A method to retrieve mixed-phase cloud vertical structure from airborne lidar

Abstract

A technique was developed to provide cloud phase information using data collected by the NASA Langley airborne High Spectral Resolution Lidar systems with a particular emphasis on mixed-phase cloud conditions, where boundaries and gradients in the distribution of ice and liquid water are critically important for microphysical and radiative processes. The method is based on the established use of depolarization to identify ice particles but incorporates a new method to separate the ice depolarization from the depolarization produced by multiple scattering in dense liquid clouds. Clouds known to be liquid-only based on ambient temperature were used to train an empirical model of the multiple-scattering depolarization that results at different ranges from the lidar. The method classifies lidar observations as liquid-dominant, mixed-phase, and ice-dominant and has an additional categorization for oriented ice. For evaluation of the retrieval, a two-aircraft approach was used with the lidar observing the same clouds that were concurrently being sampled with in situ microphysical probes. Aircraft matchups were able to track the individual cloud elements and capture marked changes in the distribution of liquid and ice across flight segments of typically 20–100 km. Qualitative features relating to localized changes in the cloud-top temperature, cloud morphology, and convective circulations were generally replicated between the lidar phase classification and the in situ microphysical data. Quantitative evaluation of the phase classification was carried out using a subset of 15 cloud scenes that satisfied strict aircraft collocation and microphysical requirements. Using the in situ microphysical data, it was found that ice extinction fractions of 14 % and 76 % most closely matched the upper and lower bounds of the lidar mixed-phase classification.