WMOD00267

• Applicant: Arlin B. Super and James A. Heimbach Jr.
• Sponsor Name: Colorado Water Conservation Board and US Bureau of Reclamation
• Project Name: Literature Review/Scientific Study
• Title: Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane
• Prepared For: Coloado Water Conservatoin Board
• Prepared By: USBR
• Date: 9/30/2005
• State: CO
• Weather Modification - Doc Type: Scientific Study

Henderson and Solak (1983) presented routine California mountain-top measurements of
SLW from a Model 871B Rosemount icing rate sensor designed for aircraft use. (A review of
article titles in the first 15 volumes of the Journal of Weather Modification suggested this was the
first article to provide winter orographic SL W observations.) More recent surface icing rate
observations have usually used a similar Rosemount icing rate sensor, Model 872B, designed
specifically for use on towers. These icing rate sensors, combined with wind and temperature
observations, provided an important new approach for assessing low-level SL W over mountain
barriers, which have been used at several locations.

Henderson and Solak (ibid.) stated that, "The measurements strongly support the conclusion
that SL W is occurring throughout larger portions of stormy periods, particularly in pre-frontal
portions, than past airborne observations over the SCPP (Sierra Cooperative Pilot Project) project
have indicated. In many cases, the SL W simply occurs over the mountainous area below the
altitudes considered safe for aircraft operations." According to Solak et al. (1988), such surface
measurements, combined with microwave radiometer measurements ofSLW, "--- influenced a
shift in the focus of SCPP research from primarily postfrontal convection to the more stratiform
and widespread cloud types." Their article also compared the aircraft type and tower type
Rosemount icing sensors, collocated on a mountaintop, and found the latter underestimated SL W
because only deicing heater cycles are output for recording. That finding was also reported by
Super et al. (1986) who noted that about 100 hours of limited but additional icing were indicated
by an aspirated aircraft type Rosemount sensor located next to a tower type sensor during a
several month period atop the Grand Mesa. However, as discussed in Appendix A, under the
review of the Solak et al. (2005) article, it is possible that many events by their aircraft sensor
may have been false.

Similar findings of near-terrain SL W have since been verified over other mountain ranges
including in Colorado and Utah. For example, similar observations over the Tushar Mountains of
southern Utah were presented by Solak et al. (1988). They made the important point that,
"Comparisons of low-altitude SL W flux values and precipitation rates suggest that increased
precipitation alone does not necessarily signal diminished augmentation potential." Other
investigations have questioned the concept that moderate or even heavy snowfall totally
eliminates seeding potential (e.g., Super and Heimbach 2005a).

The development of the microwave radiometer (Hogg et al. 1983), capable of remotely
sensing both water vapor and liquid water along a selected path, can be credited with rapid
increases in SL W documentation. Moreover, use of special waivers from the FAA, which
allowed sampling over carefully selected relatively flat mountains (Bangtail Ridge target of the
Bridger Range Experiment in Montana; Grand Mesa of Colorado; Wasatch Plateau of Utah;
Mogollon Rim of Arizona), has permitted aircraft sampling to within 1000 ft of the highest
terrain. Such low level sampling has been found by experience to typically be about 2000 ft
above the average barrier top of such semi-rugged terrain. Even occasional minor peaks force
aircraft observations well above the 1000 ft special waiver minimum above highest terrain.
Lowest practical aircraft sampling over more rugged mountains would be even higher, perhaps
3000 ft or more over the average crestline elevations. Nevertheless, use of microwave
radiometers, aircraft measurements nearer the mountains and routine surface measurements with
icing rate sensors, together have lead to a much improved portrayal of the spatial and temporal
distributions of SL W cloud over mountains.

The locations of surface icing sensors and microwave radiometers have important implications
for interpretation of resulting data. It is now recognized that subsiding air normally exists

14