Identifying multiple-scattering-affected profiles in CloudSat observations over the Oceans

Multiple scattering strongly affects the CloudSat Profiling Radar reflectivity when the satellite is overpassing moderate and heavy precipitation systems. Therefore it is important to identify profiles that may be contaminated by multiple scattering prior to interpreting the results of any application that involves the use of CloudSat data in raining scenes. On the basis of analysis of multiple-scattering Monte Carlo reflectivity simulations applied to cloud-resolving model-generated microphysical profiles encompassing a large variety of precipitating systems, a relatively straightforward criterion is proposed for flagging profiles potentially affected by multiple scattering. The path-integrated attenuation, that can be estimated from CloudSat's 2B-GEOPROF product, can be used to identify four multiple-scattering regimes: (1) the single scattering approximation is applicable to the entire Z-profile; (2) the single scattering approximation is unreliable but the second order of scattering approximation is valid; (3) the second order of scattering approximation is not valid owing to higher order of multiple-scattering effects which, however, do not affect the surface reference technique-based path-integrated attenuation estimates; and (4) the multiple scattering is affecting the surface return as well, thus spoiling the path-integrated attenuation estimates. Operational path-integrated attenuation thresholds for each of these regimes are then applied to the CloudSat data set over the global oceans, where path-integrated attenuation estimations are more accurate than over land. Case studies and global statistics of the occurrence of multiple scattering are presented: for ocean pixels, around 80% (90%) of the profiles identified as rainy can be treated in the single scattering (second order of scattering) approximation. A threshold value around 20 dB for the one-way path-integrated attenuation is suggested for the applicability of the surface reference technique to the CloudSat Profiling Radar system. This roughly corresponds to 96.5% of the rainy pixels. Owing to the different precipitation regimes, results are strongly regionally and seasonally dependent. For instance in the Inter-Tropical Convergence Zone a not negligible fraction of the raining pixels requires either second-order (10–15%) or higher-order scattering (∼10%) to accurately model the observed reflectivity profile.