Retrieval of Physical and Optical Cloud Thicknesses from Space-Borne and Wide-Angle Imaging Lidar

Abstract

Pulsed laser beams are essentially δ-functions in space, time and direction; lidar returns are therefore the remotely observable parts of the cloud’s Green function for radiative transfer. However, “observable” is not limited here a priori to quasi-backscattered radiance, so propagation away from the laser beam and back to the detector by multiple scattering through arbitrarily large angles is considered. We present heuristic arguments demonstrating that the primary information conveyed by Green functions in the spatial/angular and time domains can, in principle, be combined to retrieve the physical and optical thicknesses of stratiform clouds at about 0.5 km resolution. The photon random walk theory presented here is justified by in-cloud “diffusion domain” observations and validated by statistical analyses of real and simulated LANDSAT radiance fields, as well as a remarkable result from LITE. The remaining challenge is to detect the weak, highly-scattered signal. Strategies for ground- and space-borne designs are discussed in the framework of current lidar and low-light imaging technologies.