Water vapor Convection Atmospheric circulation Ice initiation Remote sensing Atmospheric measurements Clouds 1

In situ, airborne and satellite measurements are used to characterize the structure of water vapor in the lower tropical troposphere—below the height, z ; of the triple-point isotherm, T : The measurements are evaluated in light of understanding of how lowertropospheric water vapor influences clouds, convection and circulation, through both radiative and thermodynamic effects. Lower-tropospheric water vapor, which concentrates in the first few kilometers above the boundary layer, controls the radiative cooling profile of the boundary layer and lower troposphere. Elevated moist layers originating from a preferred level of convective detrainment induce a profile of radiative cooling that drives circulations which reinforce such features. A theory for this preferred level of cumulus termination is advanced, whereby the difference between T and the temperature at which primary ice forms gives a ‘first-mover advantage’ to glaciating cumulus convection, thereby concentrating the regions of the deepest convection and leading to more clouds and moisture near the triple point. A preferred level of convective detrainment near T implies relative humidity reversals below z which are difficult to identify using retrievals from satellite-borne microwave and infrared sounders. Isotopologues retrievals provide a hint of such features and their ability to constrain the structure of the vertical humidity profile merits further study. Nonetheless, it will likely remain challenging to resolve dynamically & Bjorn Stevens bjorn.stevens@mpimet.mpg.de 1 Max Planck Institute for Meteorology, 20146 Hamburg, Germany 2 Laboratoire Atmospheres, Milieux, Observations Spatiales, 78280 Guyancourt, France 3 Deutsches Zentrum für Luftund Raumfahrt, Institute of Atmospheric Physics, 82234 Oberpfaffenhofen, Germany 4 UPMC Univ. Paris 06, Paris, France 5 LATMOS-IPSL, Universit Versailles St-Quentin, Paris, France 6 Institute of Earth Sciences, University of Iceland, Reykjavı́k, Iceland 7 Laboratoire de Météorologie Dynamique, CNRS, IPSL, Ecole Polytechnique, 91128 Palaiseau Cedex, France 123 Surv Geophys DOI 10.1007/s10712-017-9420-8