Relationships Between Lidar Aerosol Extinction/Backscatter Coefficients and CCN Number Concentrations During the NASA ORACLES Campaigns

Abstract:

Due to seasonal biomass burning and the presence of a semi-permanent stratocumulus deck off the western coast of Africa, the Southeast Atlantic (SEA) region is particularly important in terms of characterizing aerosol and cloud impacts on radiative forcing and regional climate. Aerosols that activate to become cloud droplets, the cloud condensation nuclei (CCN), are crucial in determining aerosol-cloud interactions due to the aerosol first indirect effect. In this study, we examine the relationship between NASA Langley’s airborne High Spectral Resolution Lidar 2 (HSRL-2) measurements of backscatter (BSC) and extinction (EXT) coefficients with in-situ measured CCN number concentrations collected during the ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) campaign. The analysis is performed for various spatial and temporal collocation and different meteorological conditions. Preliminary results suggest that the relationship is strongest for a short time window and large horizontal distance between BSC/EXT and CCN, and for relative humidity below 40%. Ultimately this information will be useful when developing methods for constraining CCN concentrations with lidar measurements alone for locations where in-situ data are not available, specifically over the SEA. Additionally, aerosol microphysical properties, e.g., size distribution, as retrieved from combined sets of lidar and polarimeter observations, will be used to constrain the CCN concentration estimate based on Köhler theory and aerosol chemical compositions.