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

Abstract

An extensive set of in situ water vapor (H 2 O) data obtained by the IAGOS-CARIBIC passenger aircraft at 10-12 km altitude over 8 years (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) is analyzed. A multifaceted description of the vertical distribution of H 2 O 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, H 2 O 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 H 2 O 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%.

Long-range transport pathways of tropospheric source gases originating in Asia into the northern lower stratosphere during the Asian monsoon season 2012

“…In CLaMS at this altitude, contributions of cirrus clouds to the total water content are of minor importance. An increase of H 2 O in the northern lower stratosphere is found in our simulation during summer and autumn as reported in previous studies (e.g., Ploeger et al, 2013;Zahn et al, 2014;Müller et al, 2016). The fraction of H 2 O from different boundary tracers is indicated by different colors.…”

“…The composition of the young air masses is dominated by air masses primarily originating in India, China, and southeast Asia, in particular at the end of the monsoon season in September/October 2012. Thus, the Asian monsoon anticyclone and boundary emissions from Asia contribute to the maximum of tropospheric signatures found in the northern extratropical lower stratosphere in boreal summer and autumn (e.g., Hoor et al, 2005;Hegglin and Shepherd, 2007;Bönisch et al, 2009;Zahn et al, 2014;Müller et al, 2016), sometimes also referred to as "flushing" of the lower stratosphere (Hegglin and Shepherd, 2007;Bönisch et al, 2009).…”

Global-scale atmosphere monitoring by in-service aircraft – current achievements and future prospects of the European Research Infrastructure IAGOS

“…In the dry LS, however, both models show a considerable moist bias by 100Á150% in summer/autumn and 50Á100% in winter/spring Â2 km above the tropopause. Zahn et al (2014) studied the transport pathways of water vapour to 9Á12 km altitude and the coldest temperatures the air masses had experienced at the Lagrangian cold point (LCP). The LCP sets (by freeze-drying) the H 2 O mixing ratio that is then conserved until sampling.…”

“…The identification of processes controlling the distribution of water vapour in the UTLS by means of long-term IAGOS measurements (Gierens et al, 1999;Zahn et al, 2014) adds significantly to this subject of research.…”

How stratospheric are deep stratospheric intrusions? LUAMI 2008

“…An open question has been how much of the tropospheric air originates already from the so-called "mixing layer" around the thermal tropopause (e.g. Danielsen, 1968;Lelieveld et al, 1997;Hintsa et al, 1998;Zahn et al, 1999Zahn et al, , 2014Fischer et al, 2000;Hoor et al, 2002Hoor et al, , 2004Pan et al, 2004Pan et al, , 2007Brioude et al, 2006Brioude et al, , 2008Sprung and Zahn, 2010;Vogel et al, 2011) prior to the descent and how much of the admixture occurs during the descent of an intrusion layer into the lower troposphere. In some cases mixing of polluted or convectively lifted air into intrusions within the free troposphere has been reported (e.g.…”