WMOD00315 ‎(2)‎

• Title: Colorado River Augmentation Demonstration Program - Technical Report January 1983 - March 1985
• Date: 4/1/1986
• Weather Modification - Doc Type: Report

2.4 Spatial Distribution of
SLW

2.4.1 Aircraft Measurements. - Liquid water measurements
recorded by aircraft flights over the Mesa were classified as
either along-axis (southwesterly winds) or cross-axis (north
or northwesterly winds), based upon aircraft-measured winds
upwind. Flight altitudes were normally at 3700 m m.s.!.
(mean sea level), 300 m above the highest terrain, and some-
times at 300-m increments above that level when clouds were
sufficiently thick. Headings were normally parallel to the
winds measured upwind of the Mesa.

For each kilometer along each pass over the Mesa, liquid
water amounts were averaged and classified as: (a) no liquid
water, (b) a trace to 0.2 g/m3, (c) >0.2 to 0.4 g/m3, or (d)
>0.4 g/m3. These values were derived primarily from the J-
W probe data, which suffer from zeroing errors and other
problems [9] and; therefore, have only semi-quantitative va-
lidity. The distribution of liquid water for along-axis cases is
shown on figure 2-2. Based on about 35 passes at 3700 to
4100 m m.s.!., the highest LWC (liquid water content) was
found above the Mesa top, from the southwestern edge to
about 30 km downwind. Of the 15 passes made at 4400 m

and above, the greatest LWC was observed near the upwind
edge [10]. Coincident measurements of SLW made by the
radiometer were in reasonable agreement with aircraft values
[11].

Since the greater amounts of SLW were most often recorded
on the lowest passes, the possibility exists of the clouds being
too warm to seed with AgI (silver iodide). Surface temper-
atures recorded during SLW episodes were most often be-
tween -4 and -1OoC, and a 300-m thick cloud (height of
lowest passes above highest terrain on Mesa top) would likely
have a top temperature no colder than -6 to -120C. In the
warmer cases, seeding with dry ice might be required.

The cross-axis distribution of LWC varied depending on the
cloud type over the Mesa at the time of sampling. November
and December of 1983 were typified by orographic cap
clouds (fig. 2-3), with a narrow horizontal zone of liquid water,
and lee wave cloud downwind.

January and February of 1983 were dominated by synoptic-
scale cloudiness (fig. 2-4), and significant LWC was found
both upwind and downwind of the Mesa. Subsidence dra-
matically reduced the LWC within 7 km of the southern edge
of the Mesa top in both periods.

ALONG-AXIS LIQUID WATER

* 60
Q) 40
0
c:
Q)
...
:3 20
ClIO
>0
:;:0 0
.!!-
='0 60
E>.
='0
(.)c:
Q) 40
:3
C"
Q)
... 20
LL
I'

2

-20

o

Higher Passes

KEvI7:T.
or-49/m'

20

60 km

40

Figure 2-2. - Distribution of liquid water observed by aircraft during along-axis passes in west-
southwesterly flow over Grand Mesa. Composites of 35 passes between 3.7 and 4.1 km. m.s.!.
(lower passes) and 15 passes between 4,4 and 5.4 km m.s.!. (higher passes) are shown.

I
I
I.

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