Monday, 3 June 2002
Microphysical and optical properties of a wave-cirrus cloud sampled during the INCA experiment
During the first field experiment in the Southern hemisphere within the international INCA project (Interhemispheric differences in cirrus properties from anthropogenic emissions) a wave-cirrus cloud was sampled by the German Falcon aircraft. The aircraft observations were co-located with the vertical profiles of the backscattering properties obtained from the Lidar system based near Punta Arenas (Chile) and operated by the AWI. The cloud microphysical and optical properties were inferred from the PMS FSSP-300 and 2D-C probe and the Polar Nephelometer. The results show that the cloud exhibits a rather large number density of small ice crystals (up to 100 cm-3 with an effective diameter of 11 micrometers). The occurrence of these small ice crystals cannot be due to the shattering of large ice crystals on the probe inlets because no particles were detected by the 2D-C probe. These characteristics lead to an unusual large extinction coefficient (up to 15 km-1) which is confirmed from both the Polar Nephelometer measurements and the Lidar retrievals. Because an asymmetry parameter of about 0.77 was measured by the Polar Nephelometer, the small ice particles are not spherical. Humidity measurements reveal all the available water vapor (supersaturation excess with respect to ice) was fully consumed by the formation of the numerous ice crystals offsetting the growth of these ones. Larger ice crystals were detected (2D-C probe) only on the fringes of the cloud where the water vapor was still available for a further growth. This feature leads to large gradients in the asymmetry factor. The Lidar measurements revealed the decoupled-layer feature of the wave-cirrus sampled cloud with a cloud-base at –38°C and a top near –48°C. Because no water droplets have been detected near the cloud base this suggests the occurrence of homogeneous freezing of solution droplets at this cloud formation level.