Further Validation and Refinement of the Tenti Model for Atmospheric Lidar Backscatter

The objective of this study was to further validate and refine the Tenti Model for Atmospheric Lidar Backscatter in temperature, wavelength and scattering angle space. This was achieved through experimental Rayleigh-Brillouin scattering measurements recorded in the laboratory using a home-built setup. The setup included a high-power narrowband tuneable laser source in the ultraviolet and blue range of the spectrum, an enhancement cavity to produce some 5 Watts of power in the scattering zone, and a Fabry-Perot spectral analyzer that records the spectrum at a 90 degrees scattering angle. Scattering in the forward direction could not be tested in this activity due to technical and time constraints. RB-spectra are obtained in air for both 366 nm and 403 nm wavelengths for a pressure-temperature parameter space covering 0.3, 0.75, 1.0 and 3.0 bar and-20, 4, 24, 45 and 65 o C. These data match predictions from the TENTI S6 model to 1% accuracy, if the proper values for the macroscopic transport coefficients are included. In this model also a value for the bulk viscosity is used, which is derived from the present optical data. The TENTI model calculations can be used to retrieve the temperature levels in the experiment to within 0.4 o C. General background to the project The present study on laboratory Rayleigh-Brillouin scattering is intimately linked to future lidar missions of the European Space Agency. The immediate connection is to the ADM-Aeolus mission aiming to measure wind profiles in the Earth atmosphere on a global scale. This is pursued by active remote sensing, i.e. by measuring the spectral profile of the back-scattered light from an ultraviolet laser on board of the satellite. In the recent past it was noted that the molecular scattering functions are not Gaussian profiles and the deviations from gaussianity will influence the Doppler measurements and impact the wind profile analysis. In particular acoustic phenomena known to produce the characteristic Brillouin side-wings on the Doppler profile have a strong effect. This was identified as a major problem in a previous study (the ILIAD report 1) and it was estimated that neglecting the Brillouin effect will result in errors in the radial wind measurement of up to 10% in several cases. These estimates were made on the basis of models known in the literature since the 1970s as the TENTI models. These TENTI models had only been tested for a few measurement configurations and for a very …