The Structure and Classification of Numerically Simulated Convective Stormsin Directionally Varying Wind Shears

Abstract

Using a three-dimensional numerical cloud model, we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes. As with a previous series of experiments using unidirectional wind shear profiles (Weisman and Klemp), the current results evince a spectrum of storm types ranging from short lived single cells at low shears, multicells at intermediate shears, to supercells at high shears. With a clockwise curved hodograph, the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank. An analysis of the dynamic structure of the various cells reveals that the quasi-steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system. We use this feature along with other related storm characteristics (such as updraft rotation) to propose a dynamically based storm classification scheme. Following Browning, this scheme includes two basic storm types: ordinary cells and supercells. Multicell storm systems and squall lines would then be made up of a combination of supercells and ordinary cells. As in the unidirectional shear experiments, a convective bulk Richardson numbercharacterizes the environment conducive to producing particular storm types.