ON THE HELICITY AND THE HELICITY EQUATION

In this paper,a simplified helicity equation is derived from the basic moment um equations by dimensional analysis,and the effect factors of helicity variation are discussed.Furthermore,the charact eristics of the horizontal helicity and the vert ical helicity are investigated.The analysis results show that among these effect factors,besides the buoyancy effect emphasized in Lilly's study(1986),the solenoid,the pressur-egradient force and the vorticity have import ant effects on the variation of helicity,while the Coriolis force just makes effect when the horizontal scale is large.The effect factors of the horizontal helicity and the vertical helicity are different.The variation of horizontal helicity is affected mainly by the horizontal solenoid,vorticity and pressure-gradient force,while the variation of vertical helicity is closely related to the vertical pressure force and the buoy ancy force.Besides,they have different behavior with storm development and evolution.The positive anomaly of the horizontal helicity may be a precursor of a convective storm,while the positive anomaly of the vertical helicity may be a parameter,which may reflect the intensity and stability of a convective storm.How ever,these two kinds of helicity are relative and interactive.On the one hand,the positive anomaly of the horizontal helicity can contribute to the increase of the vertical helicity,in turn,the motion responding to high vertical helicity can affect the horizontal helicity.