Abstract
During the intensive International Cirrus Experiment conducted over the North Sea during fall 1989, natural cirrus and contrail-induced cirrus were analyzed from in situ and remote sensing measurements (lidar and infrared radiometer). These two cloud types primarily formed at the same range of altitude (8200 m, −37°C). Analysis of the measurements depicts distinctive microphysical and optical properties in the two types of cirrus. Natural cirrus exhibits sheared fallstreaks of ice crystals up to 750 µm in size near the base level. From the top to the base of this cloud the mean values of ice water content and particle concentration increase from 15 to 50 mg m−3 and from 26 to 60 L−1, respectively. The corresponding visible optical depth is around 2.0. Greatest particle concentration and smallest ice crystals are measured at all levels in contrails leading to an optical depth of 0.8 in the denser cloud despite an ice water content that never exceeds 18 mg m−3. These results are consistent with remote measurements from which the backscattering to extinction ratio k is deduced. The largest values of k (0.047 sr −1) are found in a young-life contrail and can be theoretically explained by a spherical shape of small ice crystals. Nonspherical ice particles with larger mean diameter are found in natural cirrus and lead to lower values of k (around 0.02 sr−1).
