Weather Modification – Utah has a similar program dating back to the 1970’s

Weather Modification – Utah has a similar program dating back to the 1970’s.

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.

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).

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.

Primarily, ground-based generators are used to burn a solution of silver iodide-ammonium 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.

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.

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.

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.

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) 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.
   
3. 
   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.
   

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 rawinsondes can also be used to monitor this criteria.
   
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 rawinsondes 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, exceeds the accumulation envelope defined by the following percentages to date of long-term averages on the same date:
   

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.
   

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:

1. 
   National Weather Service Public Product Service giving hourly weather conditions.
   
2. 
   DIFAX Service giving a selection of weather maps 24-hours-per-day.
   
3. 
   PC-McIDAS Service giving a selection of weather satellite maps and other products 24-hours-per-day.
   

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 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 generator operators in the Ruby Mountains region.
   
3. 
   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.
   
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 estimated by Slide Mountain or Ward Peak mountain top data, 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 station above Conway Summit.
   
3. 
   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.
   

Revised: 6/6/97

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, 1290-1300.

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.

Image size is 10M

Atmospheric and Dispersion Modeling