Abstract
This paper describes approaches to retrieve important aerosol results from the strong lidar return signals that are received by the space-borne CALIPSO lidar system after reflecting off-ocean surfaces. Relations, from which the theoretically expected values of area under ocean surface returns can be computed, are presented. A detailed description of the lidar system response to the ocean surface returns and the processes of sampling and averaging of lidar return signals are provided. An effective technique that reconstructs the lidar response to surface returns—starting from down-linked samples—and calculates the area under it, has been developed and described. The calculated area values are validated after comparing them to their theoretically predicted counterpart values. Methods to retrieve aerosol optical depths (AODs) from these calculated areas are described and retrieval results are presented, including retrieval comparison with independent AOD measurements made by an airborne High Spectral Resolution Lidar (HSRL) that yielded quite good agreement. Techniques and results are also presented on using the spectral ratios of the surface response areas to determine spectral ratios of aerosol round-trip transmission and AOD spectral difference, without need of a specific/accurate ocean-surface reflectance model.
Highlights
LIght Detection And Ranging is an optical remote sensing technology which is very often used in the study of atmospheric aerosols
A technique has been presented and applied to numerous Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)/Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations for estimating from signal samples the normalized area, ALPF, under the received surface response from CALIOP-transmitted lidar pulses reflected from ocean surfaces
Groupings of ALPF defined by binning the areas in small increments of total integrated attenuated backscatter (TIAB) and wind speed yielded tightly clustered groups with similar statistics for both wavelength channels (532 and 1064 nm) and all three ocean regions that were investigated
Summary
LIght Detection And Ranging (lidar) is an optical remote sensing technology which is very often used in the study of atmospheric aerosols. Similar in concept to radar, lidar emits highly coherent, ultra-short laser pulses at wavelengths which are comparable to the dimensions of atmospheric aerosol particles. The expected surface return area, ALPF, after being normalized by the calibration constant, peak energy and range squared (just as the lidar equation, Equation (1), is normalized for the CALIPSO level 1 data product) is given by ALPF =. C must be evaluated in units of km/μsec (c = 0.3 km/μsec) in Equation (5) to yield ALPF values consistent with the CALIPSO data product sample values of the surface response signals, and the ALPF inferred from the samples, per/characterized by the normalized lidar equation
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