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A Systenl That Monitors Blowing Snow
in Forest Canopies
R. A. Schmidt and Robert L. Jairell 1
Abstract--An electronic system that connects a desktop computer
(PC) to tower-mounted arrays of snow particle counters I!Irovides a new
approach to an old problem--why more snow accumulates In sma" openings than in the surrounding forest. Each photoelectric sensor generates
a pulse for a particle passing through its light beam. These are counted by
a microprocessor system that passes the sums to the PC. Preliminary
measurements in both a clearing and the forest upwind show counts
Increasing with height in the 15-m region centered near the mean canopy
height (18 m).
Why do small forest clearings accumulate more snow than
adjacent stands? This question has puzzled researchers since
studies began at the Fraser Experimental Forest in Colorado.
They developed two explanations: (1) snow trapped by the
forest canopy (inte.rception) evaporates in place, re.ducing
snowpack on the forest floor, and (2) wind guides more
snowfall (including interception) jnto the clearings (aerodynamic re.distribution).
Interce.ption was first favored as the likely explanation.
Goode.11 (1959) measured evaporation of interc.epted snow,
and c.alled for detailed analytical studie.s of the process
(Goodell 1963). However, intensive measurements of snowpack on the Fool Creek watershed suggested that timber
harvest did not increase total snow accumulation, compared to
the East St. Louis control watershed (Hoover and Leaf 1967).
This evidence supported the aerodynamic redistribution
hypothesis, because reducing interception loss by timber
harvest should increase total snow accumulation.
Researchers led by Charles Troendle have continued to
measure the peak water equivalent of snow on Fool Creek and
East St. Louis. 'Vith precision increased by a longer measuring
period, it now appears that total snow accumulation on Fool
Creek did increase about 9% after harvest. This increase
approaches the 12% increase Hoover and Leaf (1967) said
was expected, assuming all increased snowpack (and thus
streamflow) resulted entirely from reduced interception loss
(Troendle and King 1985).
Analytical experiments on the evaporation of blowing
snow in the plajns environment (Schmidt 1972, 1982; Tabler
and Schmidt 1972; Tabler 1975), show how strongly surface
area affects the process. The results support the likelihood
that evaporation of intercepted snow explains' the increased
accumulation in dearings. Snow held on branches presents a
huge extension of surface area exposed to moving air, compared to the snow surfac.e in the dearing.
Intensive snowboard measureme·nts reported by ""heeler
(in press) for the 1985-86 winte.r, demonstrate that accumulation differences between dearing and fore.st (1) usually occur
during storms, not between storms, and (2) are inversely
relate.d to wind speed during the storm. Both re.sults, but
especially the latter, argue against aerodynamic redistribution
as the main cause of the difference.
The electronic system described here is a step toward the
detailed studies of the distribution and evaporation of snowfall in forest canopies and clearings called for by Goodell in
1963. This paper presents a method of counting precipitating
snow crystals, and shows example results from a cut block on
the Fraser Experimental Forest.
Study Site
The cut block, 80m wide in a stand of mature spruce and
lodgepole pine with an understory of subalpine fir, extends
100m down a north-facing slope (40%) draining into West St.
Loui.s Creek, in the Fraser Experimental Forest (fig. 1). The
block is near Short Cre.ek, in the center of Section 8, T2S,
R76W, at 2925m elevation (9600ft), 106°54' west longitude,
39"52'30" north latitude. Wind during storms is most often
from the west to northwest at this site. Troendle designed the
block and erected towers for these experiments, with the
assistance of ]\.lan11al H. Martinez, who also constructed the
tower wiring harnesses and placed the cables between the
11-1ydrologist and Forestry Technician, respectively, with the 110cky
Mountain Forest and Range Experiment Station, 222 South 22nd Street,
Laramie, Wyoming, 82070·5299, in cooperation with the University of
Wyoming. Station headquarters is in Fort Collins, in cooperation with
Colorado State University.
227
MAST #3
DOWNWIND
MAST #2
CLEARING
MAST #1
UPWIND
GENERATOR
~WIND
TRAILER
Figure 1.--The experimental site on West St. louis Creek Includes a cut block In a mixed stand of
spruce, fir, and lodgepole 1,lne. This clearing Is 80m wide and extends 100m down a north-facing
40% slope. Towers are located (1) upwind, (2) center, and (3) downwind of the clearing, with 85m
separating towers 1 and 2.
