Structure and Dynamical Influence of Water Vapor in the Lower Tropical Troposphere |
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| Authors: | Bjorn Stevens Hélène Brogniez Christoph Kiemle Jean-Lionel Lacour Cyril Crevoisier Johannes Kiliani |
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| Institution: | 1.Max Planck Institute for Meteorology,Hamburg,Germany;2.Laboratoire Atmospheres, Milieux, Observations Spatiales,Guyancourt,France;3.Deutsches Zentrum für Luft- und Raumfahrt, Institute of Atmospheric Physics,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,Reykjavík,Iceland;7.Laboratoire de Météorologie Dynamique, CNRS, IPSL,Ecole Polytechnique,Palaiseau Cedex,France |
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| Abstract: | 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 lower-tropospheric 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 important aspects of the vertical structure of water vapor from space using only passive sensors. |
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