Winter "Cloud Seeding Windows" and Potential Influences of Targeted Mountain Barriers

Abstract

The concept of “winter cloud seeding windows” is a familiar theme found in a number of earlier publications. More recent feasibility studies, physical observations and analyses of existing cloud seeding programs have indicated some of this earlier thinking has considerable merit. The concept that deep winter storm systems with cold cloud tops often appear to be naturally efficient with little or no supercooled liquid water content is especially important. It appears from a variety of earlier sources of information and more recent observations that shallow, orographically induced clouds often contain supercooled liquid water and therefore offer good cloud seeding potential.

Several studies and observations suggest that shallow orographic clouds that contain supercooled liquid water frequently occur after the passage of a surface cold front and even after the passage of an upper level trough. If the occurrence of such clouds is viewed in the context of the orientation of the targeted mountain barriers, the question can be asked if mountain barrier orientations have any impact on the development of these types of “seedable” clouds? This is basically a question of the amount of up barrier flow that accompanies these shallow orographic clouds. North American Weather Consultants  has developed a conceptual model  that the barrier orientation that provides the best conditions for the formation of these kinds of clouds in the western United States (and perhaps elsewhere) are barriers with a north-south orientation since post-frontal or post upper trough passage conditions will produce considerable up barrier flow over these barriers. Fortunately, most mountain barriers in the western United States have such an orientation.

North American Weather Consultants believes that recognition and verification of the above will be important in the conduct of future winter orographic cloud seeding programs. Placing “seedabilty” in the synoptic setting and relating “seedability” to barrier orientation will be important in estimating potential cloud seeding effects in different project areas in the future.