Rain-snow discrimination with a biaxial lidar ceilometer

The Vaisala ceilometer LD-40 Tropopauser is a compact eye-safe biaxial lidar measuring continuously under all possible climatic conditions and scanning the atmosphere up to a height of 13000 m. It uses laser diodes with 855 nm wavelength that are pulsed at an average frequency of 4000 Hz. The distance of the systems range bins is 7.5 m. Its main task is reporting cloud base heights and vertical visibility for aviation safety purposes. The focus of this appear however is directed on the investigation of backscatter data from falling precipitation. The LD-40 ceilometer is able to detect falling precipitation and it can distinguish between rain and snow by examining backscatter data profiles. This ability is caused by the biaxial lidar optics in combination with the different scattering phase functions of rain droplets and snow. A simple rain-snow discrimination algorithm is introduced, routine at the same place, illustrates the backscatter profile differences between the two optical systems.