Visual Aspects of the Flanking Line in Severe Thunderstorms

Abstract

Photographs are presented that illustrate the various forms in which ?flanking lines? exist in nature. Flanking lines may appear not only as the commonly observed sloping line of cumulus congestus, but also as erect towers with a vertical face, and as a line of altocumulus castellanus above and parallel to a band of stratocumulus lenticularis. It is suggested that the slope of the tops of the flanking-line towers with respect to the ground is related to a quantity that is similar in form to the bulk Richardson number, and that the orientation of the flanking line is a function of the mean wind in the lowest 6 km and storm motion. These hypotheses need to be verified when high-resolution, rapid-scan, visible and infrared satellite imagery become available on a daily basis.