Cloud retrievals exploiting multiply scattered lidar returns

Cloud properties, in particular their extinction coefficient profile, play an important role in the radiation budget and hence climate. Spaceborne lidar returns can be used to retrieve cloud extinction profiles; however, the direct lidar return is rapidly attenuated by optically thick cloud. The attenuation is due to photons being scattered away from the direction of direct return and into the cloud. These scattered photons carry information about the optical properties of the cloud and some of these photons may be multiply scattered back towards the lidar where they can be measured by a lidar specially equipped with a wide field-of-view receiver. The multiply scattered return can complement the direct return to provide cloud optical properties to much greater optical depths than direct return alone.

We have developed a retrieval method based on optimal estimation theory that uses a fast multiple scattering algorithm to forward model the multiple field-of-view lidar observations. Using synthetic data and data from the THOR lidar (cloud Thickness from Offbeam Returns), with eight concentric fields of view, we demonstrate that combining backscatter measurements from multiple fields of view allows cloud retrievals of stratocumulus with optical depths up to 30 compared to optical depths of about 3 from direct return alone. We consider how additional constraints may be used to handle instrument noise and to incorporate prior knowledge of cloud profiles in regions with poor observations. This work will help to interpret spaceborne cloud radar and lidar returns that are often affected by multiple scattering.