Measuring the turbulence profile in the lower atmospheric boundary layer.

Optical turbulence can have a severe effect on the propagation of laser beams through the atmosphere. In free space optics and directed energy applications, these laser beams quite often propagate along a slant or vertical path. In these cases, the refractive index structure function parameter cannot be assumed constant, since it varies with height. How it varies with height, especially in the first few meters above the ground, is not well behaved. Turbulence height profiles have been measured since the 1970s, mainly for astronomical observations. These profiles are usually measured for the atmospheric boundary layer (the layer of air from the ground up to approx. 1 km during day and 100 m during night) and some kilometers above it. We have measured the temperature fluctuations in the first few meters above ground level using a system containing eight resistance thermometer devices, mounted in a row at different spacings. Measurements were made flying this system under a tethered balloon or mounted on a telescoping mast. The temperature structure function parameter, CT2, can be estimated from the temperature fluctuations measured by the 28 different probe pairs and the unique distances between the two probes. Finally, Cn2 is estimated from this temperature structure function parameter and compared to values predicted by a turbulence profile model.

Evolution of near-ground optical turbulence over concrete runway throughout multiple days in summer and winter.

Experimental data are presented that demonstrate the evolution of the anisotropy/isotropy of atmospheric statistics throughout the course of four days (two winter, two summer) near the ground over a concrete runway in Florida. In late January and early February of 2017, a 532 nm near-plane-wave beam was propagated 1 and 2 km at a height of 2 m above the runway, and irradiance fluctuations were captured on a CCD array. In August of 2017, a 532 nm Gaussian beam was propagated 100 m at a height of near 2 m, and fluctuation data were captured on a CCD array. Winter data were processed to calculate the covariance of intensity and summer data processed to calculate the scintillation index. The resulting contours indicated a consistent pattern of anisotropy early in the day, evolving into isotropy midday, and returning to anisotropy in late afternoon. Accompanying atmospheric and wind data are presented throughout the measurement days.

Near ground measure and theoretical model of plane wave covariance of intensity in anisotropic turbulence.

Experimental measurements were recently made which displayed characteristics of plane wave propagation through anisotropic optical turbulence. A near-plane wave beam was propagated a distance of 1 and 2 km at a height of 2 m above the concrete runway at the Shuttle Landing Facility, Kennedy Space Center, Florida, during January and February of 2017. The spatial-temporal fluctuations of the beam were recorded, and the covariance of intensity was calculated. These data sets were compared to a theoretical calculation of covariance of intensity for a plane wave.