Certain features of a helical vortex

It is known that atmospheric vortex structures causing extreme weather even ts are of helical nature. It is suggested that the presence of helicity pro vides for their development and longevity. Since the axial velocity compone nt is non-zero, such vortex structures can draw in dust particles. Such par ticles are retained by a vortex for a sufficiently long time due to its cer tain features caused by the velocity distribution in a vortex. The retained particles circulate in the vortex for a considerable part of its life-time , moving together with the vortex. Such a mechanism can explain scalar tran sfer to long distances. (C) 2000 Elsevier Science Ltd. All rights reserved.