Scintillation measurements with a HeNe and a CO? laser were used to derive turbulent fluxes of heat and momentum in the surface layer. This was achieved by the structure constant or dissipation technique. i.e., by relating the measured structure constants and inner scales of refractive index fluctuations to structure constants of temperature fluctuations and dissipation rates of turbulent kinetic energy. respectively, and then assuming Monin-Obukhov similarity. The resulting heat fluxes agree well with measurements using the eddy correlation technique but for averaging periods of 10min. the optical data show a much smoother and physically more plausible behaviour. The optically derived friction velocities are in good agreement with estimates derived from wind velocity and surface roughness. It was also observed that for stationary conditions. l-min averaged optical measurements already provide good estimates for longer averaged heat and momentum fluxes.Even though some uncertainty remains about the empirical constants and MoninPObukhov similarity expressions used, the method clearly pr-oses to be of great value for monitoring surface-layer turbulence

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.

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.

A model for CT, the structure constant of temperature fluctuations, for the lowest 50-500 m of the atmosphere over land has been derived from similarity expressions and simple surface flux parametrisations. Only very few data are required by the model: standard weather data and estimates of ground characteristics. Universal flux profiles of the nocturnal boundary layer lead to CT profiles which increase in value near the top of the layer. Validation of model predictions by slant path laser scintillometer and differential thermometer measurements over flat terrain gives very good agreement for daytime and good agreement for night-time. Slant path scintillometer measurements show that even in extremely mountainous terrain the daytime model gives reasonable results.

Modelling refractive index turbulence over paved surfaces is of significant importance to predict limitations on optical remote sensing systems operated over roads, airport runways, or urban areas. Since optical turbulence is generated by the turbulent heat transfer between ground and atmosphere, its magnitude depends on the thermal and dynamical properties of the surface. Compared with typical vegetated and unvegetated surfaces, the pavement has a significantly smaller roughness length, a lower albedo, and a higher heat conductivity and capacity. This results in a turbulent situation which is very different from that found over other areas and therefore needs special investigation.This paper presents a model to calculate the structure function constant C,2 and the inner scale of the refractive index fluctuations over a paved road. The basic input parameters are air temperature, surface temperature, and wind speed. We limit ourselves to the dry case.The model is verified by measurements taken in summer 1994 over a motorway near Vienna. Both C,,2 and were determined using displaced-beam laser scintillation. The optical data is compared with the model applied to on-site meteorological observations. Good agreement is observed. MODELThe concept of Monin-Obukhov similarity has been shown to be a suitable tool for the prediction of C2 and 1,2,3 In this procedure, the first step is to relate C,,2 and to the structure function constant of temperature C2 and the dissipation rate of turbulent kinetic energy E, respectively : C, = P (1) and = 7.4 v314 c 1/4Here a1 is a proportionality constant, which is slightly wavelength dependent and in the visible has an approximate value of 8x iO KIhPa. p is the air pressure, T is the air temperature, and Vis the kinematic O-8194-1824-2/95/$6.OO SPIE Vol. 2471 / 197 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/30/2015 Terms of Use: http://spiedl.org/terms