Snow Hydrology
Instructor: Randy Julander
Weather: a brief, non-technical overview
INTRODUCTION
In hydrology, we need to know some of the fundamentals of
meteorology in order to construct mathematical models of
hydrologic processes. In Snow hydrology, the most important
weather principles we deal with are: 1) temperature and its
behavior with respect to watershed characteristics, 2)
precipitation and its behavior with respect to watershed
characteristics, 3) solar radiation and ditto. Other
meteorologic characteristics such as wind, relative
humidity, etc are important but because we have very little
data, and relatively poor procedures for calculating these
variables in space and time, they are ignored.
Solar and earth radiation
Solar radiation is the Earth’s chief source of energy and it
determines the world’s weather and climates. Both the earth
and the sun radiate as blackbodies: i.e. they emit for every
wavelength almost the theoretical maximum amount of
radiation for their temperatures. Radiation from the sun is
short wave and, from the earth it is long wave.
The rate at which energy (solar radiation) reaches the upper
limits of the earth’s atmosphere on a surface to the
incident radiation and at earth’s mean distance from the sun
is called the solar constant. It is about 1.94
Langleys/minute (l langley is 1 calorie/cm squared) a large
part of solar radiation is reflected back to space by clouds
and the earth’s surface or absorbed in the atmosphere.
Radiation scattering is dependent on the wavelength: shortwave lengths scatter easily (0.45 microns) the blue range
thus accounting for the blue sky, very little in the red
range is scattered (0.65)
Since the surface area of a sphere is 4 times greater than
that of a circle, solar radiation intercepted by planet
earth averages about 1/4 the solar constant or about 0.5
langleys/min. Various points on the earth’s surface receive
different amounts of energy depending on their aspect and
watershed orientation.
In the above figure, solar radiation to the watershed
surface has been calculated as an index for the Salt Lake
city basins. In March and April, as the sun angle is still
relatively low, there are wide differences in the amount of
the radiation index. As the sun angle increases (we move
closer to the summer months and the sun becomes more
directly overhead), all watershed receive equal amounts of
energy. Watershed orientation thus becomes a large factor
in the timing of snowmelt and peak flow. The SLC watersheds
are very similar and elevation and size become more dominant
factors in the runoff scenario.
The earth’s surface radiates as a blackbody at a mean
temperature of about 15 degrees C - about 1.25 langleys
/min. This is about 2.5 times the 0.5 langleys per minute
of incoming radiation. Net loss of heat is prevented and a
heat balance maintained because the atmosphere reflects back
to the surface about 85% of the emitted long wave radiation.
Were it not for this greenhouse effect, the mean temperature
of the earth would be about -40 degrees C.
Solar radiation is the engine that produces weather systems
around the globe. This energy produces air masses of
different temperatures, moisture content, elevation and
travelling in different directions at different speeds.
These air masses bring the conditions we call weather.
Fundamental principal:
Warm air holds more moisture
than cold air. If you have warm air at 100% relative
humidity and cool that parcel of air, the moisture must go
somewhere thus it condenses and precipitates.
Fundamental principal:
Warm air is lighter than cold
air and thus it rises. Adiabatic processes (very complex
equations dependent on humidity, pressure, etc) cool warm
air as it rises. Therefore, the main mechanism producing
precipitation is warm, moist air forced to rise, cool, and
dump moisture.
Processes that move and cool warm air parcels.
Frontal systems: a frontal surface is the boundary between
two adjacent air masses of different temperature and
moisture content. Frontal surfaces are actually layers or
zones of transition. The line of intersection of a frontal
surface with the earth is called a surface front. An upper
air front is formed by the intersection of two air masses
aloft. If air masses are moving so that warm air displaces
cold air then it is called a warm front. Conversely if cold
air is displacing warm air, then it is called a cold front.
Cold fronts move faster than warm fronts and usually
overtake them. The boundary layer between air masses is
where most precipitation occurs
CONVECTION:
Warm air is capable of containing more moisture than cold
air. Warm air rises. Through a series of complex
processes, the warm air expands, losing heat... thereby
becoming colder. This process: adiabatic loss rate or the
lapse rate, under theoretical conditions, is 3 degrees per
every 1000 feet of elevation gained. An air mass rising
10,000 feet is capable of 30 degrees of cooling without
extenuating circumstances. Thus an air mass, full of
moisture at 60 degrees, forced to rise 10,000 feet is cooled
to 30 degrees and is capable of producing snow at that
elevation. Most convective activity occurs in the summer as
a result of the warming of the earth’s surface and the air
mass next to it. As these air masses move and produce rain,
at higher elevations, this may be snow.
Orographics:
Other influences cause air masses to rise, chiefly,
orographics.
This is land topography that forces air masses to rise. As
moist air masses over the pacific move along the jet stream
or prevailing wind, they encounter land. The source of
moisture is dramatically decreased, as is a great deal of
energy. As these air masses are forced to rise, they cool
and precipitate. The closer to the source of moisture along
the prevailing winds, the more precipitation falls: i.e. the
Oregon/Washington coast, the Sierras of California, etc.
Notice
southern California and how dry it is... this is a function
of the prevailing winds, storm tracks (jet stream), location
relative to warm and cool water bodies and the Coriollis
effect. The higher the landmass, the more moisture is wrung
out.