Counting Snowflakes
towers and the trailer. A 27m tower supports instruments in
the center of the dearing, 40m east from the upwind edge. In
the upwind forest, 85m from the clearing tower, a 34m tower
extends about 16m above the average canopy height of 18m. A
third tower in the forest downwind of the dearing was not used
in initial experiments with this system. (All three towers
carried triaxial anemometers at two levels as part of another
study by David ~1iller.)
For the first tests, six sensors were uniformly spaced 3m
(10ft) apart between 15 and 30m on the upwind mast during
several storms. The lowest two sensors were below the average
canopy height. AU sensors were then deployed on the clearing
tower, again at the same spacing, between 11 and 26m above
the snow surface, during two storms. Finally, three sensors
measured snowfaU at the highest (#:1), lowest (ll6), and #4
position on both towers, allowing coniparison of simultaneous
counts in the forest and clearing.
The system (fig. 2) consists of the sensors mounted on
towers, wiring harnesses on each tower to provide signal and
power interconnections, cables from each tower to a small
instrument trailer below the dearing, counting circuits that
sum each sensor's signals, and the desk-top computer (PC). A
propane.-driven generator provides satisfactory electric
power, since the system operates only during events when an
observer is at the site.
The snow sensor, called a snow particle counter (SPC), is
a device developed to measure the number and size of
particles moving in blizzards (Schmidt 1977). Two phototransistors sense shadows cast by particles passing through a light
beam (fig. 3), producing voltage pulses that are amplified at
the sensor and sent to counting circuits. Pulse amplitude is
related to the size of the particle's shadow. (For these initial
228
experiments, the system does not extract the information on
particle size, however.)
The microprocessor syste.m (fig. 2) passes counts from
runs, of 1 to 10 min duration, to the computer for printing and
storage on di.sk. Using an internal analog-to-digital converter,
the computer also monitors wind spee.d and direction from an
anemometer and vane mounted at 10m on the clearing tower.
To assure comparable sensitivity, each SPC is calibrated by
spinning a wire through the beam and adjusting amplification
for a standard output, measured as peak-to-peak pulse amplitude on an oscilloscope. This was accomplished before and
after each day's experiment for the first few events, until
experience showed such frequent calibrations were unnecessary. After that, calibrations were checked when some· change
was made in the setup, suc.h as nu;ving sensors between towers.
(l\filler equippe.d each tower with a safety rope and halyard
system, which greatly fadlitated calibration of the SPC's.)
Example Results
Although the objective of initial experime.nts was only to
test the measurement technique, transfer from our laboratory
to Dr. Troendle's study site proceeded so smoothly that he was
able to test two hypotheses concerning the snow ac.c.umulation
problem during l\farch, 1987. Details will be reported elsewhere, but the hypotheses were: (1) There is no significant
differe.nce i.n particle counts between levels on a tower, and (2)
The.re. is no si.gnificant difference between average counts at
each tower.
TOWER 2
TOWER 1
#1
#3
#2
SENSOR
POSITIONS
#4
#5
#5
#6
#6
c:
01
downwind
Figure 3.--The snow particle counter (SPC) senses the shadow of
particles passing through the light beam, producing amplified
voltage pulses. With two windows, estimates of particle speed
are possible (from Schmidt 1977).
Figures 4a and 4b show typical exam pIes of counts when all
sensors were on one tower. Counts were normalized by the
mean count of all sensors for each run, and heights we·re
normalized by the midpoint and spacing of the sensor array.
Figure 4c is a comparison of c.ounts during a run with three
sensors at each tower. We found no large di.fferences i.n
average counts between towers.
Although designed to measure the smaller pa.rticles and
greater frequency of drifting in blizzards, the SPC's provide.d
useful and a.pparently consistent mea.sures of snowfaH under
light winds in a forest canopy.
If these preliminary results withstand tests for statistical
significance, it appears that particle count usually increases
with height both in the clearing and above the c.anopy (although the opposite gradient was occasionally observed).
Average counts seem to be about the same at each tower,
however. The next question is, "Are particle sizes similar?"-that is, do these counts represent the same mass flu."t. We are
adding electronics to determine size distributions for expe.riments during the 1987-88 winter.
