Cabannes versus Rayleigh scattering and terrestrial backscatter ratio revisited in LITE in support of CALIPSO

In order to make optimal quantitative use of multi-wavelength spaceborne lidar data in the upcoming satellite lidar mission CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), it is essential that the lidar be well calibrated. In view of the use of a narrow bandwidth receiver filter in CALIPSO, it is necessary to quantitatively distinguish Cabannes scattering from the full bandwidth Rayleigh scattering for correct calibration of the 532 nm channel based on Rayleigh normalization. This also affects the 1064 nm calibration when it is implemented by the 532/1064 calibration ratio approach. Secondly, the calibration obtained by normalizing to a nearly Rayleigh background signal depends on the backscatter ratio R, defined as the ratio of the total backscattering coefficient [aerosol backscattering coefficient, β_a(z), plus molecular backscattering coefficient, β_m(z)] to the molecular backscattering coefficient, β_m(z). How well this backscatter ratio, R, can be estimated in clean air regions will play an important role in lidar measurements of aerosol and cloud by CALIPSO. The LITE (Lidar In-space Technology Experiment) mission, the first lidar designed for atmospheric studies to fly in Earth orbit, provided an important database for doing the analysis and simulations for future lidar systems on free-flying satellite platforms. This paper presents calculations of Rayleigh/Cabannes scattering cross sections that should apply for CALIPSO by incorporating different depolarization factors appropriate for different spectral bandwidths. LITE data are analyzed to recover estimates of R characteristic of clean air regions. Errors in recovering R resulting from uncertainties in the calibration constant and two-way transmission from the calibration reference height are simulated and assessed.