The farther from the source, without significant
replenishment, the less moisture is available for
precipitation.
In our case, the Sierras wring out most of the moisture,
leaving the Great Basin high and dry. Snow water equivalents
in the Sierras can easily get over 100 inches deep whereas
in Nevada and Utah 20 to 40 inches is more the norm.
The Wasatch Front is the next big rise for air masses coming
across the Great Basin. Since the air mass is already quite
dry, the snow that falls here tends to be very light in
density and have very distinct layers (avalanche potential).
The Sierras get very wet, heavy, dense snowpacks commonly
referred to as Sierra cement. Utah has a cool continental
climate, conducive to such snowpacks whereas the Sierras
have a coastal influence. A further complicating factor is a
significant moisture source in the form of the Great Salt
Lake. Early season storms of cool air travel across the
warm lake and gain moisture and energy. This effect is very
short lived however and typically the Cottonwood Canyons are
the beneficiaries. There is a great deal more snow in the
Cottonwoods than in the canyons to the north or south,
partly due to elevation and partly due to prevailing storm
direction in the early season trending form the northwest to
southeast.
Topography and snow accumulation
Topography: Orographics (or major land-forms- Mountainous
areas are clearly distinguishable in the above Precipitation
map of Utah, so much so that it is, in reality, a coarse
topographical map) cause the air mass to rise and
precipitate but micro-topography plays a huge role in the
actual accumulation pattern. Canyons can funnel and trap
moisture-laden air masses and get substantial accumulation.
Wind patterns affect snow accumulation. As air masses
compress and are forced up and over an obstacle, velocities
increase near the summit, i.e. the Bernoulli effect,
airplane wing, etc. in these areas; Snowpacks can be
scrubbed clean constantly.
Bald Mountain Pass, Uintahs – note the wind scrubbing.
However, the leeward side becomes the beneficiary of this,
because the wind densifies the snowflakes by breaking them
apart and reforming in tighter patterns of plates and
grains, etc. On the leeward side, there is a sudden
decompression and wind velocities decrease tremendously,
allowing the snow to fall to the surface. These areas often
form giant snowdrifts of considerable size with great
bonding strength. Cornices can be a hundred feet high and
sometimes as long.
Cornice formation on Farmington Peak.
Even if conditions prevent the formation of a cornice,
drifts can be very impressive such as the one that forms on
the lee side of Bull pass in the Henry Mountains. It is
often 300 to 500 feet in length and 20 to 40 feet deep. It
blocks the road opening many times until mid July and
sometimes even later.
Being able to locate areas such as
these allows one to strategically place roads and structures
to avoid the maintenance costs of plowing, etc. One can
also locate other structures to alter the shape and size of
such drifts. Any topographical feature that produces a
disturbance in the prevailing steady stream air flow is
capable of producing anomalies in the snowpack, not only in
actual precipitation amount but the other characteristics of
the pack as well: such as grain size and distribution,
bonding characteristics, etc.
Solar radiation and snowmelt
Topography has a great influence on the eventual melting of
snowpacks. Aspect has perhaps the greatest impact. A northfacing slope receives little incoming radiation for much of
the winter and early spring. What it does receive is at an
oblique angle and mostly reflected. Not till later in the
season does it start getting enough direct energy for melt
and typically this is at a time that temperatures are quite
warm for conductive and convective melt as well.
Looking north at south facing slopes, west is left and east
is right: note the difference in snow covered aspects.
A brief synopsis of snow formation
Moisture in an air mass is forced to lift, cooling it,
causing condensation and forming water vapor. In order for
this to happen, some form of nuclei must be present. Water
in an air mass may become super cooled before condensing if
these particles are not present. Water vapor condenses on a
particle, then more and more vapor condenses. If it is
rain, the drop gets larger and larger finally overcoming the
lifting and turbulent airflow and settles to the ground.
With a snowflake, the same process occurs, but snowflakes
have a much greater surface area, lighter density and can
take more time. Water vapor also forms ice crystals
directly on the particle, which grow in what appears to be
perfect 6 sided symmetry. This flake continues to grow
until it overcomes the internal storm forces, submits to
gravity and falls to the earth. During this time, collision
with other flakes can occur and sometimes 2 flakes become 1
or flakes become damaged/broken and continue to grow. Once
on the ground, the flake continues to undergo changes. The
wind can pick it up again, break its little fragile arms and
rearrange its face. Snowflake size and pattern are
dependent on the conditions of its birth. Calm cold
conditions produce large fluffy flakes. Wet, warm conditions
produce more granular crystals.
Once on the ground, the flake bonds to one degree or another
with the snow already present. The metamorphosis continues
as other flakes pile up on top. Vapor pressure within the
interstices or voids determines water migration from one
crystal to another. Crystals become less dendritic, more
faceted. During this time the crystal becomes more and more
subject to the increasing weight of snowpack above.
Compressive forces mold the crystals even further. Crystals
on or near the surface are exposed to other forces such as
sun action, wind, and temperature extremes. Depth hoar
layers may form which are crystals that grow long fine and
branched dendritic patterns. The light fluffy stuff of
skiers dreams. These layers can form on or near the surface.
