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1166 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide over the Rocky Mountains. Part III: Observations over the Grand Mesa, Colorado ARLIN B. SUPER AND BRUCE A. BOE. Bureau of Reclamation, Montrose Skywater Office, Montrose, Colorado (Manuscript received 6 April 1987, in final form 14 March 1988) ABSTRACf During March 1986, several airborne and ground-based silver iodide (AgI) seeding experiments were conducted ove~ th~ Grand Mesa, Colorado, during a three-day period of northerly flow and shallow orographic cloud. ~hlle little natural snowfall was observed during these experiments, supercooled liquid water formed over the wI~d';Yard slopes and evaporated to the lee ofthe mesa for many hours. Seeding-induced microphysical changes comcldent .WIth the. AgI plumes .were found in all eight experiments, (including two that employed ground- based seedmg) by aircraft sampling about 500 m above the mesa top. Precipitation rates estimated from ice particle images at flight levels suggested increases within the seeded volumes in all but one experiment. Surface pre~ipitation inc~e~s we~ observed in three aircraft seeding experiments and one ground-based seeding ex- penment that comclded WIth the passage of AgI plumes aloft. Surface observations were not possible during the other ground-based seeding experiment, but some increase in snowfall is thought probable. Three aircraft seeding experiments failed to show surface snowfall increases, and reasons for this are explored. 1. Introduction The objective of the Colorado River Augmentation Demonstration Program, part of the Bureau of Rec- lamation's Project Skywater, is to improve cloud seed- ing technology for the Colorado River Basin. The Grand Mesa of western Colorado was selected as an experimental area particularly well suited for physical investigations of cloud and precipitation responses to the introduction of seeding agents. Physical cloud seeding experiments were conducted over the Grand Mesa during 18-20 Mar 1986, a period characterized by northerly flow and shallow orographic cloud. The goal of these experiments was to measure directly seeding-induced microphysical changes within cloud and in precipitation at the surface. A more de- tailed discussion of objectives and the physical hy- pothesis is given in Part I (Super et al. 1988). In six experiments, silver iodide (AgI) was produced at varying distances north of the mesa along flight tracks flown approximately perpendicular to the wind. Seed- ing was conducted at the 3.8 km level (all altitudes msl), the lowest practical flight altitude, which was about 500 m above the mesa top. Subsequent sampling passes by the instrumented aircraft were also flown at 3.8 km, but parallel to the wind, through the seeded zone and over the surface target (hereafter called Snow * Present affiliation: North Dakota Atmospheric Resource Board Bismark, North Dakota. ' Corresponding author address: Dr. Arlin B. Super, Bureau ofRec- lamation, Code 0-3720, Denver Federal Center, Denver, CO 80225. @ 1988 American Meteorological Society Lab, see Fig. 1). The along-the-wind sampling passes continued until the seeded zone passed well downwind of the Snow Lab. Two other experiments used a ground-based AgI generator on the upwind slope of the mesa. Passes were flown over the top of the mesa and approximately per- pendicular to the wind, which allowed sampling of both the seeded cloud volume (hereafter called the "seeding plume" or "seeded zone") and the adjoining natural cloud. Targeting of the Snow Lab was not possible in these cases, and only aircraft measurements were re- corded. In one case, cloud base was far enough above the mesa to allow subcloud sampling by the aircraft. 2. Project area and operational limitations The Grand Mesa is a large, flat-topped barrier with an average caprock elevation of between 3.2 and 3.3 km in the study region. It has a vertical rise of about 1.6 km from the floors of the Colorado River Valley to the north, the Grand Valley to the west, and the Uncompahgre Valley to the south. Upper air flow from the north, west and south is unblocked, resulting in a high frequency of orographic storms. The major axis of the mesa is oriented WSW-ENE, so that southerly and northerly flow are especially conducive to the for- mation of orographic cloud. An all-weather highway over the mesa top is open year-round, and snowmobiles afford good access to most of the top. Much of the mesa is covered by conifer forest, and clearings provide adequate sites for precip- itation gages. Commercial power is available in a few locations on top of the mesa and on the north and