Processes Controlling Water Vapor in the UT/LS: Eight Years of Monthly Measurements by the IAGOS-CARIBIC Observatory
Thursday, 18 December 2014: 11:35 AM
An extensive set of in situ water vapor (H2O) measurements obtained by the CARIBIC passenger aircraft at cruise altitude from 2005 to 2013 is presented. A description of the vertical distribution of H2O from the upper troposphere (UT) via the extra-tropical tropopause mixing layer (exTL) into the lowermost stratosphere (LMS) is given. H2O was found to undergo considerable seasonal variation, with maxima in summer and remaining in phase from the UT up to about 4 km above the tropopause. The transport and dehydration pathway of air starting at the Earth’s surface and ending at 10-12 km altitude is reconstructed based upon (i) potential temperature, (ii) relative humidity w.r.t. ice (RHi), and (iii) back trajectories as a function of altitude relative to the tropopause. RHi of an air mass is found to be primarily determined by its temperature change during recent vertical movement, i.e. cooling during ascent/expansion and warming during descent/compression. The data show with great clarity that H2O and RHi at 10-12 km altitude are controlled by three dominant transport/dehydration pathways: (i) the Hadley circulation, i.e. convective uplift in the tropics and pole-ward directed subsidence drying from the tropical tropopause layer (TTL) with observed RHi down to 2%, (ii) warm conveyor belts and mid-latitude convection transporting moist air into the UT with observed RHi usually above 60%, and (iii) the Brewer-Dobson shallow and deep branches with observed RHi down to 1%.