These layers constitute weak bonding surfaces in the
snowpack and have the potential for large-scale avalanche
activity later in the season as the weakness persists for
long periods of time. Later in the season, as snowpacks
receive more energy, they start to process of becoming
isothermal. This is a condition that must occur preceding
melt. This is simply that the pack is at or very near 32
degrees from top to bottom and that subsequent energy may
convert solid to liquid. In this condition, free water may
start migrating through the pack, which in turn may give
some energy to lower layers. Water and energy continue to
change the snow crystals right up till the final melting
phase.
Temperature
Temperature is the most widely used driving process in
snowmelt modeling. Why? - it has been the only variable
that is widely available. There are some problems - as
always. Temperature is only an index to the major process
that drives snowmelt: solar radiation. Temperature is yet
another variable for which we need some kind of spatial
interpolation. Temperature - max, min and average are
typically collected by the NWS through many avenues but
almost always in cities and towns- not necessarily at the
top or even middle of a watershed where we need it.
Temperature, as a general rule, is much more stable over
similar elevations than is precipitation. If you have the
temperature in SLC, you can with 98% confidence, predict the
temperature in Ogden and usually, with reasonable accuracy,
even predict the higher elevations such as Alta or Brighton.
Temperature is elevationally dependent. It gets colder as
elevation increases USUALLY. The adiabatic process defines
how much colder. As air compresses without any other energy
input, the air warms. How much is dependent on relative
humidity, pressure, etc. As air is lifted, it decompresses
and cools. The range is anywhere from 1 degree to almost 6
degrees F per 1000 feet of elevation gain or loss. The most
typically used values are 3 degrees f and 3.8 degrees f per
1000 feet. Thus a parcel of air lifted 10,000 feet will
cool (on average) 30 degrees. It may cool only 10 degrees
or it could cool as much as 60 degrees. You get the idea
real fast that using an average value may, at times, really
produce bad results in hydrologic snowmelt modeling. So why
is it used in this way? Again, the only available temp data
when these models were developed and calibrated were the
lower elevation locations. (remember, you typically use
what you have and try to derive the best possible
application)
Inversions: an inversion is where cool air, which is very
heavy, pools into the lower elevations and without any wind
or thermal activity stays in place: typically associated
with fog which takes enormous energy to burn off, leaving
little for heating. The infamous Great Basin High pressure
systems can generate fairly long lasting inversions in Utah.
High elevations can have temperatures of 40 to 55 degrees
and the valleys may remain in the 30's. Fortunately, these
typically occur in January and February with minimal
snowmelt. At the peak of snowmelt, the adiabatic lapse rate
may display a wide range of values thus when modeling, it is
always best to have temperatures from several elevation
zones and calculate linear interpolations between stations
based on elevation and aspect.
Weather Modification – Utah has a similar program dating
back to the 1970’s.
A Synopsis - September 1997
PURPOSE OF PROGRAM:
Augment snowfall in selected mountainous regions of Nevada to increase the
snowpack, the resultant spring runoff and the water supplies of municipalities,
agricultural regions, recreational lakes, and environmentally threatened terminal
lakes.
AREAS OF ACTIVITY:
The drainage basins of Lake Tahoe, the Truckee River, the Carson River, the Walker
River, the Upper Humboldt River (Ruby Mountains), the South Fork of the Owyhee
River (Tuscarora Mountains), and the Reese River (Toiyabe Mountains).
HISTORY OF OPERATION:
Cloud seeding has been conducted in the Tahoe area since the 1960's. The original
seeding equipment was acquired mainly through U.S. Bureau of Reclamation (USBR)
grants to DRI (e.g., Pyramid Lake Project). The Ruby Mountain operation started in
1981 using surplus USBR generators. State funding for the program began as early as
1979-80, and continuous State funding has been available since 1984. Since 1991
Nevada has also funded the fabrication of one or two remotely controlled MK-2
generators per year. Remote Walker-Carson generators (4) and two weather stations
were added in 1992 through a special USBR grant ($300,000). The Desert Research
Institute has designed and operated the Nevada State program since its inception.
METHOD OF OPERATION:
Primarily, ground-based generators are used to burn a solution of silver iodideammonium iodide in acetone to release silver iodide (AgI) particles which create
additional ice crystals, then snow, in winter clouds. Weather conditions are selected to
optimize fallout in targeted basins. Most generators are remotely operated by radio or
cellular telephone. A seeding aircraft is also used to augment ground seeding
operations. The aircraft releases AgI from pyrotechnic flares or solution burners. Dry
ice is also used in situations when clouds are shallow and cloud top temperatures are
too warm for silver iodide to be effective.
Seeding Equipment:
Sixteen (16) remotely controlled AgI generators; one manually operated AgI
generator; and two contracted seeding aircraft with AgI burners and flare racks.
Supporting Equipment:
Two USBR-supplied weather stations in the Walker-Carson. DRI/NWS
hydrometeorological network in the Tahoe area - access to RAWS, SNOTEL and
other meteorological data networks through the DRI Western Regional Climate
Center - NEXRAD radar and GOES satellite imagery - the DRI trace chemistry
laboratory - DRI microwave radiometer for sensing cloud liquid water - an NCAR ice
nucleus counter for detecting AgI plumes.
Equipment Inventory:
About $1.4 million as of September 1997.
Supporting Personnel:
Four full time technicians who install and maintain all generator networks and
fabricate new generators - four part time professionals who forecast seeding
operations, implement design changes, evaluate operations, monitor environmental
aspects of the program and evaluate seeding effectiveness.
