WMOD00420

• Title: Corrections of Surface Particle Probe Measurements for the Effects of Aspiration
• Date: 8/4/1988
• Weather Modification - Doc Type: Report

AUGUST 1988

TERR Y DESHLER

549

TABLE 1. Comparison of simultaneous sampling of snowfall by an airborne and aspirated 2D-C.
Sample
2 3 4 All samples
Winds (m S-I) 2.3 2.8 3.2 3.9 3.0
Concentration (L -I)
aspirated
aircraft 32 18 15 22 22
9.0 7.2 5.4 5.3 6.7
}. (mm-I)
aspirated
aircraft 7.1 7.6 7.4 8.8 7.7
3.5 3.8 3.1 3.3 3.4
Ratio of IPC (aspirated:aircraft) 3.6 2.5 2.8 4.2 3.3
Size interval (mm) Ratios of IPC by size bin (aspirated:aircraft)
0.05-0.1 5.2 5.1 8.4 6.2 6.2
0.1-0.2 4.4 3.0 3.4 5.2 4.0
0.2-0.3 2.9 1.8 2.7 6.2 3.4
0.3-0.4 4.3 1.8 1.6 3.2 2.7
0.4-0.5 2.5 1.3 1.9
0.5-0.6 1.4 1.2 0.7 1.1
0.6-0.8 0.5 0.5 0.3 0.3 0.4

slightly different calibration constants compared to the
one used by Holroyd. Software changes were made to
account for the new windspeed sensor. After collection,
data from the aspirated and truck-mounted 2D-C were
processed using the same software. This software was,
in most respects, similar to that described by Holroyd
(1987) and used by Holroyd (1986). Sample volume
corrections supplied by PMS for small sizes were not
used.

3. Experiments and results

a. Comparison of airborne and aspirated 2D-C

The aircraft and ground measurements were made
at 1630 LST 12 December 1985, at the airport near
Laramie, Wyoming. A fast moving short wave crossing
southeastern Wyoming produced a snow shower across
the valley that afternoon. Cloud base was 300 m above
ground, the surface temperature was -120C and winds
were from the north at 4 m S-I in light snow. The
aircraft made four passes at 10 to 20 m overhead, lasting
20 to 30 sec each, while the aspirated 2D-C was op-
erated at the ground. Observations of snow crystal habit
were not made at the time of the experiments. An anal-
ysis of 2D-C images indicates that the snowfall was
composed of moderate to heavily rimed planar crystals.
Aggregation was not occurring.
For comparison, the ground data were averaged for
time intervals selected so that sample volumes matched
those from the aircraft passes. Time intervals from 45
to 110 sec were required. For each pass a concentration
and size distribution was calculated from the two in-
struments. An exponential function of the form N

1011

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z
o

~107
0:
I-
Z
W
U
Z
o
u

.,
1
I GROUND BASED
I
lr,
Ll
1
L,,-..,
II AIRBORNE

L~-

106

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2

I
DIAMETER (mm)

FiG. I. Size distributions of snowfall from simultaneous measurements
by an airborne and aspirated 2D-C.

= Noe-w was used to fit the size distributions where N
is particle concentration and D particle diameter. Val-
ues of IPC and ^ from the two instruments are shown
in Table 1. Also shown are ratios ofIPC in each size
bin for the aspirated and aircraft data. Ratios are pre-
sented only for size bins where more than 10 particles
were sampled by each probe. A typical size distribution
from the two instruments for these passes is shown in
Fig. 1. The results of these comparisons indicate that
the aspirated instrument measures consistently high
by approximately a factor of 3 in total concentration.

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