A Novel Methodology for Retrieving CCN Concentrations from CALIPSO Measurements.

Aerosols act as cloud condensation nuclei (CCN) and affect the cloud micro and macrophysical properties. The aerosol-cloud interactions (ACI) depend on the physical and chemical properties of the aerosol along with the ambient meteorology. The complex mechanisms involved in the ACI are still not understood, resulting in uncertainties in our climate models. Satellites provide long term global measurement of aerosols that helps in studying aerosol-cloud interactions with constrained meteorology. The frequently used satellite proxies for CCN, e.g., aerosol optical depth and aerosol index are vertically integrated parameters and may not represent the CCN particles that usually lie close to the cloud base. One of the alternatives is to switch to space-borne lidar that provides height resolved measurements of aerosol optical properties. We present a novel methodology to estimate CCN concentrations from the measurements of CALIOP lidar aboard the CALIPSO polar-orbiting satellite. We use the aerosol microphysical properties from the CALIPSO aerosol models and model the extinction coefficient. We then modify the size distributions until the modelled value becomes equal to the actual CALIPSO measured extinction coefficient. The final size distribution is used to compute cloud relevant aerosol number concentrations. We have also performed sensitivity analysis by varying the initial size distribution parameters. In addition, we have compared our results with the ground-based and aircraft in-situ measurements. The results are promising, demonstrating the potential of CALIPSO for measuring global 3D cloud relevant aerosol number concentration.