WMOD00310 ‎(2)‎

• Project Name: Sierra Cooperative Pilot Project
• Title: The Characteristics of Evolution of Supercooled Water in Wintertime Storms Over the Sierra Nevada
• Date: 9/9/1988
• Weather Modification - Doc Type: Report

SEPTEMBER 1988

MARK F. HEGGLI AND ROBERT M. RAUBER

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FIG. 2. Map of the SCPP project area in central California showing the location of
topographic contours, towns, highways. Project instrumentation discussed in the text
was located at Sheridan (SH), Lincoln (LN), Kingvale (KV), Signal Peak (SIG) and
Squaw Peak (SQP).

a. Dual-channel microwave radiometer

The dual-channel microwave radiometer is a
ground-based passive instrument that measures the
brightness temperatures emitted by water vapor and
cloud liquid water. The quantity of vapor and liquid
is determined by the magnitude of the brightness tem-
perature at specific microwave frequencies. The radi-
ometer operates at two frequencies: 20.6 GHz, sensitive
primarily to water vapor, and 31.65 GHz, sensitive
primarily to liquid water. The antenna beam width is
2.5 deg. The principles of microwave thermal emission
from Rayleigh attenuating clouds have been described
by Westwater (1972). The design and operating prin-
ciples of the dual-channel ground-based microwave ra-
diometer have been presented by Guiraud et al. (1979)
and Hogg et al. (1983). The SCPP radiometer was de-
signed by the National Oceanic and Atmospheric Ad-
ministration and manufactured by the Hughes Cor-
poration.
Path-integrated amounts of water vapor and liquid
water, expressed as depth in centimeters and milli-
meters, respectively, were calculated from the radi-
ometer measurements of brightness temperature using
statistical retrieval algorithms (Hogg et al. 1983). Spe-
cifics concerning retrieval algorithms and their use
during SCPP are given by Hogg et al. (1983) and Heggli
et al. (1987). Sources of measurement error have been
estimated theoretically by Westwater (1978) to be less
than 15%, and this estimate was confirmed in field tests
by Heggli et al. (1987). An in situ comparison between
radiometry measurements in connection with icing of
aircraft is given by Hogg et al. (1980).
Sources of data contamination were rain or mixed-
phase precipitation, which obscured the measurement

of cloud liquid. False signals could also be caused by
melting hydrometers and/or a water-coated reflector.
Periods when melting occurred or when the reflector
was wet were removed from the dataset prior to this
analysis. Ice crystals, on the other hand, have no dis-
cernible effect on emissions by water vapor and liquid,
whether from falling or resting on the reflector. (For
more details concerning the radiometer, see Hogg et
al. 1983).
The SCPP radiometer was programmed to average
vapor and liquid measurements over 2-min intervals.
It was located at Kingvale (KV), California (Fig. 2) and
was pointed vertically during the SCPP.

b. Rawinsondes

The SCPP typically had two rawinsonde stations in
operation during a single field season. The VIZ Acculok
rawinsonde was used at Sheridan (SH) California, and
at KV for nearly the entire research period. The ex-
ception was during the 1986/87 season, when the Cross
Chain Loran Atmospheric Sounding System (CLASS)
was used (Lauritson et al. 1987). During 1986/87, the
Sheridan launch site was moved 11.7 km southeast to
Lincoln, California, to accommodate the CLASS sys-
tem. Sheridan (or Lincoln) in the Sacramento Valley,
and Kingvale, near the Sierra crestline, have a cross-
barrier alignment, providing the boundary conditions
for the physical phenomena brought about by the Sierra
Nevada.

c. Icing rate detectors

The icing detector, designed by Rosemount, Inc.,
was originally intended to detect icing on board aircraft.