WMOD00293

• Title: Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1996
• Date: 9/1/1998
• State: UT
• Weather Modification - Doc Type: Report

;.

~.

I
1
t
I
I
I
,
f
t
I
I.
L
;.
1

1
~ .,.
1

I

!
i
i

I

2. AVAILABILITY OF SUPERCOOLED LIQUID WATER _

2.1 Background Information

It is well known that anecessary (but not sufficient) condition for winter orographic cloud seeding to be
effective is the availability ofSLW (Supercooled Liquid Water) in excess of that naturally converted to
snowfall.' Successful seeding also requires transport and dispersion of seeding agents into the SL W
clouds in sufficient concentrations to convert significant quantities of SL W to ice particles. Moreover,
the conversion must take place where sufficient time and distance remain for the seeded particles to grow
to snowflakes sizes which settle to the mountain surface before sublimating in the lee subsidence zone.
. All of this must happen in an ever-changing and complex airflow and cloud environment. While it is
sometimes convenient to consider winter orographic clouds as semi steady-state entities, they are actually
changing rapidly on a wide range of spatial and temporal scales.

-Many operational winter orographic cloud seeding programs and a number of experimental projects have
assumed the presence of abundant SLW. However, few programs have made significant efforts to test
this crucial assumption. The NOAAlUtah AMP put considerable effort into investigating spatial and
temporal SL W distributions, using both observations and sophisticated numerical modeling. Indeed,
documentation ofSLW was the first major scientific objective of this program.

Several of the, studies later summarized dealt with the importanttopic of SL W. These include Huggins
(1992), Sassen and Zhao (1993), Super and Huggins (1993), Super (1994), Huggins (1995), Super
(1995a), Huggins (1996), and Wetzel et at (1996). Observations ofSLW were made by tower-mounted
icing rate meters, sensors carried by the NOAA instrumented aircraft, and by both fIXed and mobile
microwave radiometers. A mobile microwave radiometer was first deployed in the field during the
NOAAlUtah AMP, as discussed by Huggins (1992, 1995) and Wetzel et al. (1996). It proved invaluable
in mapping SL W distributions over the Plateau.

The general portrayal of SL W over the Plateau in the articles just noted is similar to findings from other
mountainous regions, as reviewed by Super (1990). He noted that, "There is remarkable similarity among
the research results from the various mountain ranges. In general, SL W is available during at least
portions of many storms. It is usually concentrated in the lower layers, and especially in shallow clouds
with warm tops. Average integrated amounts are normally limited implying low cloud liquid water
contents, in agreement with aircraft observations."

The articles cited herein agree with the Super (1990) portrayal and expand upon it. Seasonally, a
significant portion of the SL W flux is not converted to snowfall during passage over the Plateau. This
finding suggests the availability of sufficient "raw material" for seeding to have a significant impact on
snowfall, provided that seeding can convert a significant portion of the SL W flux to snowfall. The
seasonal SL W flux is concentrated in a few large storms that are efficient(snowfall producers during
portions of their passages but inefficient during other phases. Super and Huggins (1993) considered the
SL W flu/( across a number of mountain barriers. They concluded that seeding may be -appropriate both
' when SL W is abundant and when it is limited. The relatively rare hours with large SL W amounts produce
significant flux. But the numerous hours with small SL W amounts also produce significant flux over the
course of an entire winter.

7

I
I LIB. D^ ~'J
~ ~ \ r\ j '1 ~

I MAV ? r- 1C.t:'Ci'
HI "- 0 )'.0',-',,",

I j i ;
I ! r.,.i'~~II',,:.r::r;:-;~..v,;:-;;~''''''''' r
\ v.1- \.l. - .,' ~, '..... {,: ".J:!
i _____;~.~c:f2~.:t.;~~.~. r:: . -."'" . :.- '~~Jer