Incorporation of aerosols into the COSPv2 satellite lidar simulator for climate model evaluation

Abstract

Abstract. Aerosols have a large impact on climate, air quality, and biogeochemical cycles. Their concentrations are highly variable in space and time, and a key variability is in their vertical distribution, because it influences atmospheric heating profiles, aerosols life-time and, as a result, surface concentrations, and because it has an impact on aerosol-cloud interactions. On the side of model-oriented aerosols research, using a lidar aerosol simulator ensures consistent comparisons between the modeled aerosols and the observed aerosols. In the current study, we present the lidar aerosol simulator implemented within the COSPv2 satellite lidar simulator. We estimate the total attenuated backscattered signal (ATB) and the scattering ratios (SR) that would be observed at 532 nm by the lidar CALIOP overflying the atmosphere predicted by the E3SMv1 climate model. The simulator performs the computations at the same vertical resolution as the CALIOP lidar, making use of aerosol optics from the E3SMv1 model as inputs, and assuming that aerosols are uniformly distributed horizontally within each model grid-box. It applies a cloud masking and an aerosol detection threshold, to get the ATB and SR profiles that would be observed above clouds by CALIOP with its actual aerosol detection capability. Our comparison shows that the aerosol distribution simulated at a seasonal timescale is generally in good agreement with observations, with however a discrepancy in the Southern Hemisphere, as the observed SR maximum is not reproduced in simulations there. Comparison between cloud-screened and non cloud-screened computed SRs shows little differences, indicating that the cloud screening by potentially incorrect model clouds does not affect the mean aerosol signal averaged over a season. Consequently, the differences between observed and simulated SR values are not due to sampling errors, and allow to point out some weaknesses in the aerosol representation in models. The use of lidar observations at several wavelengths can give further indication on the nature of the aerosols that need to be improved.