Abstract
Abstract. We develop a one-dimensional (1-D) steady-state isotope marine boundary layer (MBL) model that includes meteorologically important features missing in models of the Craig and Gordon type, namely height-dependent diffusion and mixing, lifting to deliver air to the free troposphere, and convergence of subsiding air. Kinetic isotopic fractionation results from this height-dependent diffusion that starts as pure molecular diffusion at the air–water interface and increases with height due to turbulent eddies. Convergence causes mixing of dry, isotopically depleted air with ambient air. Model results fill a quadrilateral in δD–δ18O space, of which three boundaries are defined by (1) vapor in equilibrium with various sea surface temperatures (SSTs), (2) mixing of vapor in equilibrium with seawater and vapor in subsiding air, and (3) vapor that has experienced maximum possible kinetic fractionation. Model processes also cause variations in d-excess of MBL vapor. In particular, mixing of relatively high d-excess descending and converging air into the MBL increases d-excess, even without kinetic isotope fractionation. The model is tested by comparison with seven data sets of marine vapor isotopic ratios, with excellent correspondence. About 95 % of observational data fall within the quadrilateral predicted by the model. The distribution of observations also highlights the significant influence of vapor from nearby converging descending air on isotopic variations within the MBL. At least three factors may explain the ∼5 % of observations that fall slightly outside of the predicted regions in δD–δ18O and d-excess–δ18O space: (1) variations in seawater isotopic ratios, (2) variations in isotopic composition of subsiding air, and (3) influence of sea spray.
Highlights
Stable isotopic ratios of water have been widely used to study the hydrologic cycle of the atmosphere
These include (1) surface evaporation and processes in the planetary boundary layer (PBL) through which vapor reaches the overlying free atmosphere; (2) rainout and other processes along the trajectory of air masses transported to a precipitation site; (3) nucleation, growth, coalescence, and reevaporation of hydrometeors between the moisture source area and the precipitation site; and (4) subsequent processes affecting precipitation as it falls through the air
We first show vertical profiles of isotopic properties of vapor in the marine boundary layer (MBL) for a representative set of parameters, and we present the entire set of results of 2835 calculations
Summary
Stable isotopic ratios of water have been widely used to study the hydrologic cycle of the atmosphere They have proven to be a powerful tool for understanding modern atmospheric processes (e.g., Dansgaard, 1964; Lawrence et al, 2004; Worden et al, 2007; Uemura et al, 2008; Kurita, 2011; Kopec et al, 2016). Sound interpretation of isotopic data requires a thorough understanding of all processes in the hydrological cycle that affect isotopic variations These include (1) surface evaporation and processes in the planetary boundary layer (PBL) through which vapor reaches the overlying free atmosphere; (2) rainout and other processes along the trajectory of air masses transported to a precipitation site; (3) nucleation, growth, coalescence, and reevaporation of hydrometeors between the moisture source area and the precipitation site; and (4) subsequent processes affecting precipitation as it falls through the air. This study focuses on the first of these – surface evaporation and isotopologue concentrations within and fluxes through the PBL – in particular, the marine boundary layer (MBL), where ascending air delivers water vapor to the free atmosphere
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