We report on the first simultaneous in situ and remote measurements of subvisible cirrus in the uppermost tropical troposphere. The observed cirrus, called UTTCs (ultrathin tropical tropopause clouds), are the geometrically (200300 m) and optically (Ä=10^-4) thinnest large-scale clouds ever sampled (=10^5 km^22). UTTCs consist of only a few ice particles per liter with mean radius =5 micromm, containing only 15 % of the total water. Yet, brief adiabatic cooling events only 12 K below mean ambient temperature destabilize UTTCs, leading to large sedimenting particles (r = 5 microm). Due to their extreme altitude above 17 km and low particle number density, UTTCs may efficiently dehydrate air during its last encounter with the ice phase before entering the stratosphere.
Dehydration potential of ultrathin clouds at the tropical tropopause
G Toci;P Mazzinghi;F Cairo;
2003
Abstract
We report on the first simultaneous in situ and remote measurements of subvisible cirrus in the uppermost tropical troposphere. The observed cirrus, called UTTCs (ultrathin tropical tropopause clouds), are the geometrically (200300 m) and optically (Ä=10^-4) thinnest large-scale clouds ever sampled (=10^5 km^22). UTTCs consist of only a few ice particles per liter with mean radius =5 micromm, containing only 15 % of the total water. Yet, brief adiabatic cooling events only 12 K below mean ambient temperature destabilize UTTCs, leading to large sedimenting particles (r = 5 microm). Due to their extreme altitude above 17 km and low particle number density, UTTCs may efficiently dehydrate air during its last encounter with the ice phase before entering the stratosphere.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.