Two Case Studies of Wintertime Cloud Systems over the Colorado Rockies

Abstract

The economical importance of the winter snowpack to the Colorado Rocky Mountain region (e.g., weather modification potential, ski industry, avalanche prediction, snow removal, etc.) calls for an understanding of how the mountain environment and synoptic weather systems interact to produce precipitating orographic cloud systems. This may be achieved by recognizing that each cloud system can be broken down into individual cloud components. In each of the case studies, a synoptic cloud component, an orographic cloud component and a convective cloud component were identified through the analysis of rawinsonde data, vertically pointing radar data, and visual observations. This study shows that wintertime cloud systems over mountainous terrain can be thought of as being composed of cloud components that form when vertical velocity components act on different size and time scales (synoptic, orographic, convective). The following atmospheric phenomena are shown to be important factors which contribute to wintertime orographic systems: 1) a fourth cloud forming component, 2) upwind blocking, and 3) airflow saturation. At times, a fourth cloud component, generated by the mesoscale structure of the synoptic disturbance (e.g., traveling gravity waves, moving convective bands, or organized convective lines, etc.) invigorates the entire cloud system. The orographic lift and the orographic cloud component are strongly affected by any region of stagnant air that extends upwind of the mountain ridge, i.e., upwind blocking. The orographic cloud component is also very sensitive to 1) the manner in which the upwind air is lifted and 2) the upwind profiles of temperature and moisture.