Small-Particle Size Determination by Optical Array Probe Oversampling

Abstract

Oversampling of optical array probes (OAPs) is described as a novel technique to increase the quality of small-particle data available in cloud microphysics. The slice rate of a 10-?m-resolution grayscale OAP is increased by a factor f with respect to that which would produce images with an aspect ratio of 1:1 for the given air velocity and probe resolution. The Fresnel diffraction pattern is simulated and the probe response calculated for normal (f = 1) and oversampled (f = 10) cases. The increased number of pixels imaged by the OAP permits the fraction of 25%?50% shadowed pixels to be used to obtain a much improved estimate of the actual droplet diameter for droplets between 7 and 50 ?m in diameter. The process of oversampling is also found to significantly increase the sample area for the smallest particles. Example laboratory particle size distributions are provided. The oversampling factor for an individual probe is limited by the maximum sampling frequency, the airspeed, and the probe resolution.