Accounting for multiple scattering in spaceborne radar and lidar observations

CloudSat observations of deep convective clouds reveal significant contributions from multiply scattered photons, resulting in "pulse stretching", where echos appear to originate from beyond the end of the cloud and even below the ground. Spaceborne lidar returns from liquid water clouds suffer from the same effect. It is essential to account for this phenomenon if we are to retrieve accurate microphysical profiles from space, but no satisfactory method for doing so has yet been devised.

In this talk, an efficient multiple scattering model will be described that may be incorporated into retrieval schemes as the "forward model" for both radar and lidar, thereby allowing multiple scattering to be accounted for. It splits the photons into those that have taken a near-direct path out to and back from a single backscattering event (in the case of lidar, accounting for small-angle forward scatterings on the way), and those that have experienced wide-angle multiple-scattering events leading to pulse stretching. The latter are modeled using the time-dependent two-stream approximation. The method performs well in comparison to Monte Carlo calculations, but is far more efficient. This offers the prospect not only of accounting for a troublesome effect in CloudSat and Calipso data, but also of making use of multiple scattering to extract extra information about clouds and convective systems.