Small ice particle observations in tropospheric clouds: fact or artifact?

Understanding the formation and evolution of small ice particles in clouds has been a long-standing problem in cloud physics. Early airborne measurements suggested that in glaciated clouds the number concentration of ice particles is dominated by small particles with sizes less than 100um. Recently, increasing evidence has suggested that it many cases concentrations of small ice particles may be the consequence of larger particle impacts on probe tips and inlets followed by shattering into small fragments. Environment Canada has recently undertaken efforts of modify cloud particle probes' inlets to deflect bouncing particles and shedding water away from the sample volume and optical field apertures thereby mitigating the effect of shattering on measurements. The performance of the modified and standard probe tips was then studied in the Cox wind tunnel using high speed video recording. In the spring of 2009 Environment Canada conducted the Airborne Icing Instrumentation Evaluation (AIIE) flight campaign, which attempted to quantify the effect of shattering on ice measurements and improve our understanding of the problem of small ice particles in clouds. The evaluation of this shattering effect was focused on the CIP, FSSP and OAP-2DC probes installed on the National Research Council of Canada Convair-580. The results of the AIIE project demonstrate that the contamination of particle size distributions by shattering is a significant problem for the airborne microphysical characterization of ice clouds. Shattering may contaminate the ice crystal spectrum up to sizes of ~500 um, particularly when large ice particles are present, resulting in overestimation of the total number concentration of particles of up to 100 times or more. It is shown that the existing data processing algorithms cannot effectively filter out all shattering events on CIP and OAP-2DC. It is also evident that the modified probe tips still shatter ice particles. However, the combination of modified probe tips and carefully designed data processing algorithms are shown to effectively reduce the number of shattered particles. Despite that the modified tips and the processing algorithms enables significant reduction of the shattering events, the accurate measurement of small ice crystal concentrations remains unresolved.