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A Comparison of Cirrus Cloud Visible Optical Depth Derived from Lidar

Lo, Chaomei Pacific Northwest National Laboratory
Comstock, Jennifer Pacific Northwest National Laboratory
Flynn, Connor Pacific Northwest National Laboratory

Category: Cloud Properties

Optically thin clouds (e.g. optical depth < 3) can have a significant impact on radiative heating in the atmosphere, particularly in the cold upper troposphere. Currently, there is no value-added product (VAP) in the Atmospheric Radiation Measurement (ARM) program archive that produces thin cloud optical depth, particularly at the Tropical Western Pacific and North Slope of Alaska sites. A VAP is under development to obtain the cirrus cloud visible optical depth from the MPLNOR (Micro Pulse Lidar Normalized Backscatter) data and radiosonde thermodynamic profiles. The optical depth produced by this VAP can be used to evaluate optically thin cloud occurrence and to compare with satellite observations and model simulations. Since cloud and aerosol free conditions above and below the cloud are required for accurate lidar optical depth retrievals, in this new cloud optical depth VAP, we utilize the cloud boundaries derived from the MPLNOR VAP, but carefully examine them for aerosol contamination or virga below the cloud and reliable molecular signal return above the cloud. If these criteria are not met, but the number of cloud bins is greater than 1, we use 5 points below and above the MPLNOR cloud base and top, respectively, to retrieve the optical depth, and flag the data based on failed quality checks. We compare cloud optical depth obtained from the Beer’s Law approach and from an approach that estimates optical depth using a variable backscatter-to-extinction ratio for each lidar profile, which accounts for changes in cloud microphysical properties with time. These optical depth values derived using the MPL are compared with values from a Raman Lidar VAP (rlprofdep) at the Southern Great Plains site (SGP). The SGP site C1 data are currently used for the tests, but the VAP can be extended to run on other sites/facilities in the future.

This poster will be displayed at the ARM Science Team Meeting.