Optical and microphysical properties of a cold cirrus cloud - Evidence for regions of small ice particles

Airborne lidar and scanning radiometer observations were made in 1984 on a cold cirrus cloud from an ER-2 aircraft flying at 20-km altitude. At the same time, in situ microphysical observations were made at two altitudes within the cloud. Optical properties calculated from observed particle size spectra were compared with optical properties determined from the ER-2 lidar-radiometer observations. When the two aircraft were approximately coincident in the vertical, cirrus cells with enhanced particle counts were sampled by the in situ aircraft. At an altitude where the temperature was −62.7°C, no particles with sizes greater than 70 μm were sampled by the two-dimensional instrument, and the Forward Scattering Spectrometer Probe (FSSP) instrument sampled sizes only smaller than 10 μm. At the second altitude where the temperature was −47.3°C, larger particles were sampled up to sizes of 600 μm, and 95% of the extinction occurred for particles with dimensions greater than 30 μm. Measurements of lidar depolarization ratio indicated that the clouds were composed predominantly of ice crystals. At the first altitude, the lidar and radiometer analysis gave values of backscatter to extinction ratio k and the visible extinction to infrared absorption ratio α of 0.0324 sr−1 and 2.3, respectively, using a value for multiple-scattering factor η equal to 0.76 calculated from the FSSP spectra. This compared with a value of α equal to 3.77 calculated from cloud microphysics. The value of k did not agree with values obtained in previous ground-based experiments at an equivalent temperature. The discrepancies were attributed to undersizing of particles by the FSSP. At the second altitude, using a value of η equal to 0.4 estimated from the two-dimensional spectra, values of 3.5 and 0.0112 sr−1 were measured for α and k, respectively, compared with a value of α of 2.0 calculated from cloud microphysics and 0.0127 sr−1 for k measured by previous workers. The discrepancy in α was observed in previous experimental data, where a value of 3.5 was also obtained. Direct and remote measurements were in basic agreement for the lower layer. A much less favorable comparison in the upper layer is considered to be due to limitations in the in situ observations for small crystals.