Dependence of the lidar signal depolarization on the receiver's field of view in the sounding of fog and clouds

We report an experimental study of the lidar signal depolarization as a fun ction of the relative contribution of the multiple scattering in case of op tically dense objects in the atmospheric planetary boundary layer. Results of the observation of fog and stratus clouds are presented, as well as thos e obtained by sounding of stratocumulus clouds during a snowfall. The lidar data point to a rise of the depolarization coefficient as the influence of the multiple scattering increases in consequence of both viewing angle enl argement and penetration into the object sounded. The variations of the dep olarization coefficient are studied as a function of the field of view. Tn the case of fog, this dependence is approximated by a three-parameter expon ential law; it is found that the depolarization increases steeply when the viewing angle is increased from 9 mrad to 12.5 mrad. The relationships betw een the approximation parameters and the microphysical characteristics of t he scattering medium are considered. The experimentally determined size of the area where multiple scattering occurs is in good agreement with that ca lculated according to the diffusion model. The results obtained on the mult iple scattering effect on the depolarization can also be employed in determ ining the extinction coefficient profiles in optically dense objects, as we ll as in evaluating the characteristic size of the scattering particles.