Constraining cirrus ice crystal size through observation of halos

During the Mid-Latitude Cirrus Experiment (MidCiX), the NASA WB-57F aircraft sampled cirrus at high altitudes over the central United States during the Spring of 2004. Comparisons between measurements of integrated size distributions and bulk probe total water and optical extinction showed an agreement not seen in prior field programs (e.g. CRYSTAL-FACE), and implied that about half of ice crystal mass, and 90% of ice crystal extinction could be attributed to small ice crystals < 50 μm across. However, observations of such high concentrations of small crystals remain contentious as it has been suggested that they are the result of shattering of large crystals on the inlets of cloud probes. An independent constraint of these measurements may come from observations of cirrus optical effects. During in-cloud flight near cirrus cloud top, the WB-57F backseater noted repeated occurrences of 46 and 22 degree halos. During the halo sightings, a bimodal distribution was observed with area-equivalent mode radii located at 12 μm and >100μm . From theoretical arguments, diffraction prevents halos from forming in very small ice crystals (<7μm radius) , whereas increasing aspect ratio (length/width) reduces the probability of refraction through a basal face for very large crystals (>100μm radius). Therefore, the observations suggest that ice crystals in the smaller mode were responsible for both halos and cirrus extinction implying that cloud probe measurements of small ice crystals were real.