Lidar Retrieval of Water Cloud Effective Radius from Single and Multiple Scattering

Shallow marine clouds have an important impact on climate as they act to cool the Earth system. Changes in cloud amount and microphysics can modulate cloud radiative forcing and affect global warming trends. Cloud droplet effective radius is one of the key microphysical properties of water clouds. Droplet size at cloud top influences evaporation-entrainment processes and sedimentation rates, and cloud liquid water path is commonly estimated from cloud droplet size and optical depth. Parameterizations of droplet effective radius are still under study in both cloud- resolving and global climate models. Cloud droplets are formed by water vapor condensing onto aerosol particles and their sizes are closely related to cloud evolution (i.e. condensation processes, ice nucleation processes, entrainment etc.). Effective radius could also be an indicator of the concentration of cloud condensation nuclei in the atmospheric environment, which could help us to understand cloud-atmosphere interactions and entrainment. Techniques for using lidar measurements to study water cloud effective radius have been developed for many years, in particular with the observations from the space-borne lidar CALIPSO. In this paper, we introduce methods that use both lidar backscatter and depolarization profiles together with layer-integrated measurements to estimate water cloud effective radius. As multiple scattering makes significant contributions to the signals from water clouds measured by space-borne lidar, retrievals of effective radius are investigated using Monte Carlo simulations and observations from both A-Train sensors and airborne instruments.