ESTIMATED BENEFITS OF PROGRAM:
Benefits vary with the seasonal frequency of suitable weather opportunities. Research
results have documented precipitation rate increases of 0.1 - 1.5 millimeters per hour
due to ground-based seeding during the proper weather conditions. Estimates of
augmented water from seeding have varied from 35,000 to 60,000 acre-feet over each
of the last ten years. Seasonal percentage increase estimates have varied from four to
10%; generally greater in drought years; less in above normal years. The cost of
augmented water, based on the cost of the program, has ranged from $8 to about $15
per acre-foot.
COOPERATIVE DRI RESEARCH:
The State program originated as an outgrowth of DRI weather modification research
programs funded through the USBR and the National Oceanic and Atmospheric
Administration (NOAA). Pertinent research findings are immediately applied to
operations. Research equipment is often shared with the State program at no cost.
Current DRI research is aimed at quantitative evaluation of winter storm cloud
seeding. Chemical evaluation of snowpack samples is also used to assess cloud
seeding targeting and potential environmental impacts.
Operational Guidelines and Safety Restrictions
In the event of any emergency which affects public welfare in the region
of any seeding operations being carried on by the Nevada State Weather
Modification Program, those seeding operations in that region will be
suspended until the emergency conditions are no longer a threat to the
public. Seeding suspensions are generally expected to occur due to one
or more of the following conditions:
A. When the avalanche category, determined by the U.S. Forest
Service, is designated as EXTREME.
B. When the National Weather Service (NWS) or the Project
Meteorologist forecasts a warm winter storm with the possibility
of considerable rain at the higher elevations which might lead to
local flooding.
C. When the Project Meteorologist feels potential flood conditions
may exist in or around any of the project areas he will consult
with the National Weather Service Flood Forecast Services at Reno
and Sacramento about the possibility of any of the following
warnings or forecasts being in effect.
1. Flash flood warnings by the NWS.
2. Forecasts of excessive runoff issued by the River Forecast
Center, including such forecasts for rivers on the adjoining
west slope of the Sierra Nevada.
3. Quantitative precipitation forecasts issued by the NWS which
would produce excessive runoff in or around the project
area.
In addition to the above, if any of the following conditions or
forecasts exist, seeding operations may be suspended at the discretion
of the Project Meteorologist in and around the areas of concern:
A. When the wind speed is 60 knots or more for over 30 minutes at the
700 mb level (10,000 ft). For monitoring purposes in the western
part of Nevada, the winds measured at Slide Mountain (9,650 ft)
are considered equivalent to the 700 mb level winds. The Reno and
Elko rawinsondes can also be used to monitor this criteria.
B. When wind directions lie outside of the range between 180 and 340
degrees during ground-based seeding operations on the west side of
the Sierra Nevada crest. The winds measured at Slide Mountain or
Ward Peak (8,480 ft), and the rawinsondes from Reno and Elko can
be used to monitor wind direction.
C. When the water content of the snowpack in the target area, as
measured at existing snow courses, exceeds the accumulation
envelope defined by the following percentages to date of long-term
averages on the same date:
December 1
175%
February 1
150%
April 1
140%
January 1
150%
March 1
150%
May 1
140%
Intermediate limits shall be derived by linear interpolation
between the percentages given above.
D. During major holidays such as Thanksgiving, Christmas, New Year's
Day, and President's Day, in areas and times of heavy traffic on
Highways 50 and 80, over the Sierra Nevada.
8/27/90 Revised: 6/6/97
Weather Monitoring Facilities and Procedures
The Nevada State Weather Modification Program is operated from the
Desert Research Institute's Atmospheric Sciences Center, located in the
Stead Sciences Center, Stead, Nevada. The project has 24-hour access to
a broad base of NWS weather data through UNIDATA, a program managed by
the University Corporation for Atmospheric Research (UCAR). The data are
received over the INTERNET through a contract with Alden Electronics.
Additional WEB sites on the INTERNET provide other data and forecasts.
The products are comprised, in part, of the following:
A. National Weather Service Public Product Service giving hourly
weather conditions.
B. DIFAX Service giving a selection of weather maps 24-hours-per-day.
C. PC-McIDAS Service giving a selection of weather satellite maps and
other products 24-hours-per-day.
In addition to the above National Weather Service data products
(supplied through Alden), the data from remote weather stations on Slide
Mountain, Ward Peak, Conway Summit and a site near Elko, Nev. are
continuously available in the Atmospheric Sciences Center. Data from the
Bureau of Land Management RAWS network, the Natural Resources
Conservation Service SNOTEL network, and from a local NWS
hydrometeorological network are available through the DRI Western
Regional Climate Center on a near real time basis. These sources provide
additional local information concerning surface temperature, humidity,
wind, precipitation, and snowpack accumulation. In addition, the
INTERNET provides access to a wide range of weather images, including
composites of radar, satellite and surface images. Nevada State Cloud
Seeding staff also confer directly with the National Weather Service
forecasters and National Forest Service staff when flood or avalanche
potential exists in any of the project areas. [See Operational and
Safety Guidelines.]
