Comparison of slant-path scintillometry, sonic anemometry and high-speed videography for vertical profiling of turbulence in the atmospheric surface layer

Abstract

The optical effect of atmospheric turbulence greatly inhibits the achievable range of Detection, Recognition and Identification (DRI) of targets when using imaging sensors within the surface layer. Since turbulence tends to be worst near the ground and decays with height, the question often arises as to how much DRI range could be gained by elevating the sensor. Because this potential DRI gain depends on the rate of decay of turbulence strength with height in any particular environment, there is a need to measure the strength profile of turbulence with respect to height in various environments under different atmospheric and meteorological conditions. Various techniques exist to measure turbulence strength, including scintillometry, sonic anemometry, Sound Detection and Ranging (SODAR) and the analysis of point source imagery. These techniques vary in absolute sensitivity, sensitivity to range profile, temporal and spatial response, making comparison and interpretation challenging. We describe a field experiment using multiple scintillometers, sonic anemometers and point source videography to collect statistics on atmospheric turbulence strength at different heights above ground. The environment is a relatively flat, temperate to sub-tropical grassland area on the interior plateau of Southern Africa near Pretoria. The site in question, Rietvlei Nature Reserve, offers good spatial homogeneity over a substantial area and low average wind speed. Rietvlei was therefore chosen to simplify comparison of techniques as well as to obtain representative turbulence profile data for temperate grassland. A key element of the experimental layout is to place a sonic anemometer 15 m above ground at the centre of a 1 km slant-path extending from ground level to a height of 30 m. An optical scintillometer is operated along the slant-path. The experiment layout and practical implementation are described in detail and initial results are presented.