To explain the decrease in count within the forest seems
simple.-- interception. Yet a similar gradie.nt of partide. numbers appears in the dearing measurements. Does this reflect
an aerodynamic. effect? Anemometers at each SPC location
will heJp esti.mate particle trajectories in upcomi.ng experiments with this system.
#3
#4
'0
'jj;
Plans
#1
#2
&
o
~f--L----""~~:':':":="'-­
SHIELDED CABLES
Literature Citeci
1_______________________________________ _
Goodell, B. C.1959. Management of forest stands in Western
United States to influence the flow of snow-fed streams.
In: Symppsium of Hannoversch-Munden, Publication
No. 48 of the International Assodation of Sdentific
Hydrology, Gentbrugge, Belgium. 1: 49-58.
Figure 2.--Slgnals fl'C)m ea(:h SPC on the t()VI(~rs am summed and
transferred to the computer by a microprocessor system. An
ana.log-to-digltal converter in the compute,' measures voltages
from wind speed and direction sensors at the 10-m height on
tower 2, in the clearing.
229
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2.5
UPWIND FOREST
11 MARCH 1987
RUN 14
MIDPOINT= 22.9m
SPACING= 3. 3m
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a
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16 MARCH 1987
RUN 11
MIDPOINT= 18.9m
SPACING= 3. 3m
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~
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3
3
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2.5
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SIMULTANEOUS
UPWIND
o CLEARING
20 MARCH 1987
RUN 3
c.
+
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z
1-1
Figure IJ.-Example plots of particle counts during 5~mln runs with six
sensors at (a) towel' 1 (upwind) and (b) tower 2 (clearing). Three
sensors at each towel' gave the counts In (e). Both count and
height are normalized by the respective means.
u
<:
CL
(J)
',--,.
0. 5
1.0
l-
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/
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MEAN COUNT= 763
/
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~)
Tabler, Ronald 0.1975. Estimating the. transport and evaporation of blowing snow. liz: Snow Management on the
Great Plains: Proceeding of the symposium; 1975 July 29;
Bi.smarck ND. Publication No. 73. Research Committee
of the Great Plains Agricultural Council. Agricultural
Experiment Station. Unive.rsity of Nebraska, Lincoln. 85104.
Goodell, B. C. 1963. A reappraisal of precipitation interception by plants and attendant water loss. Journal of Soil and
Water Conservation. 18(6): 231-324.
Hoover, Marvin D. and Charles F. Leaf 1967. Process and
significance of interception in Colorado subalpine forest.
In: Forest Hydrology: Proceedings of an International
Symposium; Vol. E. Soper and H. 'AT. Lull (eds.). Pergamon
Press, New York. 213-223.
Tabler, Ronald D., and R. A. Schmidt. 1972. Weather conditi.ons that determine snow transport distances at a site in
Wyoming. In: The role of snow and ice in hydrology:
Proceedi.ngs of the Banff Symposia; 1972 Septe.mber;
Banff, Alberta, Canada. UNESCO/'Vl\fO/IAHS. Vol. 1.
118-127.
Troendle, C. A. and R. M. King. 1985. The effect of timber
harvest on the Fool Creek watershed, 30 years later. Water
Resources Research. 21(12): 1915-1922.
Schmidt, R. A. 1972. Sublimation of wind-transported snow-a model. Research Paper RM-90. U.S. Department of
Agriculture, Forest Service, Rocky l\1ountain Fore·st and
Range Experiment Station. Fort Collins, CO. 24 p.
Schmidt, R. A. 1977. A system that measures blowing snow.
Research Paper RM-194. U.S. Department of Agriculture., Forest Service, Rocky Mountain Forest and Range
Experiment Station. Fort Collins, CO. 80 p.
'ATheeler, Kent. (in press). Interception and redistribution of
snow in a subalpine forest on a storm-by-storm basis. In:
55th Annual Western Snow Conference: Proceedings;
1987 April. Vancouver, British Columbia, Canada.
Schmidt, R. A. 1982. Vertical profiles of wind spee.d, snow
concentration, and humidity in blowing snow. Boundary
Layer l\feteorology. 23: 223-246.
230