Analysis of Randomized Winter Orographic Cloud Seeding Experiments in Utah

Abstract

Analysis is made of two randomized winter orographic cloud seeding experiments conducted in the northern Wasatch Mountains. In the first experiment seeding material was released from airborne pyrotechnics and in the second experiment, from three mountain-top generators. Precipitation was measured by a network of remotely interrogated precipitation gages. Interpretation of the results are aided in particular by the use of storm type classifications, precipitation estimators based on upper level data, vertical incidence radar and aircraft icing reports. An a priori hypothesis, that seeding of clouds would increase precipitation when the 500 mb temperature is warmer than ?22°C, is rejected. Also, precipitation increases are not found in orographic clouds when the cloud-top temperature is warmer than ?29°C. An a posteriori analysis fails to show increases in precipitation when the cloud-top temperature is warmer than ?24°C. On the other hand, stratification of experimental events according to degree of aircraft icing, as a measure of supercooled water concentration, indicates that marked seeding effects may be present.