In situ measurements of background aerosol and subvisible cirrus in the tropical tropopause region

In-situ aerosol measurements were performed in the Indian Ocean ITCZ region during the APE-THESEO field campaign based in Mahe’, Seychelles, between February 24 and March 6, 1999. Ultrathin layers of cirrus clouds were penetrated by Geophysica directly beneath the tropical tropopause at 17 km pressure altitude and temperatures below 190 K. The encountered ultra thin subvisible cloud layers can be characterized as horizontally extending over several hundred kilometres, persisting for at least three hours (but most likely longer) and having geometrical thickness of 100 to 400 metres. These clouds layers belongs to the geometrically and optically thinnest clouds ever observed.