Mountain Waves and Orographic Precipitation in a Northern Colorado Winter Storm

Gravity waves forced by terrain-induced vertical displacements of stably-stratified air parcels are referred to as mountain waves. There has been considerable investigation of the role of mountain waves in downslope windstorms, clear-air turbulence and orographic drag influence on the general circulation. In contrast, the influence of mountain waves on orographic precipitation has received comparatively little attention in the literature. Most of these investigations have employed a simulation approach using either quasi-analytical linear models or numerical weather prediction models. Observational studies linking mountain waves to orographic precipitation are lacking. Doppler-radar-based studies of mountain waves and orographic precipitation have provided useful insights, but are limited by not describing the field of vertical air motions across the primary barrier through the full depth of precipitating cloud. It is these vertical motions acting in concert with horizontal motions that directly impact the spatial distribution of precipitation relative to the barrier.

The present study addresses these limitations through analysis of a winter storm passing over the Park Range of northern Colorado on 15 December 2010 during the Colorado Airborne Multi-Phase Cloud Study (CAMPS) field experiment. Observations from the W-band Wyoming Cloud Radar onboard the University of Wyoming King Air (UWKA) research aircraft are used to document horizontal and vertical motions along with precipitation structure in a two-dimensional vertical plane that extends across the Park Range near Steamboat Springs from upstream of the windward slope, over the crest and downstream of the lee slope. Additional observations employed in the analysis include those from the UWKA flight-level instrumentation and ground-based observations from the Atmospheric Radiation Measurement (ARM) program mobile facility (AMF2) such as a W-band scanning radar, balloon sounding system and surface meteorology instrumentation. Surface observations from the Desert Research Institute (DRI) Storm Peak Laboratory (SPL) are also incorporated into the analysis.

The analysis reveals a column of upward air motion (1-2 m/s) above the lower windward slope of the primary Park Range barrier accompanied by a column of downward air motion (0.5-1 m/s) above the lee slope. Both vertical air motion structures are relatively erect, but tilted slightly upstream in the manner of a vertically propagating mountain wave. Westerly horizontal winds of 20-30 m/s in the 4-5 km MSL layer are deflected upward over the windward slope but dive sharply downward to below 3 km MSL over the lee slope, almost reaching the surface. There is a distinct horizontal gradient in the precipitation distribution, with higher intensities on the windward slope and noticeable weakening in the lee.