Power spectra of temperature, humidity and refractive index from aircraft and tethered balloon measurements

Abstract

Fifty-seven spectra of temperature, vapor pressure and refractive index were computed from captive balloon data taken at elevations up to 3000 feet MSL. Eight spectra of refractive index were obtained using an aircraft equipped with a microwave refractometer. It was found that atmospheric stability apparently has a pronounced effect on the variation of turbulent intensity with height. Although the spectra of all three parameters generally approach a -5/3 power law at high wave numbers, stability seems to have a controlling influence on spectral form at the low-frequency end of the wave number range studied. It is therefore concluded that methods of computing microwave scattered fields from the mean square dielectric perturbations and scale size obtained from the auto covariance are unreliable. The forms of the experimental auto covariances appear to be best represented by an exponential function or perhaps by a Modified Bessel Function of the second kind and 1/3 order. The temperature-humidity cospectrum may influence the shape of the refractive index spectrum, especially under unstable conditions. The equivalence of Eulerian space and time spectra is verified for refractive index by a series of aircraft-balloon fly-bys.