IMPROVEMENTS OF CLOUD PARTICLE SIZING WITH A 2D-GREY PROBE

The potential of the 2D-Grey optical array probe (OAP) (with 10-mu m r esolution) to determine cloud microphysical properties is studied; Sys tematic test measurements with a spinning glass disk with sample spots of various sizes between 50 and 500 mu m in diameter were conducted. These measurements show that the particle image diameter increases con siderably if the particle crosses the illuminating laser beam at incre asing distance from the object plane. Eventually, shadow images of the smaller spats lose even their circular image shape and appear fragmen ted. A method is proposed to improve the estimation of the nominal par ticle size of droplets from the recorded image by exploiting the four available shadow (grey) levels. Laboratory tests show that spherical p articles from 50 to 500 mu m in diameter can be properly sized with an rms uncertainty of less than 6%. After discussion of the concept of d epth of field in OAPs, a definition for the 2D-Grey probe is presented that is consistent with the standard definition for the 2D-C probe. T he authors' measurements show the depth of field of the 2D-Grey probe to be three times larger than the value conventionally assumed for the 2D-C probe for which similar corrections have been recently discussed in the literature. Finally, the impact of these findings on particle size distribution for in situ measurements is discussed.