Rooftop devices measure wind with sound

June 24, 1994

By Mike Wooldridge, [email protected]

Keen-eared employees who have recently noticed a faint chirping outside several Lab buildings aren't hearing a new species of bird. The sounds are coming from devices that monitor the wind by "listening" to echoes bounced off of the passing air.

The Sonic Detection and Ranging Devices (SODARs) were installed on top of Bldgs. 50 and 62 in February to chart air flows around the Lab. Another site, at Bldg. 25A, is being prepared for a third SODAR. The project was a collaborative effort of the Environment, Health and Safety Division and the Facilities Department, with Nathan Hong of Facilities Architecture Engineering and Pat Thorson of EH&S leading a team of designers. Fred Bush of Facilities Maintenance and Operations coordinated LBL construction crews for the project.

LBL takes wind readings in order to predict the movement of certain pollutants--such as radionuclides--emitted from its facilities, which is required by the Environmental Protection Agency and DOE. "The lab is not a large emitter of any of these chemicals," Thorson says. "But we do have a variety of chemicals being used at a variety of locations."

The SODARs took over the wind-monitoring task from the weather tower atop Bldg. 70. Past site audits revealed that surrounding trees had grown taller than the tower, shielding it from the prevailing southwest winds. Auditors also determined that a single tower on Bldg. 70 could not accurately measure winds on the Lab's eastern perimeter, a half mile away.

The initial plan was to build taller weather towers at several sites around the lab. This was abandoned after designers determined how extensive such a project would be--LBL would need six weather towers to monitor winds lab-wide, and three of the towers would have to be more than 10 stories tall. "Towers weren't feasible just from an aesthetics point of view," Thorson says.

The design team solved the problem with SODARs. Composed of a small panel of speakers and a processing unit, the devices are about as inconspicuous as hi-tech weather-monitoring equipment gets, with each device taking up as much space as a small dumpster. The SODARs are also able to estimate the wind currents across all of LBL with only three stations. (A short tower accompanies one of the SODAR stations for quality control.)

A barrier to using SODARs in the past at many facilities was their size and the noise they made. Earlier models, used at sites such as airports and factories, had head-high, cone-shaped speakers that boomed their signals into the air. Only recently has technology miniaturized and quieted the devices, making them reasonable for locations such as LBL. Lab designers took further precautions by surrounding two of the SODARs with sound-dampening fences.

To measure the wind, a SODAR sends out a series of three high-pitched "chirps" from its speakers. The chirps are spaced about a second apart, and each is broadcasted in a slightly different direction--a center component travels directly upward from the SODAR, while two others travel at 17-degree angles to the first.

As they travel skyward, the signals are reflected back to the device by the wind. The frequency of the returning sound waves changes depending on the wind's movement. The SODAR can calculate the speed and direction of the winds by measuring the change in frequency of the echoes.

Still, even with the most advanced equipment, wind measurement isn't exactly a breeze at LBL. Local weather currents vary considerably due to the give and take between weather on the coast and the weather inland. In the daytime, especially in the summer, air currents are usually drawn eastward over the East Bay Hills as the inland air warms and rises. At night, when the inland areas cool, the winds often die down and the cooling air mass creates downslope breezes.

The varied terrain of the East Bay Hills further complicates the situation. The canyons, trees and buildings tend to channel the winds, which also affects their speeds. A light, southwest breeze blowing at Bldg. 50 might be channeled into stronger west gusts at Bldg. 62, near Strawberry Canyon.

"To my knowledge, it is really the first time someone has stationed this many SODARs on a site with terrain as complex as LBL's," Thorson says. He would like to make the LBL SODAR data available to meteorologists trying to better understand the currents that flow over the East Bay hills. More importantly, the SODAR data will be accessible to the LBL Fire Department to use in the event of a blaze in the surrounding hills, an accidental gas leak, or other emergencies.

In addition to Facilities and EH&S, building managers and the Purchasing Department also played important roles in the two-year project to install the SODARs. "People across the Lab spent a lot of time working together to deal with all the twists and turns that arose," Thorson says. In the end, he says, the project came in on-time and under budget, and is now producing the results expected by the design team.