Forecasting - General
One of the primary operational functions is determining when seedable conditions exist, so that the experimental design can be implemented. WMI meteorologists monitor weather conditions over the target ranges constantly when there is any potential for seeding. General weather information is obtained primarily from Internet sources such as satellite imagery, synoptic surface reports, and visual observations of the ranges (during daylight hours). Expectations are honed by numerical model output, especially that from the Weather Research and Forecast (WRF) real-time four-dimensional data assimilation (RT-FDDA) modeling system, which is run specifically for the WWMPP by NCAR. The WRF RT-FDDA is nested down to 2-km (over most of Wyoming) and is nudged with observations to produce a 24-hr forecast every three hours.
Forecasting - Randomized Seeding Experiment
As low cloud develops over both the Sierra Madre and Medicine Bow Ranges, super-cooled liquid water (SLW) is detected by radiometers, one sited toward each range. These observations are reported in real-time and available via a secure link through NCAR. When SLW is detected by the radiometers sampling both ranges and satellite imagery and/or visual observations indicate orographic cloud fully covering both ranges, the SLW criterion is considered satisfied. The temperature and wind direction at 10,000 ft MSL (700 hPa) are initially obtained from prognostic model output, particularly the WRF RT-FDDA. When these criteria are believed to be satisfied, a radiosonde is released from Saratoga. If the temperature and wind direction are within the limits or criteria established for the seeding experiment, a case is declared. If not, declaration is delayed until conditions change, and another sounding is released.
Ice nucleus aerosols of silver iodide (AgI), or more specifically a AgI-salt complex, are generated via acetone solution combustion techniques from ground-based generators upwind of the barrier and then drift with the wind towards the barrier containing the target area. In the southern mountains of Wyoming (see map of areas), eight generators target the Medicine Bow Range and eight target the Sierra Madre Range. The coverage of wind direction and speed are comparable between the two ranges. Ten generators target the Wind River Range, mostly located along the western flanks. All the generators are remotely operated and monitored via satellite communications by WMI technical staff.
The sounding site is located midway between the Medicine Bow and Sierra Madre Ranges in the town of Saratoga. Soundings ("weather balloons") using Vaisala GPS sondes are taken by WMI staff for each storm in the southern target ranges to provide information on the environmental conditions and to determine whether the storm qualifies for seeding based on temperature and wind criteria. Data from these soundings (temperature and winds at 10,000 ft MSL (700hPa) in particular) are used to confirm that seeding operations should commence.
Radiometrics passive microwave radiometers are located upwind of each of the ranges. These radiometers scan to low elevation angles to intercept clouds forming over the ranges, and the data are transferred over internet connections every 10 min. The primary variable of interest measured by the radiometers is liquid water path, which is calculated in real-time and available to the forecasters for monitoring cloud conditions and making short-term forecasts.