8/27/90 Revised 6/6/97
Cloud Seeding Operations Criteria
The following weather and cloud conditions should exist to initiate or
continue cloud seeding operations in any one of the operational areas of
the Nevada State Program. Operations can also be initiated based on a 03 h forecast of these conditions existing in any of the three
operational areas. Seeding suspension criteria will always override
seeding operations criteria.
1. Cloudiness of sufficient areal extent to cover at least 50% of the
intended target area. Verification is by means of GOES visible or
infrared satellite images.
2. Clouds of sufficient depth, with cloud bases at least as low as
the highest mountain peaks, to provide the potential for
precipitation over the target areas. Verification of these
conditions can be obtained by one or more of the following:
a. NWS hourly reports of cloud conditions and precipitation at,
but not limited to, the following sites: MMH, BLU, TRK, TVL,
RNO, EKO.
b. Visual observations and/or reports of cloud conditions by
the Project Meteorologist, other Project Staff, or generator
operators in the Ruby Mountains region.
c. Observation of precipitation from any automatic recording
gauge whose data are telemetered to DRI, but in particular
the gauge at Alpine Meadows or gauges near the Mt. Rose
summit.
d. WSR-88D radar images obtained from Sacramento, Reno or Elko
NWS radar sites.
3. Wind directions that are conducive to transporting seeding
material over the target areas. This criteria will vary by area as
follows:
a. Truckee-Tahoe area: Wind direction at 700 mb, as estimated
by Slide Mountain or Ward Peak mountain top data, from
(clockwise) between 180 and 340 degrees.
b. Carson-Walker: For ground seeding cloud level wind
directions from 135 to 270 degrees as verified by the
weather station above Conway Summit.
c. Ruby Mountains: Wind directions in the cloud layer from 210
to 330 degrees as verified by the NWS Elko rawinsonde,
visual observations from generator operators, or remote
weather station data.
4. Wind speeds at or near 700 mb should not exceed 30 m s-1 (~60 kts)
in order that adequate time be available for growth of ice
crystals initiated by seeding. Slide Mountain, Ward Peak, and
Conway Summit weather stations, and NWS Reno and Elko rawinsondes
will provide verification of wind speed.
5. The existence of supercooled liquid water in clouds is a condition
necessary for successful cloud seeding. This quantity is not
routinely measured over the target areas, but the observation of
icing at Slide Mountain (or other mountain top site), or the
observation of liquid water from one of DRI's microwave
radiometers should be given strong consideration in the decision
to initiate a seeding operation in any area where these data are
available. When available these data will be used in postseason
evaluations of seeding operations.
6. To increase the likelihood of ice crystal formation by AgI seeding
aerosols from ground generators, the temperature near 10,000 ft
should be -5°C, or colder, as verified by data from the Slide
Mountain weather station, or Reno and Elko soundings. Operations
may be initiated at a temperature as warm as -3o C, provided the 5o C threshold is forecast to be met within 0 to 3 hours.
7. For aircraft seeding in the Truckee-Tahoe or Carson-Walker
regions, winds can have either westerly or easterly components.
The airborne seeding contractor, in coordination with the DRI
Project Meteorologist, will determine suitable wind conditions
based on radar observations, soundings, or NWS upper air charts.
Flight levels will be selected to ensure that seeding material is
released at temperatures colder than -5°C. The presence of
supercooled liquid water must be verified for aircraft seeding
operations to be initiated or continued.
The Nevada State Program Meteorologist is responsible for forecasting
and verifying seedable conditions, and also initiating and terminating
operations. Logs documenting the weather conditions during an operation
will be kept by the meteorologist and included in the report on each
season's operations.
Revised: 6/6/97
Historical Perspective and References
Operational weather modification projects have benefitted from the
results of numerous research experiments that have been conducted since
the 1950's and 1960's. Many of the early experiments relied on
statistical evaluation of precipitation data to determine if cloud
seeding was having a positive impact. As techniques and instrumentation
evolved, the impacts of cloud seeding began to be documented from the
initiation of ice in clouds to the measurement of precipitation at the
surface.
Wintertime cloud seeding for snowpack augmentation has historically
involved a variety of techniques, seeding materials and dispensing
methods. The research has been conducted in numerous mountainous areas
of the western U.S., including the Rocky Mountains of Colorado and
Montana, the Cascade Mountains of Washington, and the Sierra Nevada of
California. Research results can be found in the references listed
below:
Hess, W. N., 1974:
Weather and Climate Modification. John Wiley & Sons, Inc., 842 pp.
Hobbs, P.V., 1975:
The nature of winter clouds and precipitation in the Cascade Mountains and their
modification by artificial seeding. Parts I and III. J. Appl. Meteor., 14, 783-804 and
819-858.
Dennis, A. S., 1980:
Weather Modification by Cloud Seeding. Academic Press, 267 pp.
Braham, R., Jr., 1986:
Precipitation Enhancement - A Scientific Challenge. Amer. Met. Soc., Meteorological
Monographs, 31, 171 pp.
Reynolds, D.W., 1988:
A report on winter snowpack-augmentation. Bull. Of the Amer. Met. Soc., 69, 12901300.
Super, A.B. and J.A. Heimbach, Jr., 1988:
Microphysical effects of wintertime cloud seeding with silver iodide over the Rocky
Mountains. Part II. Observations over the Bridger Range, Montana. J. Appl. Meteor.,
27, 1152-1165.
