MICROPHYSICAL AND OPTICAL-PROPERTIES OF CIRRUS AND CONTRAILS - CLOUD FIELD-STUDY ON 13 OCTOBER 1989

During the intensive International Cirrus Experiment conducted over th e North Sea during fall 1989, natural cirrus and contrail-induced cirr us were analyzed from in situ and remote sensing measurements (lidar a nd infrared radiometer). These two cloud types primarily formed at the same range of altitude (8200 m, -37 degrees C). Analysis of the measu rements depicts distinctive microphysical and optical properties in th e two types of cirrus. Natural cirrus exhibits sheared fallstreaks of ice crystals up to 750 mu m in size near the base level. From the top to the base of this cloud the mean values of ice water content and par ticle 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 me asured at all levels in contrails leading to an optical depth of 0.8 i n the denser cloud despite an ice water content that never exceeds 18 mg m(-3). These results are consistent with remote measurements from w hich 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 nat ural cirrus and lead to lower values of k (around 0.02 sr(-1)).