51 An airborne Raman lidar and modeling study of a dryline and convection initiation over the high plains of southeast Wyoming

Thursday, 8 August 2013
Holladay-Halsey (DoubleTree by Hilton Portland)
Philip T. Bergmaier, University of Wyoming, Laramie, WY; and Z. Wang, P. Campbell, B. Geerts, and B. Liu

Handout (3.5 MB)

An observational and modeling study of a dryline and associated initiation of deep convection over the high plains of southeastern Wyoming is presented. Operational radar and station measurements show that the dryline is a strongly-convergent humidity boundary with a modest density (i.e. buoyancy) gradient. The dryline becomes well-developed in the afternoon when a deep convective boundary layer is present. Its westward progression is inhibited by high terrain (the Laramie ridge, ~2400 m MSL). Instead, the dryline boundary moves eastward in the late afternoon, as low-level westerly wind on the dry side intensifies with an advancing weak upper-level trof. At least one of the thunderstorms that emerged from the dryline became severe. Weather Research and Forecast (WRF) model simulations accurately reproduce the dryline, as well as the timing and location of convection initiation. The WRF output is used further to examine how deep convection initiated along the dryline. A dryline bulge over the Cheyenne Ridge appears to be an essential mesoscale ingredient in convection initiation on this day, as it is associated with the strongest convergence and buoyancy gradient. Horizontal convective rolls and small dryline vortices are evident in the model simulation; they may have aided the initiation. Vertical transects of Raman lidar water vapor, collected along flight legs in close proximity to the emerging thunderstorm, reveal a highly-convergent, sharp humidity boundary with denser air on the humid side and strong vertical wind shear across the humidity boundary. The Raman lidar reveals sheared plumes of high mixing ratio, several km in diameter, just east of the surface dryline boundary, emerging above the top of the moist wedge and approaching the LCL. An airborne back-scatter lidar reveals some convective clouds above flight level. Their cloud base indicates that these clouds emerge from the moist wedge, not from the much drier air in which the cloudy parcels are embedded. In other words, these cumuli are moisture plumes detached from their shallow source in the dry westerly current above
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