| How well can a co-located lidar and radar retrieve a drop size distribution in drizzling clouds? To answer, we mimic scattering from a laboratory cloud to retrieve a lidar-radar effective diameter D'eff = (⟨D6⟩/⟨D2⟩) 1/4. Using only the shape parameter of the gamma-distributed drops, the mean diameter of the drops can be estimated from D'eff to within a few percent of the true mean. In practice, the shape parameter of the gamma distribution is not known. To set bounds, mean diameters were calculated from D'eff using a range of in situ measured gamma shape parameters. The estimated means varied within 13% below to 18% above the true mean. To put this range of inherent uncertainty for lidar-radar retrievals in perspective, a decrease of 15-20% in drop size is argued to be sufficient to offset a doubling of carbon dioxide concentrations (e.g., Slingo 1990). |
