Streamwise Vorticity: The Origin of Updraft Rotation in Supercell Storms

Linear (small amplitude) theory of shallow, inviscid, isentropic convection in a dry, unstably stratified, nonrotating atmosphere is used to investigate the rotational characteristics of an isolated, incipient convective storm in strong environmental shear. Environmental winds veering with height are associated with streamwise vorticity (i.e., a component of vorticity along the mean wind direction). We demonstrate that a roughly circular storm acquires net cyclonic (anticyclonic) rotation within its updraft (downdraft) when the storm-relative winds veer with height, or equivalently when the environmental flow possesses streamwise vorticity in a reference frame moving with the storm. A formula for the correlation coefficient between vertical velocity and vertical vorticity is obtained. The physical explanation for the correlation is as follows. Initially, the isentropic surfaces and vortex lines are horizontal. After the onset of convection, air parcels and vortex lines remain in their original isentropic surface, which is continuously deformed by the convective motions. Tilting of the vortex tubes produces cyclonic (anticyclonic) vertical vorticity on slopes of an isentropic surface that face toward (away) from the mean vorticity vector (as viewed from above). Because air parcels are constrained to remain within their original isentropic surface, vertical velocity maxima (minima) are displaced upstream—with respect to the storm-relative mean wind—from the high (low) points of the isentropic surface due to the upslope (downslope) effect. When the mean vorticity has a storm-relative streamwise component, the cyclonic (anticyclonic) regions tend to coincide with the upslope (downslope) regions of the isentropic surface, and vertical vorticity and vertical velocity on the surface are positively correlated. We believe that the above mechanism is the origin of organized rotation in supercell storms, leading eventually to the formation of mesocyclones. Implications of our findings for short-term forecasting are discussed.