WMOD00263 ‎(2)‎

• Applicant: CWCB
• Sponsor Name: USBR
• Project Name: Response to RFP
• Title: Numerical Simulations of Snowpack Augmentation for Drought Mitigation Studies in the Colorado Rocky Mountains
• Prepared For: USBR
• Prepared By: Joe Busto, CWCB
• Date: 8/20/2003
• Weather Modification - Doc Type: Application

.

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

where Nid (1-1) is the number of pristine ice crystals predicted due to deposition-

condensation freezing, a = -0.639, b = 0.1296, and S; is ice saturation ratio. In addition,

Young's estimates of contact nuclei concentration were replaced with estimates from
laboratory experiments by Vali (1974, 1976), Cooper (1980), and Deshler (1982). The
formula for contact nucleation takes the form,
N;c = exp[a + b(273.15 - I:)]

where a = -2.80 and b = 0.262, is used [units of N. are inverse liters (1-1)]. In version
IC
4.3 we have added an option to predict the vertical and horizontal transport of ice-
forming nuclei (IFN). Thus in cases where soundings of IFN are available we now have
the capability of predicting the vertical mixing and transport of IFN from high to low
concentration regions and vice versa. This also permits the addition of an IFN source
such as ground-based generators to simulate cloud seeding.

The secondary ice particle production model in RAMS is based on Mossop (1978) in
which he derived the empirical relation:
Ni = c N24 (N'3)m (2)
where N1 is the number of ice particles produced per second, N24 is the number of cloud
droplets larger than 24 microns in diameter that are collected by ice each second, N13 is
the number of cloud droplets smaller than 13 microns in diameter that are collected by ice
each second, m is an exponent equal to 0.93, and c is a constant of proportionality. This
relationship was derived from a laboratory apparatus in which supercooled cloud droplets
were collected on a glass rod, and experiments were conducted for a wide range of
droplet concentrations, leading to many different values of N24 and NI3'

We re-interpreted the empirical results ofMossop by accounting for the cross-sectional
area of the glass rod, which was 5.4 cm2. In RAMS, we thus compute N24 and NI3 per
5.4 cm2 of cross sectional area of ice particles that collect cloud water, and apply the
above formula. In MKS units, the formula is:

Nj = 9.1e-l0 x B x N_24 X (N13}93 (3)

where B increases linearly from 0 to I as ice temperature T increases from -8 C to -5 C,
B decreases linearly from 1 to 0 as T increases from -5 C to -3 C, and B is zero at other
ice temperatures.

At colder temperatures such as found in cirrus clouds, primary nucleation by
homogeneous freezing of supercooled cloud drops and haze particles was introduced
(DeMott et aI., 1994). Ice is categorized in RAMS into pristine ice, which is pure vapor-
grown crystals, and snow, which is larger vapor-grown crystals and crystals that undergo
moderate riming (Harrington et aI., 1995). Aggregates remain as a separate category, as
well as low-density graupel particles. Hail is an additional category that represents high-
density frozen particles such as frozen raindrops and hailstones.

II-IO