Processes Controlling Water Vapor in the UT/LS: Eight Years of Monthly Measurements by the IAGOS-CARIBIC Observatory

Abstract:

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%.