Study of the diffraction pattern of cloud particles and the respective responses of optical array probes

Thibault Vaillant De Guélis,Bastien Laurent,Pierre Coutris,Christophe Gourbeyre,Régis Dupuy,Alfons Schwarzenböck,Valery Shcherbakov,Christophe Duroure

doi:10.5194/amt-12-2513-2019

Abstract

Abstract. Optical array probes (OAPs) are classical instrumental means to derive shape, size, and number concentration of cloud and precipitation particles from 2-D images. However, recorded 2-D images are subject to distortion based on the diffraction of light when particles are imaged out of the object plane of the optical device. This phenomenon highly affects retrievals of microphysical properties of cloud particles. Previous studies of this effect mainly focused on spherical droplets. In this study we propose a theoretical method to compute diffraction patterns of all kinds of cloud particle shapes in order to simulate the response recorded by an OAP. To check the validity of this method, a series of experimental measurements have been performed with a 2D-S probe mounted on a test bench. Measurements are performed using spinning glass discs with imprinted non-circular opaque particle shapes.

Highlights

In Earth’s atmosphere, the evolution of clouds is highly dependent on interactions of a number of dynamical, radiative, and microphysical processes (Boucher et al, 2013)
A striking result is that measurements obtained with the 2D-S probe are in really good agreement with the theoretical diffraction simulation results, in terms of diffraction pattern, and with respect to the depth of field (DoF) limit which is illustrated by the disappearing image at approximately the same Z value in theory and the corresponding measurement
We presented in this study a first comparison of theoretical diffraction simulations of non-spherical cloud particles and respective image responses of Optical array probes (OAPs) probes

Summary

Introduction

In Earth’s atmosphere, the evolution of clouds is highly dependent on interactions of a number of dynamical, radiative, and microphysical processes (Boucher et al, 2013). Therein, optical array probes (OAPs; Knollenberg, 1970) are classical instrumental means to measure the shape, concentration, and number size distribution of cloud and precipitation particles. OAP probes are based on the principle of a linear array of photodetectors illuminated by a laser to image cloud particles crossing the laser beam. These probes are subject to several uncertainties. The 2-D image is subject to distortion due to the diffraction effect of light when cloud particles are imaged out of the object plane of the optical device (e.g., Thompson, 1964) The latter phenomenon highly affects smaller cloud particles up to several hundred micrometers in particle diameter. Common OAP image processing algorithms include items such as the reconstruction of truncated images (Korolev and Sussman, 2000), elimination of noisy pixels (Lawson, 2011) and splashing

Methods

Results

Conclusion