WMOD00443

• Title: An Investigation of Precipitating Ice Crystals from Natural and Seeded Winter Orographic Clouds
• Date: 11/18/1974
• Weather Modification - Doc Type: Report

~~.

- ..,.

1~._--~

i

Reprinted from the Preprint Volurre, 4th Conference on
weather M::x1ification, Nov. 18-21,1974, Ft.Lauderdale,
Fla. Published by the AIreL Heted,r. Soc., Boston, MA.

AN INVESTIGATION OF PRECIPITATING ICE CRYSTALS

FROM NATURAL AND SEEDED WINTER OROGRAPHIC CLOUDS

Larry Vardiman and Gerald Mulvey

Colorado State University
Ft. Collins, Colorado

and

Curtis Hartzell

Western Scientific Services, Inc.
Ft. Collins, Colorado

1.

INTRODUCTION

An investigation of ice crystals
which precipitate from winter orographic clouds
over Wolf Creek Pass is being conducted by
Wes~ern Scientific Services, Incorporated (WSsr)
to examine the effectiveness of the seeding
opetations of the Colorado River Basin Pilot
Project. The Colorado River Basin Project is
a major cloud seeding research effort of the
Bureau of Reclamation's Atmospheric Water
Resources program. The target area for this
research project embraces 1,300 square miles in
the San Juan Mountain Range of southwestern
Colorado. Wolf Creek Pass is centrally located
in the target area. The rationale of the
investigation is that seeding to increase snow-
fall in orographic clouds will cause changes
in the characteristics of the ice crystals in
seeped clouds compared to natural clouds. At
Wolf Creek Pass, these changes are expected to
app,ear as an increase in the concentration of
cry:stals, a decrease in crystal size and amount
of rime, and a possible change in crystal type.

The primary goals of this investigation
are to (1) characterize the precipitation from
natural storms, (2) detect to what extent seeding
at Wolf Creek Pass is modifying the ice crystals
relative to natural storms, and (3) investigate
the extent seeding potential is being utilized
based on the ice crystal characteristics. A
reasonable number of natural storms and seeded
storms are required to adequately characterize
the difference due to seeding under various
cloud conditions.

2.

DATA SOURCES

A photographic ice crystal replica tor
designed and built by WSSI has been used during
the winters of 1972-73 and 1973-74 to obtain the
ice crystal data necessary for this investigation.
The replicator is shown in Fig. 1 set up for
operation in the field. When in use it has a
wind shield constructed of 55-gallon barrels
surrounding the instrument to allow representative
collection of ice crystals. The photographic
replica tor consists of a modified 35-mm radar
camera mounted over a moving black velvet belt.
The belt carries the ice crystals from an adjust-
able settling area to the camera where they are

-

Figure 1. Photographic Ice Crystal
Replicator at Wolf Creek Pass

photographed automatically. The belt is continually
cleaned with a rotating brush qn the under side of
the instrument. The belt moves at a rate of
slightly more than three inches per minute, and
the slit width at the settling area can be varied
from one to three inches. The camera can be set
to photograph one frame per minute, three frames
per minute, or six frames per minute. At six
frames per minute, the frames overlap so that
about half of anyone frame has already appeared
in the previous photograph. In addition to the
crystals, a frame number, a clock, and an infor-
mation slate appear on one side of each photograph.
A typical frame from the photographic replica tor
is shown in Fig. 2. The optic~ of the camera are
adjusted so that the ice cryst~ls appear 0.60 times
actual size on the film and particles down to 50
microns can be identified. The camera holds
100-foot rolls of 35-mm high r~solving microfilm,
allowing data to be taken continuously for over
24 hours When set to photograph one frame per
minute.

468

:\It''.