Incorporation of aerosol into the COSPv2 satellite lidar simulator forclimate model evaluation

Atmospheric aerosol has substantial impacts on climate, airquality and biogeochemical cycles, and its concentrations are highlyvariable in space and time. A key variability to evaluate within models thatsimulate aerosol is the vertical distribution, which influences atmosphericheating profiles and aerosol-cloud interactions, to help constrain aerosolresidence time and to better represent the magnitude of simulated impacts. Toensure a consistent comparison between modeled and observed verticaldistribution of aerosol, we implemented an aerosol lidar simulator withinthe Cloud Feedback Model Intercomparison Project (CFMIP) Observation SimulatorPackage version 2 (COSPv2). We assessed the attenuated total backscattered(ATB) signal and the backscatter ratios (SRs) at 532 nm in the U.S.Department of Energy's Energy Exascale Earth System Model version 1(E3SMv1). The simulator performs the computations at the same verticalresolution as the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP),making use of aerosol optics from the E3SMv1 model as inputs and assumingthat aerosol is uniformly distributed horizontally within each modelgrid box. The simulator applies a cloud masking and an aerosol detectionthreshold to obtain the ATB and SR profiles that would be observed aboveclouds by CALIOP with its aerosol detection capability. Our analysis showsthat the aerosol distribution simulated at a seasonal timescale is generallyin good agreement with observations. Over the Southern Ocean, however, themodel does not produce the SR maximum as observed in the real world.Comparison between clear-sky and all-sky SRs shows little differences,indicating that the cloud screening by potentially incorrect model cloudsdoes not affect the mean aerosol signal averaged over a season. Thisindicates that the differences between observed and simulated SR values aredue not to sampling errors, but to deficiencies in the representation ofaerosol in models. Finally, we highlight the need for future applications of lidar observations at multiple wavelengths to provide insights into aerosol properties and distribution and their representation in Earth system models.