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A first observational and modeling study of a dryline and associated initiation of deep convection over the high plains of southeastern Wyoming is presented. Radar and station measurements show that the dryline is a well-defined convergent humidity boundary with a modest density (i.e., buoyancy) gradient. Its development, intensity, and movement are regulated by the terrain, diurnal land surface and boundary layer processes, and synoptic-scale evolution. At least one of the thunderstorms that emerged from the dryline became severe. Weather Research and Forecasting Model (WRF) simulations accurately reproduce measured aspects of this dryline, as well as the timing and location of convection initiation. The WRF output is used further to characterize the dryline vertical and horizontal structures and to examine convection initiation processes.Adryline bulge over a local terrain ridge appears to be an essential ingredient in convection initiation on this day: just north of this bulge the surface convergence and buoyancy gradient are strongest, and deep convection is triggered. In this region especially, the WRF simulation produces horizontal convective rolls intersecting with the dryline, as well as small cyclonic vortices along the dryline. In fact, the primary storm cell initiates just downwind of one such vortex. Part II of this study describes the finescale vertical structure of this dryline using airborne Raman lidar data.




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