The influence of aperture averaging on the two-wavelength intensity covariance function was experimentally determined for visible (0.63 microm) and infrared (1.06 microm) collinear, approximately spherical beams which propagated through the earth's turbulent atmosphere. Range varied from 1300 to 3250 m, and due to the prevailing atmospheric conditions, most measurements were made in the strong turbulence regimes. Results show that (1) the covariance function monotonically decreases as the receiver aperture size increases; (2) the correlation coefficient attains high values > or = 0.7) even for a relatively small aperture size of 5 mm; (3) while the single wavelength probability distribution of the intensity is approximately lognormal, the experimental two-wavelength conditional probabilities are higher than those predicted by the lognormal model.

Presents a method for the determination of the path-averaged value of the inner scale of atmospheric turbulence, l0, using an optical scintillometric method. The proposed method involves the measurement of the bichromatic correlation of two laser beams of different wavelengths propagating in overlapping paths. Results obtained using this technique are compared with the ratio of the log-amplitude variances of the two laser beams and with l0 values derived from vertical wind velocity fluctuation measurements. The results of the correlation method agree well with theory based upon Hill's refractive index spectrum. Results obtained by the ratio of variances match those obtained by the correlation technique after correction for the refractive index dispersion due to humidity fluctuations. These two methods can be used jointly for the simultaneous determination of l0, CT2 and CQ2.

No abstract

Structure constants and inner scales of temperature fluctuations were derived from ultrasonic anemometer -thermometer measurements taken at heights of 48 m and 80 m above the ground. They were shown to follow local Monin -Oboukhov similarity from very unstable to very stable atmospheric stratification. A direct empirical expression for the stability dependence of the nondimensional inner scale is given.A bichromatic scintillometer based on the wavelengths 0.63pm and 10.6 pm was operated near the ground. Comparison between optically measured path averaged inner scales to those derived from point measurement of vertical velocity fluctuations yielded excellent agreement. The observed dispersion of refractivity fluctuations was used to separate structure constants of temperature and humidity. Surface fluxes obtained from parameterizations requiring only simple meteorological input data were applied to scale optically measured structure constants of temperature and inner scale. The parameterized fluxes were shown to be sufficiently accurate to be used in Monin -Oboukhov similarity based models.

No abstract