Microphysical proporties of subvisible cirrus

In situ microphysical observations of subvisible cirrus between 8 degrees north latitude and the equator were collected using the NASA WB-57F research aircraft during the Costa Rica - Aura Validation Experiment (CR-AVE) in January 2006. The observations were taken near -85 C (17 to 18 km) and included ice particle size, shape, concentration, ice water content and relative humidity. Instrumentation included a cloud particle imager (CPI), a 2D-S (stereo) probe, a forward and backward scattering probe (CAS) and a tunable diode laser (TDL) water vapor probe. These are the first in situ observations of subvisible cirrus that include a large dataset of digital images of the size and shape of tropical ice particles at -85 C. The images reveal that the large majority of the ice particles are quasi-spherical in shape, with only a small percentage of the crystals exhibiting crystalline shapes (e.g., plates and columns). The effective radius of the ice particles is on the order of 20 microns, but particles up to 100 microns were observed. The ice particles are almost always observed when the ambient relative humidity is between 120% and 160% w.r.t. ice. The paper discusses possible explanations of the shape of these ice particles and begs the question of how such large particles form and dwell at these high altitudes. These unique measurements are useful in validation of satellite retrievals and in parameterizations of radiation models.