1. 21:50 UTC western dryline On the dynamics of drylines Fine-scale vertical structure of drylines during the International H 2 O Project (IHOP) as seen by an Airborne Doppler Radar Qun Miao and Bart Geerts University of Wyoming P2R.3 Summary Dryline structure and dynamical characteristics are examined by means of aircraft and airborne Doppler radar observations collected in the central Great Plains in late spring. 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.   Of particular use is the Wyoming Cloud Radar (WCR) aboard the University of Wyoming King Air aircraft (UWKA). The reason is that the radar, in profiling mode, gives vertical structure information, which can be interpreted by means of in situ data.   The UWKA conducted several traverses perpendicular to drylines as they became more defined, sometimes prior to convective initiation (CI).   The WCR operated in profiling mode, with beams both below and above the aircraft, and in vertical-plane dual- Doppler (VPDD) mode. western dryline eastern dryline 00:12 UTC SPOL May 22 dry side is less dense (warmer) On May 22, UWKA flew across a dryline in the OK Panhandle. Flight levels are from 150 m to 1500 m AGL from late afternoon to early evening. The dryline retrogresses from east to west. Obvious mixing ratio (MR) gradient The vertical tilt of the dryline fine-line, the vertical velocity couplet, and the density temperature gradient are all consistent with a solenoidal circulation, i.e. the basis of a density current. Note:  v ’ is proportional to buoyancy solenoidal tilt solenoidal circulation solenoidal tilt solenoidal circulation June 19 has Clear skies, no deep convection in any direction apparent, at least before 20:30 Z. The dryline progresses from west to east. Small θ v gradient Composite variations across dryline Different variables are averaged in 200-m bins across the dryline for 9 cases on May 22 and 7 cases on June 19. May 22 Zm: close-flight-level mean reflectivity (14 gates) Wm: close-flight-level mean vertical velocity (14 gates) Dotted lines are individual cases and bold lines are averages of all cases. Lighter colors correspond later times. June 19 Gentle θ v gradient. Gentle θ v gradient. MR change is less than May 22 as well. MR change is less than May 22 as well. Confluence is much stronger than May 22. Confluence is much stronger than May 22. Variations with height Differences of variables across dryline are defined as: [mean of 3 km east of dryline] – [mean 3 km west of dryline] [mean of 3 km east of dryline] – [mean 3 km west of dryline] May 22 Lighter colors correspond to later times. Lighter colors correspond to later times. June 19 θ v gradient reverses sign in time, consistent with a change vertical echo tilt, a flip in sign of the solenoidal vorticity, and a change in boundary propagation speed. WEST EAST Flight level Dual-Doppler Note sloping boundary and solenoidal circulation Conclusions May 22 June 19 Time (UTC) 22:00 to 00:00 19:30 to 21:30 Direction of movement westwardeastward Orientation of dryline SSE to NNW Mean MR difference (g/kg) 2.01.5 Mean θ v difference (K) -0.8-0.1 sign of θv difference opposite to MR difference reverses from early time Mean confluence (m/s) -1.4-6.7 Change in mean wind direction across dryline (W to E) SSE to S Tilting of the dryline towards the denser air (lower θv) towards the denser air Comparison between May 22 and June 19 The echo plume and updraft plume at the dryline tilt towards the denser air (lower θ v ). The tilted updraft/downdraft couplet and the convergent flow are part of a solenoidal circulation. The θ v gradient shows that this circulation is thermally directed. The echo plume and updraft plume at the dryline tilt towards the denser air (lower θ v ). The tilted updraft/downdraft couplet and the convergent flow are part of a solenoidal circulation. The θ v gradient shows that this circulation is thermally directed. On May 22, MR difference decreases with height and so does θ v difference as well. This decrease is consistent with a density current. On May 22, MR difference decreases with height and so does θ v difference as well. This decrease is consistent with a density current. On June 19, strongly confluent flow (synoptically driven) happens to On June 19, strongly confluent flow (synoptically driven) happens to concentrate the regional moisture gradient and produce a dryline. Later in the day, surface sensible heating makes the cooler western side hotter and reverse the θv gradient. Although large-scale convergence and the regional slope of the terrain is important, on much smaller scales, the dryline definition appears to be driven by density current dynamics. Although large-scale convergence and the regional slope of the terrain is important, on much smaller scales, the dryline definition appears to be driven by density current dynamics. ground level