Interferometric laser imaging for in-flight cloud droplet sizing

Abstract

A non-intrusive particle sizing method with a high spatial distribution is utilized for the estimation of cloud droplet spectra during flight test campaigns. The Interferometric Laser Imaging for Droplet Sizing (ILIDS) method derives particle diameters of transparent spheres by evaluating the out-of-focus image patterns. This sizing approach requires a polarized monochromatic light source, a camera including objective lens with a slit aperture, a synchronization unit and a processing tool for data evaluation. These components are adapted to a flight test environment to enable the microphysical investigation of different cloud genera. The herewith presented work addresses the ILIDS system design and specifications, the flight test preparation and selected results obtained in the lower and middle troposphere. The research platform was a commuter aircraft of type Dornier Do228-101 of DLR Flight Operation Center in Braunschweig. It was equipped with the required instrumentation including a high energy laser as light source. A comprehensive data set of around 71.800 ILIDS images was acquired over the course 5 flights. The data evaluation of the characteristic ILIDS fringe patterns relies among others on a relationship between the fringe spacing and the diameter of the particle. The simplest way to extract this information from a pattern is by fringe counting, which is not viable for such an extensive amount of data. A brief contrasting comparison of evaluating methods based on frequency analysis by means of Fast Fourier Transform (FFT) and on correlation methods such as Minimum Quadratic Difference (MQD) is used to encompass the limits and accuracy of the ILIDS method for such applications.