Model studies on the retrieval of aerosol properties beneath cirrus clouds for a spaceborne HSRL

Space based lidar measurements are affected by multiple scattering effects due to the large footprint of the field of view. This effect is particularly strong in the presence of ice clouds. An accurate retrieval of aerosol layers below ice clouds is therefore only possible under a full consideration of multiple scattered light, such as in the lately developed Monte Carlo-based Exact Lidar Retrieval Algorithm (ARLEM) [1]. We have tested ARLEM on its capability to retrieve aerosol properties beneath cirrus clouds. To this end, we simulated spaceborne HSRL measurements with the Monte Carlo-based forward model of ARLEM. Special attention was paid to adding realistic signal and instrument noise. We found that the relative uncertainty in the retrieved aerosol mass concentration from layers beneath cirrus clouds with an optical depth smaller than 0.5 was below 25%. The main source for the retrieval uncertainty arises from an ambiguity in the retrieval of the cirrus properties: An increase in the ice water content and a simultaneous decrease in effective radius of the ice particle leads to similar backscatter signals. The resulting uncertainty in the amplitude of multiple scattering contribution affects the retrieved aerosol mass concentration.