Cristina Flesia

LIDAR MULTIPLE SCATTERING FROM CLOUDS

Multiple-scattering LIDAR return calculations obtained by seven different models for the same specified numerical experiment are compared. This work results from an international joint effort stimulated by the workshop group called MUSCLE for MUltiple SCattering Lidar Experiments. The models include approximations to the radiative-transfer theory, Monte-Carlo calculations, a stochastic model of the process of multiple scattering, and an extension of Mie theory for particles illuminated by direct and scattered light. The model solutions are similar in form but differ by up to a factor of 5 in the strength of the multiple-scattering contributions. Various reasons for the observed differences are explored and their practical significance is discussed.

Analytical multiple-scattering extension of the Mie theory: The LIDAR equation

Multiple scattering of light in aerosol media is described in a simple picture within the framework of Mie theory. Our approach leads to an analytical expression of then-fold scattered electromagnetic field and then to an analytical derivation of multiple-scattering LIDAR equation form transport theory. This approach differs from both the descriptions of multiple scattering based on the approximation of radiative-tranfer theory and from statistical approaches mainly based on Monte-Carlo calculations. The physical quantities of interest are expressed by straightforward generalization of the corresponding single-scattering quantities. Therefore, the multiple-scattering contributions are calculated without losing the advantage of working with analytical expression in the frame of Mie theory.

Multiple scattering and lidar returns

Experimental evidence of multiple-scattering effects on lidar returns from clouds, as well as the calculated significance of second and higher orders of scattering, as can be found in the literature, are summarized, together with results of intercomparisons of theoretical and experimental results obtained by different researchers. New comparisons between analytic results and Monte Carlo data are reported, indicating the level of accuracy of predictions relevant to multiple scattering and lidar

Multiple scattering from clear atmosphere obscured by transparent crystal clouds in satellite-borne lidar sensing

The contribution of multiple scattering to a spaceborne lidar return from clear molecular atmosphere obscured by transparent upper-level crystal clouds is assessed by the use of the variance-reduction Monte Carlo technique. High anisotropy of scattering in the forward direction by polydispersions of ice crystals is the basis of a significant effect of multiple scattering for small values of the lidar receiver field of view. Because of scattering by large nonspherical crystal particles, the lidar signal backscattered from the molecular atmosphere under the cloud increases significantly compared with the single-scattering return. The ratio of the multiple-to-single-scattering contributions from the clear atmosphere hidden by the clouds is greater than from the crystal clouds themselves, and it is proportional to the values of cloud optical thickness.

Effect of Signal Dynamics: A Comparative Study Between PRN and Pulse Illuminated LIDAR Systems

The effect of the dynamic range of the atmospheric response is studied in the case of Pseudo-Random Noise modulation Continuous Wave (PRN-CW) Backscatter LIDAR, and compared to the traditional pulse illuminated system. Using pseudo-random sequences, the measured signal consists of the sum of atmospheric response corresponding to several altitudes. Depending on the scattering volume location, the elements of the summation have generally several orders of magnitude difference. Small variations in the larger atmospheric response do not allow one to discern the atmospheric response of the same order of magnitude from other atmospheric layers. Another aspect of the dynamic range is the saturation of the detectors. Both reasons make the PRN systems more sensitive to the atmospheric response dynamic range than the traditional pulse illuminated systems, which are not affected by such a summation process.

Strong Localization of Light in Spongy Agglimerates of Carbon Grains

Ultraviolet extinction spectra of spongy agglomerates of amorphous carbon grains have been measured in the spectral range 100-300 nm. These spectra present two dips which can be explained in terms of strong localization of light in a disordered medium. The particular spongy structure of the carbon grains samples determines random fluctuations of the refraction index. When a light beam crosses samples having a granular structure, the fluctuatuions of n produce optical resonant modes which are localized along the disordered medium. Therefore, the transmission is enhanced for particular resonant frequences corresponding to the positive interferences of the light within an agglomeration of carbon grains having, in our experiment, a width of about 100 nm.

Anomalous dips in extinction spectra of disordered carbon grains

Abstract Ultraviolet extinction spectra of spongy agglomerates of amorphous carbon grains have been measured in the spectral range 100–300 nm. These spectra present two dips which can be explained in terms of harmonic resonant modes of electromagnetic waves in a disordered medium. This effect is due to the random fluctuations of the refraction index of our samples which have a disordered granular structure.

Double-Edge Molecular Technique for Doppler Lidar Wind Measurement

The double-edge lidar technique for measuring the wind based upon using molecular backscatter is described. The technique uses two high spectral resolution edge filters which are located in the wings of the Rayleigh-Brillouin profile. This doubles the signal change per unit Doppler shift, the sensitivity, and gives nearly a factor of two improvement in measurement accuracy relative to the single edge technique. The use of a crossover region is described where the sensitivity of a molecular and aerosol-based measurement are equal. This desensitizes the molecular measurement to the effects of aerosol scattering over a frequency range of plus or minus 100 m/s. We give methods for correcting for short- term, shot to shot, frequency jitter and drift using a laser reference frequency measurement and methods for long-term frequency correction using a servo control system. The effects of Rayleigh-Brillouin scattering on the measurement are shown to be significant and are included in the analysis. Simulations for a conical scanning satellite-based lidar at 355 nm show an accuracy of 2 - 3 m/s for altitudes of 2 to 15 km for a 1 km vertical resolution, a satellite altitude of 400 km and a 200 km X 200 km spatial resolution. Results for recent wind measurements, which show an accuracy of 1 m/s up to an altitude of 10 km, are given.

Depolarization of lidar returns from clouds: comparison between spherical and Chebyshev particulate

The effect of multiple scattering on received power and its polarization state is examined by considering clouds made of spherical water droplets and non-spherical Chebyshev particles. A Monte Carlo code was used and its capability of dealing with homogeneous and stratified clouds is shown by a series of examples.

Simulated multiply-scattered lidar returns from nonspherical particles

An investigation of the characteristics of multiply-scattered lidar returns from homogeneous layres of nonspherical Chebyshev particles is presented. A Monte Carlo procedure has been employed to simulate lidar measurements in a ground-based configuration. Total detected power and depolarization of the return signal have been calculated for a variety of particle sizes and deformations, as well as for different fields of view of the instrument and optical thicknesses of the medium. As far as depolarization is concerned, particles characterized by a high backscattering depolarization ratio have shown a peculiar behavior in multiple scattering. Results have been checked, for double scattering, employing an analytical formula previously developed.