Radar Observations of Wintertime Mountain Clouds over Colorado and Utah

Abstract

Ludlam (1955) postulated that seedable clouds for initiating snowfall are the extensive, shallow orographic clouds. He referred to these as "extensive low clouds". He specifically excluded clouds that contain persistent vertical motions not associated with localities where the airstream flows over mountains. The orographic, randomized Climax, Colorado cloud seeding experiment conducted during the 1960’s followed the seeding hypothesis for orographic clouds as proposed by Ludlam (Grant, 1987). This paper presents the results of recent radar observations of the characteristics of differing types of clouds that form over the mountains of Colorado and Utah. Cloud radar echo observations show that deep, stable and deep, convective cloud systems in the interior areas of the western United States during winter generally extend to elevations higher than the 50 Kpa pressure level where temperatures during winter are sufficiently cold to permit efficient ice nucleation processes to occur. Shallow, orographically forced clouds, on the other hand, almost always occur in their entirety at elevations below the 50 Kpa level where wintertime temperatures are variable with respect to temperatures at which natural ice nucleation can be either efficient or inefficient.