Cloud seeding generator near the headwaters of the Walker River

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 (Pyramid and Walker).

AREAS OF ACTIVITY:

The 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.  Four remotely-controlled generators 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 iodide-sodium iodide and salt in acetone to release silver chloro-iodide particles which create additional ice crystals, then snow,  in winter clouds.  Weather conditions are selected to optimize fallout in targeted basins. All generators are remotely operated by radio or cellular telephone. A seeding aircraft is at times used to augment ground seeding operations. The aircraft releases AgI  from pyrotechnic flares or solution burners. Dry ice is also used in airport fog clearing operations.

 

Seeding Equipment:

Nineteen (19) remotely controlled AgI generators.

 

Supporting Equipment:

Weather stations on at least one seeding generator in each targeted basin. DRI/NWS hydro-meteorological 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 radiometers for sensing cloud liquid water - an NCAR ice nucleus counter for detecting AgI plumes - laser imaging probes for measuring cloud particles.

 

Supporting Personnel:

Three full time technicians who install and maintain all generator networks and fabricate new generators - two 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 20,000 to 80,000 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 $7 to about $18 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 and model simulations of seeding plume transport.

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:

1. When the avalanche category, determined by the U.S. Forest Service, is designated as EXTREME.
2. When the National Weather Service (NWS) forecasts a warm winter storm (freezing level >8000 ft.) with the possibility of considerable rain at the higher elevations which might lead to local flooding.
3. When the Project Meteorologist determines that 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 or 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.

1. 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 radiosondes can also be used to monitor this criteria, as can the Doppler winds from the Reno and Elko NEXRAD radars.
2. 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 radiosondes and NEXRAD Doppler winds from Reno and Elko can be used to monitor wind direction.
3. When the water content of the snowpack in the target area, as measured at existing snow courses or SNOTEL sites, 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.




4. 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.

The Nevada State Weather Modification Program is operated from the Desert Research Institute Division of Atmospheric Sciences, located in the Northern Nevada Science Center (NNSC), Reno, Nevada.  The project has 24-hour access to a broad base of National Weather Service (NWS) weather data through UNIDATA, a program managed by the University Corporation for Atmospheric Research (UCAR). The data are received over the INTERNET. Additional WEB sites on the INTERNET provide other data and forecasts. A partial list of weather data products is as follows:

    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) A selection of weather satellite maps and other products 24-hours-per-day.
    D) Near real time NEXRAD radar images from all NEXRAD sites in the western U.S.

In addition to the above National Weather Service data products, the data from remote weather stations on Slide Mountain, Ward Peak, Conway Summit,  a site near the Ruby Mountains, a site near Tuscarora, NV, and a site in the Wassuk Mountains are continuously available through the Internet or telephone modem at the NNSC.  Data from the Bureau of Land Management RAWS network, the Natural Resource Conservation Service SNOTEL network, and from a local NWS hydro-
meteorological network are available through the DRI Western Regional Climate Center on a near real time basis. These sources provide additional local information concerning air 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 6/18/01

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 0-3 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:
1. NWS hourly reports of cloud conditions and precipitation at, but not limited to, the following sites:  MMH, BLU, TRK, TVL, RNO, EKO.
2. Visual observations and/or reports of cloud conditions by the Project Meteorologist, other Project Staff, or contacts in any of the project areas.
3. Observation of precipitation from any automatic recording gauge whose data are telemetered by telephone modem or the Internet to DRI. The DRI has access to many such gages in the Sierra Nevada and other mountain ranges throughout Nevada.
4. 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:
1. Truckee-Tahoe area:  Wind direction at 700 mb, as measured by the Reno NWS soundings (or soundings launched by the Sacramento Municipal Utilities District “SMUD” from Fresh Pond, Calif.) or estimated by weather stations close to the 10,000 ft altitude level, from (clockwise) between 180 and 340 degrees.
2. Carson-Walker:  For ground seeding cloud level wind directions from 135 to 270 degrees as verified by the weather stations near Conway Summit and in the Wassuk Mountains .
3. Ruby Mountains:  Wind directions in the cloud layer from 190 to 330 degrees as verified by the NWS Elko radiosonde, or remote weather station data.
4. Tuscarora area: Wind directions in the cloud layer from 90 to 270 degrees as verified by the NWS Elko radiosonde, or remote weather station data.
5. Toiyabe Mountain area: Wind directions in the cloud layer from 90 to 270 degrees as verified by remote weather station data, or upper air weather maps.
4. Wind speeds at or near 700 mb should not exceed 30 m s^-1 (~60 kt) in order that adequate time be available for growth of ice crystals initiated by seeding.  Mountain top weather stations, NWS Reno and Elko radiosondes and SMUD soundings 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 all 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 mountain top weather stations, or Reno, Elko, or SMUD soundings.  Operations may be initiated at a temperature as warm as -3°C, provided the -5°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 a wide variety of wind directions. 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 .  The presence of supercooled liquid water must be verified for aircraft seeding operations to be initiated or continued.