WMOD00420

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

Reprinted from JOURNAL OF ATMOSPHERIC AND OcEANIC TECHNOLOGY, Vol. 5, No.4, August 1988
American Meteorological Society

Corrections of Surface Particle Probe Measurements for the Effects of Aspiration

TERRY DESHLER
Bureau of Reclamation. Auburn. California
(Manuscript received I June 1987, in final form II January 1988)

ABSTRACT

Particle probes from Particle Measuring Systems are routinely designed to be mounted on an aircraft or other
mo.vable. plat~orm. There are attempts, however, to apply this technology to stationary surface measurements
by indUCing 81rflow through th: sample area. U~ o~ th~ measurements quantitatively requires an understanding
of how the meas~rements are distort~ by the art1fiClal81rflOW. In an application to surface snowfall measurements
a 2D-C probe With a horn-shaped aspIrator has been used for six winter seasons in the Sierra Nevada of California'
~o co?,e~ m~asurements from this instrument, three other methods to measure ice particle concentrations and
sIze dlstnbutlo?s at the ground are presented. and compared to simultaneous measurements by the aspirated
2D-C. The ~?Irated 2D-C was found to routinely overestimate ice particle concentration by factors of 2.4 to
3.~. In addltton, the amount of overestimation was found to be a function of particle size and surface
Wind speed.

1. Introduction

In-cloud measurements of hydrometeors have ben-
~fite~, in. the past decade, from an array of particle
Im~gIng mstruments manufactured by Particle Mea-
sunng Systems (PMS) (Knollenberg 1981). Unfortu-
nately, similar upgrades in surface measurements have
not followed, although there are attempts to use particle
prob.es (~axena et al. 1986; Humphries 1985). A direct
apphcatIOn of PMS-2D instruments to surface mea-
sure~ents is limited by the need to know the speed of
partIcles as they pass through the sampling area so
that they can be imaged properly. In a PMS-2D pr~be,
a focused laser beam illuminates a linear array of pho-
todiodes which are scanned, or clocked, at a set fre-
quency to determine their state. The size and number
of photodiodes determine the size resolution and range
of the instrument. As a particle falls through the laser
beam it shadows, the photodiodes. The Y dimension
of the particle is determined by the number of diodes
s~adowed while the X dimension is determined by the
tIme, or number of scans, a diode remains shadowed.
To correctly size a particle in the X dimension the
scan rate of the photodiodes must be set such th~t for
e~ch scan, the ~article moves a distance equal to the
SIZe of a photodiode. If a particle moves slower or faster
t~an th~s it will be stretched or compressed in the X
dImenSIOn. For an instrument in motion, airspeed
controls the residence time of a particle in the imaging
beam and can thus be used to control the clock rate
of the photodiodes. However, with a stationary instru-

Co"esponding aut~or ~ddress: Dr. ~erry Deshler, Dept. of Physics
and Astronomy, Umverslty of Wyoming, Laramie, WY 82071.

men~, particle .speed is controlled by wind speed and
termmal velOCIty of the particle. The resultant images
ll!e stretched along the direction of particle motion,
smce the clock rate of the photodiodes is constant and
usually faster than the slow and variable fall speed of
the particles. To remedy this problem, probes have been
fitted with parabolic horns and suction fans to draw
p~cles through the sampling area at a constant speed,
while the clock rate of the photodiodes is held constant.
A 2D-C optical array probe, with parabolic horn-
shaped aspirator held vertically, has been used to mea-
sure ~nowfall at the ground for six winter seasons during
the S.lerra Cooperativ~ Pilot Project (Humphries 1985).
The mstrument prOVIdes a continuous record of snow-
f~l and requires ~inimum attention while in opera-
tion. After collectIOn, the data are in a form that can
be easily managed with computer processing. These
advantages are in stark contrast to other techniques of
recording ice particle concentrations (IPC) and size
distributions during snowfalL at the ground. Other
techniques for recording similar data require photo-
graphic or replication methods to preserve sedimen-
tation slides (Reinking 1975; Hobbs and Radke 1975).
After collection, particles preserved in the samples must
be counted and sized. Both the data collection and
data reduction methods are highly labor intensive. Un-
fortunately, the errors introduced by aspiration into
the computer recorded measurements of IPC and size
distribu~ions by the 2D-C are not easily obtained, yet
are reqUIred before the data can be used quantitatively.
Preliminary evidence collected by the first users of
an aspirated 2D-C suggests that it overestimates IPC
(Holroyd 1986). This evidence stimulated several at-
tempts at quantifying the overestimation. Norment

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