The relative importance of lower-level and upper-level shear on the intensity of squall lines

Using a series of idealized numerical simulations, we investigate the relative roles of lower-level and upper-level shear on the intensity of squall lines. We use several different measures for system intensity, including: the maximum magnitude of surface winds; the total area affected by severe winds at the surface; total precipitation; total upward mass and moisture transport; and maximum vertical displacement of low-level parcels. The effects of upper-level shear are mixed. Increases in upper-level shear can enhance vertical displacements and total upward mass transport, and thus can have a positive effect on precipitation. However, increases in upper-level shear are detrimental to the intensity of surface winds and to the areal extent of severe winds (although, increases in upper-level shear do not prevent the production of severe surface winds in this study). In contrast, for conditions that are typical of derechoes, an increase in lower-level shear has a beneficial effect on the intensity of squall lines for all measures of squall line intensity investigated in our study. Furthermore, in the portion of the parameter space where both lower-level and upper-level shear are beneficial, increases in lower-level shear are 2-10 times more effective than increases in upper-level shear. When combined with recent observational studies, these results suggest that forecasters should be especially aware of regions with enhanced low-level shear as possible locations for severe wind (or derecho) production, particularly when the near-surface wind is in the same direction as the low-level shear vector.