Abstract: Coupled equations of rotational and divergent Kinetic energy are used to study the mechanisms for generating and maintaining low-level jets in two real cases with and without heavy rain. As they are accountable by the relative height at which upper and lower jets are lying, individual terms in the equations are analysed with respect to their horizontal distribution. Particularly, it is the direction of enetgy converted between the rotational and divergent components rather than the intensity of the jet that detemines whether it will be acconipanied by heavy rain. A positive Conversion from divergent to rotational kinetic energy appears to be the neceseary but not sufficient condition for the heavy rain to take place. The intensity and direction of energy conversion depends not only on the relative position of vorticity and diversity feild, but also the vertical profile of the jet.