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Visible Optical Properties

Separation of Equation 21 into Equations 23 and 24 allows solution of various atmospheric properties. Given a well mixed atmosphere, one can deduce the molecular backscatter cross-section per unit volume, tex2html_wrap_inline3021, (Piironen, 1994)


at the laser wavelength of 532 nm. A local radiosonde yields the vertical temperature and pressure profiles. Knowledge of the molecular backscatter cross-section per unit volume as a function of range thus provides a calibration target for the lidar.

Substitution of tex2html_wrap_inline3023 into Equation 23 for two atmospheric levels, tex2html_wrap_inline3025 and tex2html_wrap_inline3027, yields the optical depth of that layer at the lidar wavelength,


The scattering ratio, aerosol to molecular signal, is defined as


Using this definition and taking the ratio of Equation 24 to Equation 23, the aerosol backscatter cross-section can be written as


Figure 7 illustrates the HSRL measured aerosol and molecular backscattered return as a function of altitude (lower plot). The column integrated visible optical depth (upper plot) is due to aerosol attenuation of the molecular signal, determined from Equation 26. Aerosol backscatter, represented by the dashed curve, increases with cloudcover (5.5 to 7.5 km) and haze (3.5 to 5 km).

Figure 7: Upper plot illustrates HSRL measured column integrated visible optical depth relative to inverted aerosol (dashed line) and molecular (solid line) backscatter returns shown in the lower plot.

Backscatter depolarization is monitored to discriminate between spherical and non-spherical particles. Spherical particles (e.g., liquid, water-vapor laden solids) backscatter photons with a small change in the polarization. However, non-spherical particles (e.g., ice crystals, dust) backscatter light with a large change in polarization. Range resolved depolarization provides analysis of cloud phase and discriminates between spherical and non-spherical aerosols.

Daniel DeSlover
Sun Aug 11 10:02:40 CDT 1996