Cloud chamber laboratory investigations into scattering properties of hollow ice particles

Helen R Smith,Paul J Connolly,Anthony J Baran,Evelyn Hesse,Andrew R.D Smedley,Ann R Webb

doi:10.1016/j.jqsrt.2015.02.015

Abstract

Measurements are presented of the phase function, P11, and asymmetry parameter, g, of five ice clouds created in a laboratory cloud chamber. At −7°C, two clouds were created: one comprised entirely of solid columns, and one comprised entirely of hollow columns. Similarly at −15°C, two clouds were created: one consisting of solid plates and one consisting of hollow plates. At −30°C, only hollow particles could be created within the constraints of the experiment. The resulting cloud at −30°C contained short hollow columns and thick hollow plates. During the course of each experiment, the cloud properties were monitored using a Cloud Particle Imager (CPI). In addition to this, ice crystal replicas were created using formvar resin. By examining the replicas under an optical microscope, two different internal structures were identified. The internal and external facets were measured and used to create geometric particle models with realistic internal structures. Theoretical results were calculated using both Ray Tracing (RT) and Ray Tracing with Diffraction on Facets (RTDF). Experimental and theoretical results are compared to assess the impact of internal structure on P11 and g and the applicability of RT and RTDF for hollow columns.

Highlights

Measurements are presented of the phase function, P11, and asymmetry parameter, g, of five ice clouds created in a laboratory cloud chamber
For the hollow particle based on particles seen at À7 1C, both Ray Tracing (RT) and Ray Tracing with Diffraction on Facets (RTDF) predict an increase in the asymmetry parameter compared to a solid column of the same aspect ratio 1
For the hollow particle model with the stepped internal structure, both RT and RTDF predict a decrease in asymmetry parameter compared with a solid column of the same aspect ratio

Summary

Introduction

Measurements are presented of the phase function, P11, and asymmetry parameter, g, of five ice clouds created in a laboratory cloud chamber. Surface roughness and internal structure have gained recognition as important factors in the scattering properties of ice crystals [42,43,44,45,46,47] These small scale features cannot be accurately determined from cloud probe images and may be overlooked in particle models. Other sources show varied and complex cavities [17,55], the structure of which are difficult to determine using two-dimensional images and may be missed entirely with instruments of limited resolution This results in over-idealized geometries being used for hollow ice crystals in particle models. Further laboratory investigations are pivotal in determining internal structure and its impact upon the single scattering properties, and to test light scattering models

Methods

Results

Discussion

Conclusion