Part I of this study describes the mesoscale structure of a dryline over southeastern Wyoming. This dryline formed just east of the western rim of the high plains on 22 June 2010 and became more defined as it progressed eastward during the afternoon. Part I also describes the numerically simulated structure and evolution of this dryline and the observed initiation of deep convection in the vicinity of the dryline. An instrumented aircraft, the University of Wyoming King Air, repeatedly flew across this dryline, mostly low enough to penetrate the moist-air wedge east of the dryline. Flight-level in situ data along these low-level penetrations indicate relatively high values of convective available potential energy (CAPE; >1500 J kg~(-1)), yet low convective inhibition, within a few kilometers of the dryline.Water vapor transects obtained from a compact nadir-pointing Raman lidar aboard the aircraft reveal an extremely sharp humidity gradient below flight level along the dryline, coinciding with the fineline seen in operational weather radar base reflectivity imagery. They also reveal several plumes of higher specific humidity within the dry elevated mixed layer above the moistair wedge, possibly precursors of cumulus clouds. The vertical structure of the dryline revealed by Raman lidar and the flight-level data correspond well to that in the high-resolution numerical simulation.
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