Processes controlling water vapor in the upper troposphere/lowermost stratosphere: An analysis of 8 years of monthly measurements by the IAGOS‐CARIBIC observatory

Abstract

AbstractAn extensive set of in situ water vapor (H2O) data obtained by the IAGOS‐CARIBIC passenger aircraft at 10–12 km altitude over 8 years (2005–2013) is analyzed. A multifaceted description of the vertical distribution of H2O from the upper troposphere (UT) via the extratropical tropopause mixing layer (exTL) into the lowermost stratosphere (LMS) is given. Compared to longer‐lived trace gases, H2O is highly variable in the UT and exTL. It undergoes considerable seasonal variation, with maxima in summer and in phase from the UT up to ~4 km above the tropopause. The transport and dehydration pathways of air starting at the Earth's surface and ending at 10–12 km altitude are reconstructed based upon (i) potential temperature (θ), (ii) relative humidity with respect to ice (RHi), and (iii) back trajectories as a function of altitude relative to the tropopause. RHi of an air mass was 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 poleward directed subsidence drying from the tropical tropopause layer with observed RHi down to 2%; (ii) warm conveyor belts and midlatitude 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%.