Ultrathin Subvisible Cirrus Clouds at the Tropical Tropopause

Subvisible cirrus clouds with a vertical thickness of only 100-300 m but horizontal extent of thousands of square kilometers have been detected at the tropical tropopause around 17 km altitude during the European-Union-funded APE-THESEO campaign. The cloud layers have been characterized by measurements on board of two aircraft: the Russian high-flying research aircraft Geophysica, which performed in situ measurements of the cloud layers; and a German Falcon research aircraft flying up to 13 km altitude and directing the Geophysica into these clouds, which remained invisible for the Geophysica pilot even during level flight within the layer. Both in situ and remote measurements suggest that the condensed phase volume ranges between 1 and 5 mu m(3)cm(-3). If the particles consisted of water ice, this would correspond to 10-40 ppbv condensed water. Concerning the condensed mixing ratio this would correspond to the thinnest ice cloud ever observed. As a matter of fact, to condense only 10-40 ppbv of H2O in equilibrium requires temperatures to remain in an interval of 20-70 mK below the frost point. As the cloud layers have an extensive horizontal size this requirement can hardly be satisfied, leading to the conclusion that the clouds are not composed of water ice.