A statistical analysis for pattern recognition of small cloud particles sampled with a PMS-2DC probe

Abstract

Abstract. Although small particles (size between 25 lm and 200 lm) are frequently observed within ice and water clouds, they are not generally used properly for the calculation of structural, optical and microphysical quantities. Actually neither the exact shape nor the phase (ice or water) of these particles is well de®ned since the existing pattern recognition algorithms are only e cient for larger particle sizes. The present study describes a statistical analysis concerning small hexagonal columns and spherical particles sampled with a PMS-2DC probe, and the corresponding images are classi®ed according to the occurrence probability of various pixels arrangements. This approach was ®rst applied to synthetic data generated with a numerical model, including the e ects of di raction at a short distance, and then validated against actual data sets obtained from in-cloud¯ights during the pre-ICE'89 campaign. Our method allows us to di erentiate small hexagonal columns from spherical particles, thus making possible the characterization of the three dimensional shape (and consequently evaluation of the volume) of the particles, and ®nally to compute e.g., the liquid or the ice water content.

Measurement errors in cirrus cloud microphysical properties

“…Typically the probe covers a particle size range of 25±800 lm with a pixel size of 25 lm. The analysis techniques use speci®c algorithms applied to the image of each particle to identify that particle's shape and size (Heyms®eld and Parrish, 1978;Duroure, 1982;Heyms®eld and Baumgardner, 1985;Duroure et al, 1994;Fouilloux et al, 1997) and to determine particle concentration.…”

“…Their shapes, and hence their optical properties and their mass are not adequately determined as a function of size, and their contribution to the derived cloud parameters cannot be evaluated and may be neglected. Recent work by Fouilloux et al (1997) oers some improvement in shape analysis.…”

“…This limitation aects the con®dence which can be placed in both the description of particle shapes present, and the estimates of size ± integrated quantities such as ice water content. While recent analysis techniques oer some improvement (Duroure et al, 1994;Fouilloux et al, 1997), the accuracy of estimates of derived quantities sensitive to particle shape, such as radiative parameters and mass-integral quantities, remains limited. …”

Measurement errors in cirrus cloud microphysical properties

“…Their shapes, and hence their optical properties and their mass are not adequately determined as a function of size, and their contribution to the derived cloud parameters cannot be evaluated and may be neglected. Recent work by Fouilloux et al (1997) o ers some improvement in shape analysis.…”

“…This limitation a ects the con®dence which can be placed in both the description of particle shapes present, and the estimates of size ± integrated quantities such as ice water content. While recent analysis techniques o er some improvement (Duroure et al, 1994;Fouilloux et al, 1997), the accuracy of estimates of derived quantities sensitive to particle shape, such as radiative parameters and mass-integral quantities, remains limited. …”