Super, A.B., and B.A. Boe, 1988:
Microphysical effects of wintertime cloud seeding with silver iodide over the Rocky
Mountains. Part III. Observations over the Grand Mesa, Colorado. J. Appl. Meteor.,
27, 1166-1182.
Deshler, T., D.W. Reynolds and A.W. Huggins, 1990:
Physical response of winter orographic clouds over the Sierra Nevada to airborne
seeding using dry ice and silver iodide. J. Appl. Meteor., 29, 288-330.
Some Newer Methodologies
More recent research has dealt with trace chemistry techniques for
detecting seeding effects in the snowpacks of mountainous areas, the use
of microwave radiometers for evaluating cloud seeding potential, and the
use of numerical models for simulating the dispersion of cloud seeding
material. Examples are as follows:
Warburton, J.A., L.G. Young and R.H. Stone, 1995:
Assessment of seeding effects in snowpack augmentation programs: Ice nucleation
and scavenging of seeding aerosols. J. Appl. Meteor., 34, 121-130.
Huggins, A.W., 1995:
Mobile microwave radiometer observations: Spatial characteristics of supercooled
cloud water and cloud seeding implications. J. Appl. Meteor., 34, 432-446.
Long, A.B. and A.W. Huggins, 1992:
Australian Winter Storms Experiment (AWSE) I: Supercooled liquid water and
precipitation-enhancement opportunities. J. Appl. Meteor., 1041-1055.
Holroyd, E.W., III, J.A. Heimbach, Jr. and A.B. Super:
Observations and model simulation of AgI seeding with a winter storm over Utah's
Wasatch Plateau. J. Wea. Mod., 27, 36-56.
Model Animation of a Seeding Plume in the Sierra
Nevada
Atmospheric and Dispersion Modeling
Image size is 10M
A Case Study of AgI Seeding Effects in a Winter Storm
The following case study of a ground-based silver iodide (AgI) seeding
experiment was developed from data collected during a field research
project conducted by the NOAA-Utah Atmospheric Modification Program (a
cooperative venture between the State of Utah and the National Oceanic
and Atmospheric Administration). The experiment took place on the
Wasatch Plateau of central Utah. The State of Utah Department of Water
Resources, the U. S. Bureau of Reclamation, the Desert Research
Institute, the NOAA Air Resources Laboratory, the University of North
Carolina - Asheville, and North American Weather Consultants
participated in the project. The results shown here were taken from two
sources (Huggins, 1996a and 1996b)*, as part of the Desert Research
Institute's contributions to program.
Figure 1. Shown here is a southwest to
northeast cross-section of Utah's
Wasatch Plateau. The locations of the
main instrument sites and the cloud
seeding generator site are noted. The
NOAA research aircraft flew tracks
through this cross-section at AC1, AC2
and AC3, and one track along the crosssection (AC4).
Figure 2. Data from the RRS site at the top
of the Wasatch Plateau show the atmospheric
conditions in which the cloud seeding
experiment was conducted. The temperature was
relatively steady at about -12° C (top
panel), the wind was relatively light and
blowing from the southwest (along the crosssection in Figure 1), and supercooled cloud
liquid detected by a microwave radiometer was
present in small amounts (4th panel). These
conditions indicated that the potential for
producing a seeding effect was very good. The
seeding material being used (AgI) can produce
ice crystals in the presence of supercooled
cloud water, provided the temperature is
colder than about -5° C. The relatively slow
wind speed suggested there would be adequate
time for ice crystals created by seeding to
grow and fall out over the top of the
plateau. The third data panel shows ice
nucleus counts and the occurrence of icing at
RRS. The low ice nucleus counts indicate that
the AgI seeding material did not pass across RRS (see Figure 3). The
indication of icing shows that the RRS site was frequently in the cloud
that formed over the plateau.
Figure 3. The transport and dispersion of
an aerosol plume across the Wasatch Plateau
from a single ground seeding source at HAS
is depicted. The seeding experiment
consisted of the simultaneous release of
AgI and a tracer gas, sulfur hexaflouride
(SF6). An instrumented van was operated on
roads on top of the plateau and a NOAA
research aircraft was flown 300 to 600 m
above the plateau (see Figure 1). The van
and aircraft were equipped with sensors to
detect both ice nuclei (AgI) and SF6, and
documented the seeding plume at the
locations shown here. Seeding occurred from
0800 until 1020 PST. Although both ice
nuclei and SF6 were detected, the figure
shows only the SF6 plume interceptions. The
top of the seeding plume determined from
all aircraft passes through the plume is shown by the dashed green line
in Figure 1. Note that four of the project precipitation gages were
within the area covered by the seeding plume.
Figure 4. An example of a seeding
effect detected by the research
aircraft is shown here at a location
about 38 minutes downwind of the
seeding generator. The left panel
shows the aircraft flight track with
the box indicating where the aerosol
seeding plume was detected. The panels
on the right show the aircraft height
and air temperature (top); the liquid
water content in the cloud (2nd); the
concentration of ice particles greater
than 0.1 mm in diameter (3rd) as
measured by an optical array probe
(OAP); and the SF6 and ice nucleus
concentrations (bottom). The ice
nuclei counter takes about 1 minute to
process an air sample, resulting in
the time difference in detection of
the two plumes. The dashed red lines
in the right panels show the seeding
aerosol plume locations. The seeding
effect is clearly seen as an
enhancement in the ice crystal
concentration (about 5-20 crystals per liter of air) within the plume,
as compared to cloud regions on either side of the plume.
