The ultrasonic anemometer is a very special instrument that is used in micrometeorology, the rib of meteorology that deals with the smallest and infinitesimal motions and dynamics of the atmosphere. In particular, it is used to study the turbulence of the air, measuring the three components of the wind and the speed of sound .
To measure turbulence, that is the very small motions inside the boundary layer, it is necessary to perform many more measurements than traditional anemometers, such as those with cups, and in addition it is necessary to measure the three-dimensional motions of the air (on the horizontal plane and towards the top).
The many measures required for this kind of investigation are not compatible with the moving parts of traditional instruments (such as the cups and the wind vane) which lengthen the response times, in fact they must physically start up, and are unable to suddenly change their behavior as the direction of the air mass changes. The choice then fell on measuring the eddies generated by turbulence with sound packets. In fact, sound is a pressure wave, which propagates using air as a medium. In practice it is a succession of denser areas to more rarefied areas. Hence the operating principle of the sonic anemometer, a succession of sound packages in the three directions of space.
How the ultrasonic anemometer works
Without wind the sound waves propagate in a rectilinear way and to travel a certain path they will take a time proportional to their speed. However, if the wind is not zero it will deflect the sound packet and it will take a different time to reach its destination than expected.
Three-dimensional ultrasonic anemometers usually consist of three pairs of transducers which in turn send and listen to the sound packets. They are placed in such a way as to be able to resolve the three-dimensional space and not to disturb the air-flow too much. The image at the beginning of the article shows the anemometer installed at Alpe Veglia (Gill, R2-Research).
Physically the principle of operation between a pair of transducers is exemplified in the figure above.
A and B are the two transducers, at instant 1 the sound package will go from A to B (t 1 red), at instant 2 another sound package will go from B to A (t 2 green).
Here V is the wind speed, which can be decomposed into a perpendicular to the packet path (between A and B), V n , and one along the packet path , V d. Since the sound is a pressure wave in the air, it will be physically displaced, or deflected, by the V wind. In the first flight the packet will be favored, it proceeds with the “wind in the back”, and therefore it will take less time than calculated in the absence of wind to cover the known distance d ; on the contrary, in making the reverse path it will travel against the wind and it will take longer (t2).
The equations that solve this problem are given below.
The distance d between the two transducers (A and B) will be covered by multiplying the time of the outward or return journey by the sum of the components of the speed of sound c and of the speed of the wind V along the joining of the two transducers (subscript d).
The speed of sound is the same, so by solving the system of these two equations we can obtain the component of the wind speed along the joining A and B.
The anemometer has three pairs of transducers, monthed in a non-orthogonal way, knowing their arrangement, and the three measured speeds it is possible to obtain, with rotation matrices, the three components of the wind in a Cartesian system of axes (x, y and z ).
The speed of sound can also be solved and obtained. However, it will be the same for the three pairs of transducers, if the system has no operating defects.
Very frequently it is easier to find the sonic temperature , instead of the speed of sound, it is actually not measured by the instrument, in fact it only measures velocities (or flight times). The sonic temperature can be calculated during the data analysis phase, or more often it is directly calculated by the anemometer itself (some models produced by Metek or Campbell directly return the Ts ) .
As we have seen, the sonic anemometer has a very simple operating principle, which however allows, having no moving parts, to perform up to 100 measurements per second, and therefore to resolve turbulence down to the smallest scales.
Which quantities are measured with the ultrasonic anemometer?
With the sonic anemometer it is possible to calculate the sensible heat flux , using the sonic temperature and vertical velocity fluctuations. While, if you have a fast hygrometer, which measures the specific humidity fluctuations, you can get in similarly the latent heat flux linked to the changes in the state of the water in the surface layer and to the evapotranspiration of the plants.
By combining the fluctuations of the other components of the wind speed, it is possible to obtain momentum flows linked for example to the transport of energy or the momentum of turbulence.
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