THE 1990 VALENTINE DAY ARCTIC OUTBREAK .1. MESOSCALE AND MICROSCALE STRUCTURE AND EVOLUTION OF A COLORADO FRONT RANGE SHALLOW UPSLOPE CLOUD

The mesoscale and microscale structure and evolution of a shallow, ups lope cloud is described using observations obtained during the Winter Icing and Storms Project(WISP) and model simulations. The upslope clou d formed within a shallow arctic air mass that moved into the region e ast of the Rocky Mountains between 12 and 16 February and contained si gnificant amounts of supercooled liquid water for nearly 30 h. Two dis tinct layers were evident in the cloud. The lower layer was near neutr al stability( boundary layer air) and contained easterly upslope flow. The upper layer (frontal transition zone) was thermodynamically stabl e and contained southerly flow. Overlying the upslope cloud was a dry, southwesterly flow of 20-25 m s(-1), resulting in strong wind shear n ear cloud top. Within 10 km of the Rocky Mountain barrier, easterly lo w-level flow was lifted up and over the mountains. The above-described kinematic and thermodynamic structure produced three distinct mechani sms leading to the production of supercooled liquid water: 1) upslope flow over the gently rising terrain leading into the Colorado Front Ra nge, up the slopes of the Rocky Mountains and over local ridges, 2) up glide flow within a frontal transition zone, and 3) turbulent mixing i n the boundary layer. Supercooled liquid water was also produced by 1) upward motion at the leading edge of three cold surges and 2) vertica l motion produced by low-level convergence in the surface wind field. Large cloud droplets were present near the top of this cloud (approxim ately 50-mu m diameter), which grew by a direct coalescence process in to freezing drizzle in regions of the storm where the liquid water con tent was greater than 0.25 g m(-3) and vertical velocity was at least 10 cm s(-1). Ice crystal concentrations greater than 1 L(-1) were obse rved in the lower cloud layer containing boundary layer air when the t op of the boundary layer was colder than -12 degrees C. The upper half of the cloud was ice-free despite temperatures as low as -15 degrees C, resulting in long-lived supercooled liquid water in this region of the cloud.