Optical design of low-cost polarimetric back-scatter sondes

The optical design of a simple low-cost polarimetric balloon-borne backscatter sonde for detecting cloud thermodynamic phase is described. Simulations are performed to determine the best choice of polarization as well as the layout of the detectors with respect to the emitter. The variety of scattering angles of the detected photons, due to finite size detectors, has at least as great an effect as multiple scattering in determining the ratio of cross-polarized to co-polarized signal. The problem with aligned flat-plate ice crystals giving similar depolarization to spherical scatterers, noted in the context of cloud lidar, also occurs here even with a cloud of randomly oriented large plates. Nevertheless, we find that configurations of the instrument that can distinguish liquid droplets from a wide variety of ice crystal habits, including plates, are possible.