Uncertainty of Backscatter Coefficients from In Situ Cloud Probe Measurements in Cirrus Clouds

A major aviation hazard is power loss caused by ice particle accumulation within jet engines. Ice particles from high-altitude cirrus clouds are especially dangerous since pilots are often unable to see the hazard. The commercial aerospace industry is currently investigating several approaches to mitigate the risks posed by high-concentration ice crystal conditions. One such approach uses specially-designed airborne lidar systems to quantify high concentrations of ice crystals, which enables pilots or engine control systems to enact appropriate counter-measures. A key factor in the development of any ice crystal measurement system is the availability of reliable reference measurements of ice crystal size distributions. State-of-the-art cloud probes and advanced processing methods are used to determine backscatter coefficients within ice clouds with concurrent measurements of an airborne lidar system. Research aircraft flights in Florida anvil cirrus clouds on 31 July 2015 and 1 August 2015 provide segments that have different temperatures, habits, and particle size distributions. Wing-mounted probe (the Cloud Droplet Probe (CDP), the Two-Dimensional Stereographic (2D-S) probe, and the High Volume Precipitation Spectrometer Version Three (HVPS3)) measurements are processed to obtain the particle size distributions with a corresponding measurement uncertainty. Additionally, there are typically several crystal habits that are possible given the cloud temperature and low-resolution (10 µm) particle images. An automated algorithm determines the most reasonable habit for the environment using 1 Hz (~100 meter) samples of particle aspect ratio, effective diameter and temperature. Backscattering coefficients are derived from the microphysical measurements by assuming that the backscattering cross-section is equal to the geometric cross-section and using an assumed value for the backscatter coefficient efficiency for each given habit. These derived backscatter coefficients are compared to measured backscatter coefficients from an on-board lidar system known as the Optical Ice Detector (OID) to assess agreement.