Quiescent and synoptically-active drylines: A comparison based upon case studies

Abstract

¶A comparison of various aspects of drylines that occur in quiescent and synoptically-active environments is carried out based on review of two observational case studies. Dryline motion in the quiescent case occurs over a relatively short distance compared to that in the synoptically-active case. The latter case includes formation of a synoptic-scale bulge that is diagnosed to be a result of advection of very dry air aloft above the dryline in combination with vertical mixing in the boundary layer. In both cases the dryline at times exhibits discontinuous motion (more pronounced in the synoptically-active case). In each case the total difference (cross-line) in moisture over the region occupied by the dryline is similar, and the change takes place through two or more steps rather than a single step. The along-dryline location of convective initiation is determined by processes in the dry air in both cases. These processes include along-line differences in sensible heating in the dry air leading to variations in the efficiency of vertical mixing of heat, moisture, and momentum. Consequences of these mixing and heating differences include the formation of cloud lines in the dry air and local areas of lowered pressure near the dryline. Sensible heating differences in the dry air are linked to whether precipitation recently occurred over a given region, to the natural character of the underlying surface, and to whether regional-scale irrigation is practiced in cultivated areas.