Marco Gai

Laboratory simulations of lidar returns from clouds

The results of lidar measurements on laboratory-scaled cloud models are presented. The lidar system was based on a picosecond laser source and a streak camera. The cloud was simulated by a homogeneous aqueous suspension of calibrated microspheres. Measurements were repeated for different concentrations of diffusers and for different values of the receiver angular field of view. The geometric situation was similar to one of an actual lidar sounding a 300-m-thick cloud at a distance of 1200 or 7800 m. The results show how the effect of multiple scattering depends on the extinction coefficient of the sounded medium and on the geometric parameters. The depolarization introduced by multiple scattering was also investigated. Measurements were carried out in well-controlled conditions. The results can thus be useful to validate the accuracy of numerical or analytical procedures that have been developed to study multiple-scattering contribution in lidar returns.

Numerical codes for multiple scattering in the atmosphere

The features of a code for calculation of lidar returns from clouds are recalled. Some laboratory measurements on models showed the validity of the code with regard to polarization of received power. Reference is also made to a code for calculations of turbidity effects on optical systems.

Monte Carlo code for mixed MIE and molecular scattering in lidar returns

In several Lidar techniques molecular backscattering from atmospheric gases is considered, e.g. Raman lidar technique, and elastic molecular scattering by HSR1 technique are employed. When larger particles are also present in the sounded zone, Mie scattering contributions are superposed to the Lidar molecular signal. Because the efficiency of the Mie scattering is very high with respect to the molecular one, the effect of Mie scattering can be very strong for typical tropospheric clouds or aerosol structures, when multiple scattering contribution becomes important. In this case the incident, or the molecular scattered radiation, can be multiply scattered from the aerosols. There is no significant multiple molecular scattering, but larger particle scattering processes occur in addition to one molecular scattering. A Monte Carlo code developed for studying this effect is described by means of a flow diagram showing the details of the procedure. The code allows the consideration of depolarization for the molecular return due to the intervening effect of Mie scattering. Some results in some cases of realistic models of atmospheric structures are presented, showing increase of the molecular returns by factors even of the order of a few units. Comparisons with published data by other authors will be shown.

On Monte Carlo for nonspherical and chiral particles

The paper addresses the problem of extending conventional Monte Carlo procedures to cases of multiple scattering in media with suspensions of non-spherical or chiral particles. Extinction coefficients of the media depend on polarization of radiation. Along the propagation path polarization of radiation changes, unless the field is polarized according to one of two particular modes. The relationship between these modes and the elements of the amplitude scattering matrix for the type of particle is shown by means of a simple formalism, tested with reference to simple shapes and orientation of the particles. Some possibilities for extending Monte Carlo procedures are suggested. A case of small chiral spheres is considered.

Monte Carlo for multiple scattering and nonspherical particles

This note describes a possible Monte Carlo procedure to deal with propagatiaon of optical radiation in a medium containing non-spherical particles, without the limit of isotropic orientation of their axes. It is based on the choice of two polarization states which does not change during the propagation in the turbid medium. An approximate procedure is considered for permitting the use of only two polarization states.

Lidar return from clouds: comparison between calculations and measurements

The purpose of this paper is to present and summarize the activity of the Florence group, relevant to the effect of multiple scattering in the lidar technique of sounding clouds or fog.