STREAMWISE VORTICITY: THE ORIGIN OF UPDRAFT ROTATION IN SUPERCELL STORMS.

Abstract

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). The author demonstrates 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.