Grant and Elliott, 1974 postulated that The greatest potential for seeding with artificial nuclei to augment precipitation should occur with cloud summit temperatures in the range from about -10C to -25C.
Reynolds, 1986 concluded that seeding conditions often existed in shallow orographic clouds that frequently formed after the passage of an upper cold front.
More recent feasibility studies, physical observations and analyses of existing cloud seeding programs have indicated some of the earlier thinking, as indicated in the above references, has considerable merit. The concept that deep systems with cold cloud tops often appear to be naturally efficient with low or no supercooled liquid water contents is especially important. It appears from a variety of earlier sources of information and more recent observations that post frontal conditions with shallow orographically induced clouds often contain supercooled liquid water and therefore offer cloud seeding potential.
If the occurrence of such clouds is viewed in the context of the orientation of the targeted mountain barriers some interesting insight into seedablity of winter orographic clouds emerges. Several studies and observations suggest that these shallow orographic clouds frequently occur after the passage of a cold surface front and even after the passage of an upper level trough. The question then becomes what types of barrier orientations favor the development of these clouds? This is basically a question of the amount of up barrier flow. North American Weather Consultants (NAWC) has developed a conceptual model that barrier orientations that provide 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 (west to north west). Fortunately most mountain barriers in the western United States have such an orientation.
There are exceptions, however, where some barriers have west-east orientations. Examples of such barriers include the San Juan Mountains in Colorado, the Uinta Mountains in Utah and the Centennial, Lions Head and Henrys Lake Mountains in north-eastern Idaho/southwestern Montana. Our contention is that these types of barriers will experience the best orographic enhancement during storms that have a pre-frontal, pre-trough southerly flow component. Observations indicate such storms are typically deep with cold cloud tops and therefore have limited seeding potential. There is one mountain barrier that is a mix of these two types; the Sierra Nevada in California. This barrier has more of a northwest to southeast orientation. Post frontal, post trough flow over the Sierras therefore does not have as strong an up barrier component to the lower level wind flow as do the more north-south oriented barriers. As a consequence of the above, the seeding potential is assumed to be lower over these west-east or northwest-southeast oriented barriers. Results from earlier/present seeding programs and several feasibility/design studies will be summarized which support this hypothesis.
It should be noted that some mountainous areas of the west seem to experience a large percentage of precipitation occurring in prefrontal conditions. Based upon the high, cold cloud tops that typically occur in prefrontal conditions, seeding effects would be expected to be less than in areas that experience substantial post-frontal precipitation independent of barrier orientation considerations.
NAWC 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 effects from different program areas. Some targeting issues will also need attention. For example, if we are dealing with a west-east oriented barrier, placing a majority of the ground generators or conducting aircraft seeding flights north of these barriers may be appropriate. An interesting aspect related to this approach is that the north sides (upwind slopes) of such barriers will see increases in precipitation with the assumption that such increases will also carry over to the southern (downwind slopes) of the barrier. Following through with this thinking, it would then be desirable for the north slopes of these barriers to be part of the intended target area and local sponsors from these areas identified to contribute financially to the conduct of the cloud seeding operations. Similar reasoning for north-south oriented barriers might indicate the desirability of more generators (seeding flights) northwest instead of southwest of the target area(s).