Fine-Scale Radar Observations of Orographic Precipitation during Cold-Frontal Impingement on a Complex Three-Dimensional Mountain Barrier

The prediction of precipitation in mountainous terrain is complicated by the non-linearity of topography that does not act as an ideal barrier normal to the prevailing flow. While many have researched orographic precipitation over large or idealized mountain barriers, small-scale (~1–10 km) precipitation gradients and structures over very complex terrain are still poorly understood.

Here we examine the structure and evolution of a stable winter storm over the complex three-dimensional topography surrounding Little Cottonwood Canyon, a narrow, east-west oriented incision within Utah's Wasatch Mountains that is flanked by steep ridges that rise more than 2000 m above the canyon mouth. The analysis is based on observations collected during IOPs 6 and 7 of the student-directed Storm Chasing Utah Style Study (SCHUSS) in November 2011. During these IOPs, which featured the passage of a shallow surface-based cold front followed by a secondary midlevel (750-600 mb) trough, the Center for Severe Weather Research dual-polarimetric X-Band Doppler on Wheels (DOW6) performed a series of PPI and RHI scans directly up Little Cottonwood Canyon and over the surrounding topography. Upper-air observations were collected using a mobile GRAW GPS-sounding system over the adjacent Salt Lake Valley, and the University of Utah HYVIS snowflake imager and North Carolina State University Micro Rain Radar provided additional storm data within upper Little Cottonwood Canyon.

During the prefrontal phase, the radar reflectivity exhibited a wave-like structure sloping eastward over the west-facing slopes, with a reflectivity maximum located just east of the major peaks and above the upper canyon. Little variation in structure was observed over the major peaks and ridges compared to the canyon. This orographic reflectivity feature dissipated, however, as the shallow cold-frontal precipitation band penetrated into the Salt Lake Valley and Little Cottonwood Canyon. During this period, the frontal precipitation band was capped below crest level and radar reflectivities were higher within the lower canyon rather than over the upper canyon and higher peaks. With the arrival of the post-frontal secondary trough, the structure of radar reflectivity returns became deeper, with distinct enhancement over the higher peaks. The evolution of these small-scale precipitation features poses a challenge for operational weather prediction, avalanche mitigation efforts, and winter road maintenance in Little Cottonwood Canyon and may be broadly applicable to other areas of complex terrain.