WMOD00483

• Title: Microphysical Observations of Winter Clouds Over the Southern Canadian Rockies
• Weather Modification - Doc Type: Report

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Cloud liquid Water Content. February 9.1984

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Cloud liquid water content (g/m3)

Figure 2. Frequency distribution of "in-cloud"
liquid water content for cloud penetrations made
on February 9. 1984.

~.3 Cloud Precipitation Conversion Efficiency

Estimates of the precipitation conversion
efficiency of the winter clouds for selected days
were made as indicators of the general seeding
potential. Conversion efficiency was defined as
that portion of the upwind llOisture converted to
precipitation-sized particles in the clouds over
the mountain region. This percentage was
expressed as the ratio of the average ice water
(r.lelted ice mass) of precipitation-sized
particles within the clouds to the available
vapour supply. This approach is sirni1ar to that
of a previous case-Study analysis of an
orographic cloud systeD measured in 1982
(Krauss et al., 1983). It is interpreted as a
relative-esfimate, and not of the overall
efficiency of the removal of condensed water from
the clouds (e.g. Dirks, 1973).

Three days were selected for this
analysis, one fro~ each of the three field
seasons: March 15th and December 9th, 1983 and
February 9th. 19R4. These three days were
characterised by clouds lIfith bases at or below
the mountain ridge line, extending from the
western edge of the Rocky Mountain massif and
across the continental divide. The clouds of the
F ehruary and December cases were genera lly
shallow stratifom while the March case include
SOllIe cumulus type clouds. Winds at cloud level
were predominantly westerly. Surface
meteorological stations located along the flight
tracks reported light snow during the aircraft
measurement periOdS.

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Aircraft "in-cloud- .easurements were
along the east-west flight track (Fi gure 1,
leg C), venera lly at the 11,000 to 12,000 feet.
level and with temperatures ranging from _130C
to about -20oC. Cloud precipitation-sized
particles were Measured using a PHS 2D-P probe.
A precipitation-sized particle was defined in
this study as any particle greater than 20U
microns in diall\eter, the minimum detectable size
of this instrument. All 20-P data was processed
for a one-second averaging period, and the
complete data set for one east-to-west pass was
then averaged to derive a mean precipitation
particle diameter and concentration for the
"in-cloud" data.

The 2D-P measured mean particle paraneters
can be use~ to estimate the mean ice water
content of the cloud regi(\n. Particles were
assumed to be thick plate crystals. The mass of
the individual particles was estimated using the
empirical diaMeter 0 (c~) to volume V (c~3)

relation suggested by Pruppacher and Klett (1978,
p. 40):

V .. 0.0897 D2.778

Assuming an average density of 0.9 g/c~3 for
hexagonal plates (ibid, p. 41), the diameter to
mass M (g per particle) relation is:

H .. 0.OB07 02.778

The average total unit volume ice water mass "t
(g/m3) of the cloud sample leg is then expressed
as:

Ht .. M · Np

where Np (particles per
precipitation particle
sample.

m3) is the
con cent rat i on

average
of the

Total ~isture available to the cloud was
est imated using the Ilost representative upwind
radiosonde sounding coincident with aircraft
measurements. For three case studies, the
selected sounding was made from the Cranbrook
British ColUlllbia upper air site within two to
three hours prior to the aircraft measurenents.
This valley site is approximately 100 kilometres
west of the continental divide and is the western
endpoint of the flight leg. The calculated
upwind atmospheric water vapour content for the
cloud layer of ~ach of the three cases was 1.0,
2.2 and 2.0 gIn' respectively. These values do
not account for entrainnent effects and are
viewed as upper limits of the parcel water vapour
available to the cloud.

Calculations for the three days yielded
the following estimates of cloud precipitation
conversion efficiency:

March 15, 1983 - ~ to 13~
December 9. 1983 - 16~
February 9, 1984 - <IS to 1J

The ranges for the March and February cases
reflect the two data sets used for each date and
the Decet'lber value represents the single
continuous "in-cloud" data set used.