Figure 5. Once ice crystals grow to
sufficient size they can be detected by
radar. The radar used for this Utah project
was the DRI Ka-band radar, a special short
wavelength (8.6 mm) radar used primarily
for cloud physics studies, and capable of
detecting cloud particles even before they
reach precipitation size. This image shows
the radar echo plume that evolved as a
result of ice crystals created by the AgI
seeding from HAS. The plume-like radar echo
was found to coincide with both the aerosol
plume (Figure 3) and the aircraft ice
crystal plume locations (Figure 4). The
aircraft SF6 plume location matching this
radar image time is shown by the line
segment near the TAR site. This low level scan detected ice particles
within a few hundred meters of the surface, and therefore also gives an
indication of the areal coverage of precipitation that was being
produced by the seeding.
Figure 6. This final figure compares
precipitation from gages beneath the
seeding plume (as determined by the data
in Figures 3 and 5) to precipitation from
gages to the north and south of the
seeding plume. The seeding during this
experiment accounted for a precipitation
rate increase of about 1-1.5 mm h-1. Using
1.25 mm h-1 as the average increase, the
two hour experiment would have produced
2.5 additional millimeters of snow water
over the area affected by the plume. The
radar plume areal dimension was about 50
km2. The volumetric amount of snow water
produced by seeding can then be estimated
as being 125,000 m3, or about 100 acre-ft.
* Huggins, A. W., 1996a:
Use of radiometry in orographic cloud studies
and the evaluation of ground-based cloud
seeding plumes. Preprints 13th Conference on
Planned and Inadvertent Weather Modification, Atlanta, Georgia, Amer. Met. Soc.,
142-149.
Huggins, A. W., 1996b:
Investigations of winter storms over the Wasatch Plateau during the 1994 NOAAUtah Field Research Program. Final Report to State of Utah Dept. of Water
Resources, Desert Research Institute, Reno, Nev., 133 pp.
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A WINTER CLOUD SEEDING PROGRAM IN
UTAH
Don A. Griffith, John R. Thompson, and Dan A. Risch
North American Weather Consultants
Salt Lake City, Utah
ABSTRACT
A winter orographic cloud seeding program has been conducted over the higher elevation
areas of central and southern Utah for 13 winter seasons. The primary seeding mode has
been ground-based, manually operated "silver iodide generators" located in valley and
foothill locations upwind of the intended higher elevation target areas. The goal of this
program is to augment naturally occurring snowfall in these higher elevation areas.
Resulting augmented spring and summer streamflow is utilized for irrigation. A
target/control evaluation of this program indicates approximately an average 11 percent
increase in precipitation in the target area over that predicted from the control area
precipitation. A similar evaluation of precipitation falling in a lower elevation region
downwind of the intended target area indicates an average excess of approximately 15
percent.
1.0 INTRODUCTION
A cloud seeding program has been conducted in portions of Utah every winter season
since the 1973-74 season, except for 1983-84. The goal of this program has been to
augment naturally occurring snowpack in the higher elevation regions. The augmented
snowpack is intended to augment surface streamflow used for irrigation. This program
has been administered by a non-profit organization known as the Utah Water Resources
Development Corporation. This program was initiated at the county level for the first two
years that cloud seeding was conducted. The State of Utah, through the Division of Water
Resources, has provided financial cost sharing assistance to the participating counties
since the 1975-76 winter season. County participation in the cloud seeding program has
been determined each fall through either the County Board of Supervisors or County
Water Conservancy Districts. North American Weather Consultants (NAWC) has been the
contractor selected to conduct this program by the Utah Water Resources Development
Corporation each winter of operation.
2.0 BACKGROUND
The Utah winter cloud seeding program originated in the central and southern portions of
the State, partially in response to drought conditions that affected the State in the early
1970's. Several organizational meetings were conducted with county officials in the
summer and fall of 1973. Methods of financing an operational cloud seeding program
were discussed and a formula established which allocated the proposed contract costs to
the participating counties on the basis of county assessed valuation. Seeding first began
in January of 1974. The normal period of operation after these first two years has been
November 15 through April 15. This period was selected to concentrate on the primary
period of snowpack accumulation. The number of counties that have participated in this
program has fluctuated from year to year. Typically more counties have been involved in
the program in years following dry years than ones following wet years. Heavy
precipitation in Utah in the spring of 1983 resulted in project operations being suspended
in early February. No operations were conducted the following winter, 1983-84, because
Sufficient supercooled water is present in storms over Utah to permit seeding
material to nucleate and grow additional precipitation.
Seeding material released from ground generators can reach seedable locations in
sufficiently dilute but wide-spread concentrations to affect a significant portion of a
storm.
Growth times and trajectories of natural and augmented precipitation are
appropriate to intercept the intended target area.
