and offsets the shortage of data in the boundary and sparse vertical layers. Thus the calculation is simplified largely and conveniently applied to operation. Based on the above definitions and theory analyses, a case study is performed. A northeastern cold vortex rainfall event is simulated. By using the mesoscale model output, shearing wind helicity and thermal wind helicity are analyzed. It is found that, large-value centers of precipitation are located at the interface between positive and negative shearing wind helicity values. The intensity variation of shearing wind helicity is identical with that of precipitation. Similar to shearing wind helicity, by using the thermal wind approximation, the torsion term in the shearing wind helicity formula (i.e., thermal wind helicity) has indicative sense to precipitation and the development of convection, especially for strong precipitation and convection. Furthermore, it corresponds with precipitation and the development of strong convection better than the traditional helicity does.