WMOD00263

• Project Name: Journal of Weather Modification Abstract
• Title: Simulations of Snowpack Augmentation in the Colorado Rocky Mountains
• Prepared For: Weather Modification Association
• Prepared By: William Cotton, Colorado State University
• Date: 5/1/2006
• State: CO
• Weather Modification - Doc Type: Scientific Study

April 2006

COTION ET AL.

63

3.0 RESULTS

.

The model control simulations produced a
reasonable qualitative pattern of total pre-
cipitation and its topographic dependence
for the 30 selected days. The 30-day simu-
lated precipitation total showed only light
precipitation over the entire SE leg and
south half of the SW leg of the target area.
Thus the model suggests little orographic
precipitation potential and perhaps little
cloud seeding potential over the two south
legs of the target area.

We briefly summarize the results of this project.
For further details the reader is referred to the final
technical report (Hartzell et aI., 2(05) at the website:
http://rams.atmos .col ostate.edu/clseedin2/orog-
reports.html

The major results of this research project are as

follows:

.

WWC (Larry Hjermstad) pointed out the
forecast model exhibited a warm tempera-
ture bias at 700 mb which reduced its effec-
tiveness as a decision tool for determining if
seeding operations should proceed. Causes
of the warm bias were determined and fixes
were made in mid-February 2004. The entire
winter season was re-run to provide a better
estimate of natural and seeded precipitation.
However, the model fixes did not entirely
eliminate the low-level warm bias.

· The model forecast precipitation data were
evaluated against SNOTEL data using
MRBP statistical analysis procedures. The
results from the evaluation show that the
model is describing the non-seeded and
seeded simulation equally well. While the
signal of the fits is strong (all P-values about
1.0E-6 or less), the agreement measures are
not outstanding (all fall between 0.18 and
0.26).

.

WWC (Larry Hjermstad) found that after the
model fixes had been implemented in mid-
February 2004 and the RAMS real-time
forecast ??oo UTC cycle was run on the
new PC cluster, the forecast output that was
posted on the Web site was very useful. The
low-level warm temperature problem had
been greatly reduced and the model pro-
vided timely input for operational cloud
seeding decision making. There were nu-
merous forecast products and parameters to
evaluate. In addition to the 2-hr forecast
presentations, the animated forecast loops
provided a quick visual picture of changes
over time.

· Comparison of model-predicted non-seeded
precipitation (control) versus seeded precipi-
tation revealed that there was essentially no
difference between the 86-day seed and con-
trol average totals (difference of -1.0 mm)
for the 30 days selected for model precipita-
tion evaluation seed and control average to-
tals (difference of -0.2 mm).

· Lagrangian trajectory analyses of six se-
lected days of the subset of 30 days selected
for precipitation evaluation revealed that
particles are generally being transported to
the target area as intended. On average, 54%
of those particles are 50-500 m AGL, with
another 34% in the layer 500-1000 m AGL,
which are levels suitable for AgI seeding.

.

The thirty cloud-seeding days were selected
for use in detailed post-season research
evaluations. The 30 days were chosen as the
"best" representative examples of cases with
potential seedability, with a characteristic
"targeting wind" for each case ranging from
south-southwest through west to north-
north-west. When compared to measured
24-hr precipitation at 61 SNOTEL sites the
model exhibited a mean precipitation bias of
1.88. The highest bias areas included the
Target Area. The lowest bias areas were in
more upwind areas in northwesterly and
southwesterly events. Possible sources of
those biases are discussed in the final report
and are currently still under investigation.

· The Lagrangian analyses confirm that gen-
erators should not be used when the target-
ing wind would not carry their plumes over
the target area. Low level trapping of parti-
cles can become moderate in nocturnal in-
versions, but significant numbers of particles
escape the inversions and are transported by
the targeting wind as intended. It appears
that generators located on the lee side of
mountain ranges may be in stagnation zones
or rotors associated with high amplitude
mountain waves, leading to moderate local
trapping.

- Reviewed -