NAWC personnel, along with a number of other groups, developed an effective means of
generating minute crystaline particles of silver iodide in the early 1950's. Such particles
had been demonstrated to be effective ice nuclei in research conducted in the late
1940's. The basic "generator" consists of a stainless steel tank that contains a solution of
silver iodide dissolved in acetone, using ammonium iodide as a solubilizing agent, with a
two percent by weight concentration of silver iodide. This solution is pressurized by a
propane tank that forces the silver iodide-acetone-ammonium iodide solution through a
spray nozzle. The generator is also vented through the spray nozzle. The propane is lit by
a local operator. This operator also adjusts the flow of seeding solution into the propane
flame. Silver iodide particles produced by this generator have been tested at the
Colorado State University Cloud Simulation Laboratory (Finnegan, 1982). Garvey, 1975
provides a discussion of the procedures used in testing silver iodide generators at this
facility.
This generator is typically operated to dispense six grams per hour of silver iodide when
burning 0.1 gallons per hour of seeding solution with 0.5 gallon per hour of propane
consumption. A network of these manually operated generators has been utilized in the
Utah cloud seeding program. Generators have typically been located upwind (west) of
the major mountain barriers which comprise the target area. Local residents are trained
in the operation of the generators and turn them on or off as instructed by the project
meteorologists. Generator sites have normally been at lower elevation valley or foothill
locations. The number of generators installed are a function of the size of the target
area(s). Typically 50-60 ground generators have been utilized each season in the central
and southern Utah seeding program. Up to four higher elevation, remotely controlled
silver iodide-acetone ground generators were utilized in the program in the early 1980's.
Aircraft seeding, using up to four aircraft, was utilized in the late 1970's and early
1980's. These seeding modes were utilized in recognition that certain "seedable" storm
periods were probably unseedable using lower elevation generators due to atmospheric
stability conditions. These seeding modes have not been used in more recent years due
to budget constraints. Seeding criteria have been utilized in the conduct of the Utah
cloud seeding program. These criteria were adopted from the analysis by Vardiman and
Moore (1978) and that of Shaffer (1983). These criteria are summarized in below.
Warmer than -30 degrees C at the 500 mb level
No Low-level Trapping Inversions
Cloud Base Mixing Ratios > 3 g Kg-1
Height of the -5 degree C Level at or Below Mountain Crest Height
Analysis of data collected from past seasons of the conduct of National Oceanic and
Atmospheric Research sponsored research programs in Utah in the 1983, 1985, and 1987
winter seasons have indicated that supercooled liquid water (observed by a microwave
radiometer) was present 88 percent of the time when these seeding criteria were
satisfied (Risch, et al., 1988). A weather forecast laboratory, normally located in Salt
Lake City, has served as the operations center for the conduct of the cloud seeding
program. This laboratory has been equipped to receive standard National Weather
Service observation and forecast products as well as weather satellite observations
(GOES). The project meteorologists utilize this information to determine whether portions
of naturally occurring storms meet the seeding criteria and also to determine which
generators should be activated or de-activated in an attempt to target the seeding effects
based upon windflow and stability considerations.
4.0 EVALUATION OF SEEDING EFFECTIVENESS
Evaluating the results of an operational cloud seeding program is unfortunately rather
difficult. The seemingly simple problem of determining the effects of cloud seeding has
received considerable attention over the years. The primary reason for the difficulty
stems from the large natural variability in the amounts of precipitation that occur in a
given area. Since cloud seeding is only feasible when there are clouds and usually only
when there are clouds that are near to or are already producing precipitation naturally,
the question then becomes, "Did the seeding increase (or decrease) the precipitation that
was observed, and if so, by how much"? The ability to detect a seeding effect becomes a
function of the size of the seeding increase compared to the natural variability in the
precipitation pattern. Larger seeding effects can be detected easier and with a smaller
number of seeded cases than are required to detect small increases.
Historically, the most significant seeding results have been observed in wintertime
seeding programs in mountainous areas. The apparent differences due to seeding are
relatively small, however, being on the order of a 5-15 percent seasonal increase. The
relatively small percent increase, in part, accounts for the significant number of years
required to establish these results (often five years or more). In spite of the difficulties
involved, there are techniques available to evaluate the effects of operational seeding
programs. The techniques are not as rigorous or scientifically acceptable as is the
randomization technique used in research, where roughly one half the sample of storm
periods is randomly not seeded. They do, however, offer the potential of at least
establishing an indication of the effects of seeding on operational programs.
Probably the most commonly employed evaluation technique, and the one that NAWC
has utilized, is the "target" and "control" comparison. This technique is based on the
selection of a variable that would be affected by seeding (such as liquid precipitation or
snow). Records of the variable to be tested are acquired for an historical period of several
years duration (20 or more if possible). These records are divided into those that lie
within the designated target area of the project and those in a nearby control area.
Ideally the control area should be selected in an area which would be unaffected by the
seeding. All the historical data, e.g., precipitation, in both the target and control areas
are taken from a period that has not been subject to cloud seeding activities, since past
seeding could affect the development of a relationship between the two areas. These two
sets of data are analyzed mathematically to develop a regression equation which predicts
the amount of target area precipitation, based on observed precipitation in the control
area. This equation is then used during the seeded period to estimate what the target
area precipitation should have been based on that observed in the control area. A
comparison can then be made between the predicted target area precipitation and that
which actually occurred. Any resulting difference can be tested for its significance
through statistical tests.
This target and control technique works well where a good correlation can be found
between target and control area precipitation. Generally, the closer the two areas are
together, the higher will be the correlation. Areas selected too close together, however,
can be subject to contamination of the control area by the seeding activities. This can
result in an underestimation of the seeding effect. For precipitation and snowpack