WMOD00268

• Applicant: CWCB
• Sponsor Name: USBR
• Project Name: Final Report Weather Damage Mitigation Program
• Title: Numerical Simulations of Snowpack Augmentation for Drought Mitigation Studies in the Colorado Rocky Mountains
• Prepared For: USBR - WDMP
• Prepared By: Curt Hartzell, Dr. William Cotton, Joe Busto
• Date: 9/1/2005
• State: CO
• Weather Modification - Doc Type: Scientific Study

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and seeded precipitation. However, the model fixes did not entirely
eliminate the low-level warm Qias.
. The best 30 cloud-seeding days were selected for use in post-season
research evaluations. 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 includ~d 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 text and are
currently still under investigati'on.
. The model control simulations produced a reasonable qualitative pattern
of total precipitation and its topographic dependence for the 30 selected
days. The 30-day simulated 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.
. 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).
. Comparison of model-predicted precipitation (control) versus seeded
precipitation revealed that there was essentially no difference between the
86-day seed and control average totals (difference of -1.0 mm) or the 30
days selected for model precipitation evaluation seed and control average
totals (difference of -0.2 mm).
. Langrangian trajectory analyses of six selected days of the subset of 30
days selected for precipitation evaluation revealed that particles are
generally being transported to the target area by the targeting wind 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
Agl seeding.
. The Lagrangian analyses confirm that generators should not be used
when the targeting wind would not carry their plumes over the target area.
Low level trapping of particles can become moderate in nocturnal
inversions, 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, and their particles are also subject to moderate local trapping.

The very small differences between seed and control precipitation predicted by
the model were very disappointing and not expected at the onset of this project.
Possible causes of such low seedability: