Abstract
Physical storm characteristics during the operational period of the Santa Barbara Cloud Seeding Project have been studied. It is shown that the vertical storm structure, particularly the depth of the low-level convective layer, is of importance in determining (1) the area distribution of precipitation, (2) the transport of seeding material from the ground to the nucleation levels, and (3) the existence of supercooled liquid water at nucleation levels. Item (1) above influences the correlation between target and control precipitation amounts, and items (2) and (3) influence the effective seeding of the storm. The seeded and unseeded storms of the Project have been treated using these concepts in order to investigate their influence on the inconclusive statistical results of the Project.
On the basis of qualitative seedability criteria, it is estimated that approximately one-half of the precipitation in the Project period occurred under relatively poor seeding conditions. This was determined by classifying storms into convective and stable flow types. It is also shown that the convective and stable cases have differing orographic precipitation characteristics and that, as a consequence, the target-control relationship is a function of vertical storm stability.
The study suggests that the possibility of detecting seeding effects can be improved by elimination of poor seeding cases through development of better seedability critera and by stratifying target-control relationships according to storm type. Also indicated is a need for improved understanding of natural rainfall variations before substantial progress can be made in detecting detailed variations caused by seeding.
