Sonic thermometry today

Sonic anemometer-thermometers began appearing in field studies over 50 years ago. They have since been used extensively in field experiments by atmospheric scientists in their studies of turbulent flow in the surface layer—the first 100 feet or so above the ground. The sonic approach promised better high-frequency response and far less flow distortion than what past sensors could offer. The idea of measuring vertical wind and temperature fluctuations along the same vertical acoustic path held great appeal.

The 1968 Kansas Experiment saw the first deployment of sonic anemometerthermometers in a major field program. Conducted by scientists at the Air Force Cambridge Research Laboratories with three newly delivered Kaijo Denki, Inc. sonic anemometer-thermometers mounted at three levels on a 100-ft tower, the experiment was a success. The large amounts of sonic wind fluctuation data collected over several weeks helped redefine the nature of turbulent transport near the ground. But the sonic thermometer data was disappointing. They departed significantly from fast-response temperature data provided by back-up platinum fine-wire thermometers mounted within the frame of the sonic probes, behind the vertical axis. There was no reason to doubt the accuracy of the platinum sensors. The sonic temperature data were never used.

Sonic thermometry fell out of favor in the United States for the next 20 years until the arrival of a new generation of non-orthogonal sonic anemometers. Their three axes were tilted 60 deg from the horizontal with the three paths intersecting in the middle. They offered a common sampling volume and an open aspect to horizontal winds but the vertical wind component had to be derived from measurements along tilted axes. Several versions of this design came on the market with each manufacturer offering sonic temperature measurements as well. Soon large numbers of users in the forestry and agricultural research communities were opting for the new non-orthogonal sonic anemometer-thermometers. But no serious efforts were made to check the accuracy of the temperature measurements. Several intercomparison experiments have been conducted to resolve issues with their vertical wind measurements, but sonic temperature takes more than side-by-side comparisons to sort out sensitivity to factors other than temperature. This prompted us at ATI to study the problem and come up with ways to make the sonic temperatures more accurate.