Optimizing In-Situ Measurements of Wind, Temperature and Turbulence in Urban Environments by Multi-Path Ultrasonic Anemometry

Ultrasonic anemometers (sonics) have been used for decades for automatic unattended measurements of meteorological variables such as wind, temperature and turbulence. Sonics are using short ultrasonic pulses transmitted between pairs of transducers along three different measuring paths to determine three corresponding radial wind components and to retrieve the 3D wind vector. Due to the absence of inertial masses sonics can resolve turbulent fluctuation down to scales of 0.1 m and 20 Hz. Sonics are affordable and cost-efficient with negligible maintenance while being robust and operating reliably under practically all-weather conditions.

The accuracy of the derived wind data is mainly determined by flow modifications caused by the sonic transducers (shadowing) and/or by the sensor head structure (distortion). Wind tunnel derived correction schemes are typically applied on the measured data to compensate such deviations to the best extent. However, as the 3D-wind flow within an urban environment can fluctuate extremely in space and time such compensation schemes bear some limits, especially in case of strong up- und downward wind components. As a consequence, it is obvious that the original flow modification must be kept at a minimum.

With the uSonic-3 MP an optimized sensor head was designed which provides a minimum of flow modification effects while preserving the general benefits of sonics and a sufficient responsiveness to the wind flow. It uses an innovative Multi-Path technology, i.e. an array of 3 transducers exchanges ultrasonic pulses with an opposite array of 3 transducers. This yields 3 x 3 = 9 radial wind components including three directly measured vertical wind components. Only windward positioned paths are selected for the retrievals of wind components avoiding deterioration of measured wind data.

The concept of the Multi-Path technology and wind tunnel measurements will be presented together with a six-month field comparison against other sonic types.