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A polarization (0.694-μm wavelength) lidar dataset for subvisual and thin (bluish-colored) cirrus clouds is drawn from Project FIRE (First ISCCP Regional Experiment) extended time observations. The clouds are characterized by their day–night visual appearance; base, top, and optical midcloud heights and temperatures; measured physical (ΔZ) and estimated optical (τc) cloud thicknesses; integrated linear depolarization ratios; and derived k/2η ratios. A subset of the data supporting 30 NOAA polar-orbiting satellite overpasses is given in tabular form to provide investigators with the means to test cloud retrieval algorithms and establish the limits of cirrus detectability from satellite measurements under various conditions. Climatologically, subvisual–thin cirrus appear to be higher, colder, and more strongly depolarizing than previously reported midlatitude cirrus, although similar k/2η that decrease with height and temperature are found. A class of radiatively distinct subvisual cirrus with k/2η ≲ 0.005 sr−1 and possible day-night differences in k/2η are indicated. Optical midcloud heights tend to occur somewhat below the actual cloud center in this sample. On average, subvisual to barely visible cirrus display, τc ≲ 0.03, 0.03 ≲ τc ≲ 0.3 are indicated for thin cirrus, and an upper limit of τc ≈ 3.0 is suggested for lidar attenuation-limited opaque cirrostratus (i.e., altostratus).
Sassen et al. (Sun,) studied this question.