On the dynamics of drylines

Drylines in the southern and central Great Plains have received considerable attention in the literature. One key reason is that sometimes severe thunderstorms tend to break out along the dryline. The focus here is on late morning to early evening, when the convective boundary layer (CBL) is well-developed. Recent field observations have shown that at fine scales, not one but multiple dryline boundaries may occur, with successively drier air towards the west. In order to develop a sharp transition of a conserved property, such as water vapor, sustained convergence needs to occur with the CBL. The mechanism of this convergence has been the subject of much discussion. Certainly synoptic-scale convergence can be a factor, and the large-scale slope of the terrain allows westerly shear to produce convergence when the sheared flow is mixed in the CBL. On much smaller scales, the dryline definition appears to be driven by density current dynamics. This conclusion arises from several case studies collected during the International H2¬O Project (IHOP_2002). Of particular use is the Wyoming Cloud Radar (WCR) aboard the University of Wyoming King Air aircraft (WKA). The UWKA conducted several traverses perpendicular to drylines as they became more defined, sometimes prior to convective initiation (CI). The WCR mainly operated in profiling mode, with beams both below and above the aircraft. In situ thermodynamic data were interpreted in the context of the vertical structure of the radar ‘fine-line'. In each case we find that the virtual potential temperature (theta-v) gradient within the CBL is consistent with the slope of the echo plume, the solenoidal circulation (retrieved from dual-Doppler synthesis below the aircraft), and the direction and speed of propagation of the dryline. In one case the dryline alters course (from progressive to retrogressive), an event marked by a change in echo slope and a change in sign of the theta-v gradient. As such drylines are very similar to other mesoscale boundaries propagating in the CBL, such as outflow boundaries, sea breezes, and fronts. In fact it is quite possible that all fine-lines seen by radar in the CBL are associated with some theta-v gradient, whose origin may be hard to determine.