DUAL-WAVELENGTH LIDAR OBSERVATION OF TROPICAL HIGH-ALTITUDE CIRRUS CLOUDS DURING THE ALBATROSS 1996 CAMPAIGN

Dual wavelength aerosol lidar observations of tropical high-altitude c irrus clouds were performed during the ALBATROSS 1996 campaign aboard the research vessel ''POLARSTERN'' on the Atlantic ocean in October-No vember 1996. On the basis of 57 hours of night-time observations betwe en 23.5 degrees N and 23.5 degrees S we find in 72% of the altitude pr ofiles indications of the presence of cirrus cloud layers. This percen tage drops to 32% at subtropical latitudes (23.5 degrees-30 degrees) b ased on 15 hours of data. About one-half of the subtropical and tropic al cirrus layers are subvisual with an optical depth of less than 0.03 at a wavelength of 532 nm. In general the clouds exhibit high spatial and temporal variability on scales of a few tens of meters vertically and a few hundred meters horizontally. No clouds are observed above t he tropopause An abrupt change in the relation between the color ratio s of the parallel and perpendicular backscatter coefficients at about 240 K is interpreted in terms of changes of particle shape and/or size distribution. At temperatures between 195 and 255 K only a small frac tion of the observations are consistent with the presence of small par ticles with dimensions of less than 0.1 mu m.