Modification of the High Spectral Resolution Lidar for the measurement of multiply scattered lidar returns

The High Spectral Resolution Lidar (HSRL) provides robust profiles of the scattering cross section in clouds. In addition, a receiver channel with a computer controlled angular field of view provides measurements of multiple scattering. Using the measured scattering cross section profile and a computer model describing the dependence of the multiply scattered lidar return on the width of the diffraction peak, the multiply scattered signal can provide particle size information. Unfortunately, the multiply scattered lidar return is also a function of the weighted average of the scattering phase function near 180°. The weighting function is the probability distribution of scattering angles for the near backscatter event that sends the, multiply scattered photon back towards the receiver. Since the particle size distribution is unknown, it is not easy to estimate this value. This is especially true when the cloud is comprised of ice crystals and both particle size and shape are unknown. To avoid this problem, the authors have implemented an additional data channel. Photons which fall outside of the field stop are directed through an I₂ absorption filter and then to PMT 4. Only photons which have been deflected out of the field of view by multiple scattering are detected, The I₂ filter transmits only the spectral wings of the Doppler broadened molecular backscattering. Photons backscattered from cloud particles are removed. Thus, this channel detects photons which have encountered one or more forward scatterings by cloud particles coupled with a single backscatter described by the Rayleigh phase function