Comparison of atmospheric optical turbulence measurements from a scintillometer and a sonic anemometer

Fluctuations in the refractive index of air, known as optical turbulence, may have a deleterious effect on system performance in free space laser applications. Optical turbulence is often quantified by the coefficient of the second order structure function of the refractive index, C²_n. While a range of measurement techniques for determining C²_n exist, they may be broadly categorized as those that directly interrogate the optical medium, and those that measure the effects of the medium on some propagation characteristic of an optical wave. In this work, we compare estimates of C²_n obtained from temperature fluctuations via a sonic anemometer to measurements from irradiance fluctuations via an adjacent optical scintillometer. Both instruments are located in a near-maritime environment (coastal estuary) within the atmospheric surface layer, several meters above the surface. Our data set consists of observations that span more than one year, which allows us to examine a wide range of atmospheric conditions and to examine seasonal effects on the measurements. As a single-point measurement, C²_n from sonic anemometers is frequently used in calculations that require path-averaged C²_n values. This work explores the conditions under which this connection is valid and the parameter space where anemometry provides reliable C²_n values.