Use of the CALIOP Vertical Feature Mask in Evaluating Global Aerosol Models

Abstract:

The space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) provides aerosol-cloud distributions and vertical profiles of aerosol total attenuated backscatter and depolarization ratio. With additional information (e.g., layer altitude, geographic location) the CALIOP vertical feature mask (VFM) has been used to assign aerosol types for observed aerosol features. This in turn has been used to assign lidar ratios for conversion of the assigned backscatter profiles to extinction profiles, most useful for verification of aerosol models. Here we show how the VFM also serves as a useful tool in and of itself for evaluating our recent aerosol reanalysis, called MERRAero. MERRAero is based on the Goddard Earth Observing System (GEOS-5) Earth system model, driven by meteorology from the Modern-Era Retrospective Analysis for Research and Applications (MERRA), and incorporates an aerosol module and assimilation of aerosol optical thickness (AOT) from observations made by the two Moderate Resolution Imaging Spectroradiometer (MODIS) instruments. In this study, we construct two VFMs using MERRAero aerosol distributions for comparison to CALIOP: a “direct” one where we emulate the CALIOP VFM by applying its algorithm directly to our simulated total attenuated backscatter and depolarization ratio profiles, and an “explicit” one where we map the simulated, compositionally resolved extinction profiles to the aerosol types in the CALIOP VFM. The “directly” calculated VFM hones in on errors in our assumed aerosol optical properties, while our “explicitly” calculated VFM helps uncover transport and composition errors in MERRAero. Additionally, by comparing our MERRAero VFMs to one another, we can identify shortcomings in the CALIOP VFM algorithm in instances where the aerosol types differ. We find that MERRAero dust depolarization ratios are biased low and need to be increased by 50% to match those observed by CALIOP. We also suggest that the CALIOP VFM algorithm be modified to require a minimum total attenuated backscatter for polluted dust, as the algorithm tends to flag polluted dust as the dominant aerosol type in regions with low aerosol loadings and a minimal contribution from dust relative to other aerosol types.