Investigations of a Winter Mountain Storm in Utah. Part II: Mesoscale Structure, Supercooled Liquid Water Development, and Precipitation Processes

Abstract

A comprehensive analysis of a deep winter storm system during its passage over the Tushar Mountains of southwestern Utah is reported. The case study, drawn from the 1985 Utah/NOAA cooperative weather modification experiment, is divided into descriptions of the synoptic and kinematic properties in Part I, and storm structure and composition here in Part II. In future parts of this series, the turbulence structure and indicated cloud seeding potential will be evaluated. The analysis presented here in Part II focuses on multiple remote sensor and surface microphysical observations collected from a midbarrier (2.57 km MSL) field site. The collocated remote sensors were a dual-channel microwave radiometer, a polarization lidar, and a Ka-band Doppler radar. These data are supplemented by upwind, valley-based C-band Doppler radar observations, which provided a considerably larger-scale view of the storm. In general, storm properties above the barrier were either dominated by barrier-level orographic clouds or propagating mesoscale cloud systems. The orographic cloud component consisted of weakly (?3° to ?10°C) supercooled liquid water (SLW) clouds in the form of an extended barrier-wide cap cloud that contained localized SLW concentrations. The spatial SLW distribution was linked to topographical features surrounding the midbarrier site, such as abrupt terrain rises and nearby ridges. This orographic cloud contributed to precipitation primarily through the riming of particles sedimenting from aloft, and also to some extent through an ice multiplication process involving graupel growth. In contrast, mesoscale precipitation bands associated with a slowly moving cold front generated much more significant amounts of snowfall. These precipitation bands periodically disrupted the shallow orographic SLW clouds. Mesoscale vertical circulations appear to have been particularly important in SLW and precipitation production along the leading edges of the bands. Since the SLW clouds during the latter part of the storm were based at the frontal boundary, SLW and precipitation gradually diminished as the barrier became submerged under the cold front. Based on a winter storm conceptual model, we conclude that low-level orographic SLW clouds, when decoupled from the overlying ice cloud layers of the storm, are generally inefficient producers of precipitation due to the typically warm temperatures at these altitudes in our region.