WMOD00388

• Title: Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane
• Date: 9/30/2005
• State: CO
• Weather Modification - Doc Type: Report

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Appendix A: Brief Summaries of Selected Published Articles and
Reports Dealing with SL W Cloud Characteristics.

The summaries are presented in order of publication date to provide historical context.

Ludlam, F. H., 1955: Artificial snowfall from mountain clouds. Tellus, 7, 277-290.

Without providing observational support, the classic and farsighted article by Ludlam (1955)
stated that, "During the winter in Scandinavia extensive low clouds often occur which are only
several hundred meters thick and which give no precipitation. The clouds are composed of
supercooled droplets, but the temperature is not low enough for an abundant natural formation of
ice crystals, which could lead to the development of snow." His Fig. 1 provided a conceptual
portrayal of these clouds which is very similar to current understanding. The cloud base is below
the crestline and extends upwind for a considerable distance. The SL W cloud top has its
maximum elevation directly over the crestline, and SL W quickly evaporates downwind of the
crest. Calculations were given which suggested the optimum concentration of seeded ice crystals
would be about 20 per liter, but it was shown that the concentration could be several times higher
without seriously reducing the efficiency of the seeding operation. The thin orographic clouds
described by Ludlam (ibid.) are admittedly a "simple case" over minor mountains compared to
the rugged and complex Colorado Rockies.

Boe, B. A. and A. B. Super, 1986: Wintertime characteristics of supercooled liquid water over the Grand
Mesa of western Colorado. J. Weather Modification, 18, 102-107.

Microwave radiometer observations of vertically-integrated SLW were made over the Grand
Mesa of west-central Colorado during November and December 1983 and January through March
1985. The Grand Mesa is a high, generally flat-topped barrier averaging about 3200 m msl
(10,500 ft) in elevation. It is located 125 miles southwest of the Park Range. The radiometer
location was just south of and 400 ft below the mesa top. Almost all SL W episodes in the 5-
month data set had 700 mb wind directions from the southwest quadrant with typical speeds near
10 m S.l (22 mph). The local topography of the east-west oriented Grand Mesa often caused the
near-surface winds to turn to southerly which blew directly upslope. Virtually no SL W was
observed when mesa top temperatures were below -140C, and greatest SLW production was
associated with mesa top temperatures in the -4 to -1 OOC range, in good agreement with the
findings of Hindman (1986) for the Park Range.

Another similar finding was that 29% of all 3351 hours with valid data from 5 months of
sampling had some SL W present over the mesa. However, many of these hours had vertically-
integrated amounts less than 0.05 mm. Such low amounts can be safely ignored as only trace
precipitation could result from seeding them as shown by Super (1999a). He estimated that about
17% of all hours with radiometer observations had values of 0.05 mm or greater; that is,
significant SL W amounts, over lhe five month dataset. This is still a relatively high frequency
when one considers the many fair weather days included in the large sample.

Aircraft observations of L WC over the Grand Mesa, also presented by Boe and Super (1986),
showed greatest amounts on the lowest passes made 1000 ft above the highest terrain (about 2000
ft over the average mesa top terrain), and near the mesa's upwind edge. Subsidence dramatically
reduced the aircraft-sampled L WC within about 4 miles of the southern edge. It was estimated