Effective bulk shear in supercell thunderstorm environments

Supercell thunderstorm environments, both from observations and numerical simulations, have been characterized by relatively large buoyancy and vertical shear through a substantial depth of the troposphere. Based largely on the numerical simulations of Weisman and Klemp (1982, 1984), the depth of the vertical shear layer relevant to supercells has been defined as the lowest 4-6 km above ground level. While measures of vertical shear such as 0-6 km bulk shear and the Bulk Richardson shear term have proven successful in identifying supercell potential from environmental soundings, each of these shear parameters represents an arbitrary fixed layer. Such fixed layer parameters become less reliable when attempting to characterize environments of very tall storms (e.g., high equilibrium level heights), very short storms, or storms not rooted near the surface (so-called "elevated" thunderstorms). As an alternative to fixed layer shear depths, vertical shear can be defined with respect to a measure of the depth a particular storm (i.e., the lifted parcel height to equilibrium level height). In this way, vertical shear measures can be normalized such that very tall storms, relatively shallow storms, and elevated storms can be compared in a consistent and potentially meaningful way.

The RUC model close proximity sounding sample described in Thompson et al. (2003) has been augmented to include additional storm cases from 2003 and 2004, increasing the entire sample size to nearly 1000 soundings. The most unstable lifted parcel height and the resultant equilibrium level height was calculated for each proximity sounding, and these heights were used as lower and upper bounds to the storm depth, respectively. Bulk vertical shear was then calculated for ten equally deep layers within the storm, and the bulk shear through these layers is referred to as the "effective shear". Effective shear tends to increase through the depth of the storm for both supercells and nonsupercells, though the effective shear is usually much stronger for the supercells. The effective shear through the lowest half of the storm depth corresponds to the 0-6 km layer above ground level for typical storms with equilibrium level heights near 12 km, and this layer of effective shear discriminates strongly between supercell and nonsupercell storms. The effective shear also normalizes the shear values for shallow and very tall storms. Finally, the use of the most unstable parcel height allows elevated supercell environments to be treated similarly to surface-based storm environments. This approach better identifies the relevant shear impacting elevated storms.