Observations Related to the Rotational Dynamics of the 20 May 1977 Tornadic Storms

Abstract

Observations of the 20 May 1977 tornadic storms are used to evaluate recent theories on the initiation of rotation at mid-and low levels and to verify recent thermodynamic retrieval results. Using the lengthy data record from a variety of sensors available for this day, it appears that the mechanism that initiates low-level rotation is different from that at midlevels. Attempts to identify the source of the low-level rotation as vertical tilting baroclinically generated horizontal vorticity were inconclusive. The recent thermodynamic retrieval results of Hane and Ray and of Brandes for these storms are in good agreement with independent measurements where available. However, verification is hindered by the sparseness of these measurements. Noticeable differences in the region of the rear-flank downdraft suggest that there is room for improvement in the retrieval methods. Investigation of the cyclic generation of rotation along gust fronts indicates that the source of low-level rotation is not derived from baroclinically generated horizontal vorticity as seems to be the case with the initial mesocyclone core. Instead, vertical vorticity amplification along the gust front leading to successive generation of mesocyclone cores and discrete mesocyclone propagation is the result of the concentration of low-level preexisting vertical vorticity through convergence.