Estimate of the incoherent-scattering contribution to lidar backscatter from clouds

Lidar backscatter from clouds in the Delft University of Technology experim ent is complicated by the fact that the transmitter has a narrow beam width , whereas the receiver has a much wider one. The issue here is whether rece ption of light scattered incoherently by cloud particles can contribute app reciably to the received power. The incoherent contribution can come from w ithin as well as from outside the transmitter beam but in any case is due t o at least two scattering processes in the cloud that are not included in t he coherent forward scatter that leads to the usual exponentially attenuate d contribution from single-particle backscatter. It is conceivable that a s izable fraction of the total received power within the receiver beam width is due to such incoherent-scattering processes. The ratio of this contribut ion to the direct (but attenuated) reflection from a single particle is est imated here by means of a distorted-Born approximation to the wave equation (with an incident cw monochromatic wave) and by comparison of the magnitud e of the doubly scattered to that of the singly scattered flux. The same ex pressions are also obtained from a radiative-transfer formalism. The ratio underestimates incoherent multiple scattering when it is not small. Correct ions that are due to changes in polarization are noted. (C) 1999 Optical So ciety of America.