A Model of Atmospheric Vapor Isotopes at Their Source: the Marine Boundary Layer

Abstract:

The stable isotopes of water vapor and precipitation are widely used for studying modern and past climates, but the framework for interpreting isotope variations remains incomplete. The most significant gap is a full description of vapor isotopes and transport in the marine boundary layer (MBL) connecting the sea surface and the free troposphere. Increasingly available vapor isotope measurements in the MBL highlight the need to fill this gap.

We introduce the first moderate complexity, vertically resolved MBL model that incorporates several important processes, including 1) entrainment of subsided mid tropospheric air with original mixing ratio r_E, 2) height-dependent vapor diffusivity that is purely molecular at the surface and increases linearly with turbulence to a maximum K_max in the mid MBL, and 3) vertical velocity w_a.. Furthermore, the model does not require specification of either humidity or isotope ratios above the laminar layer, or kinetic fractionation within it. It computes all of these values as well as evaporation rate, isotopic profiles and fluxes, while the isotope flux ratios are the only output from earlier evaporation models.

Analytical solutions are found for the profiles of δD, δ¹⁸O, and d-excess in the MBL. Simulations coincide remarkably well with the region of the δD vs. δ¹⁸O plane populated by global marine observations. Numerical experiments create a family of straight lines in the δD vs. δ¹⁸O plane corresponding to different combinations of conditions. These “vapor lines” are mixing lines between isotopically enriched vapor above the laminar layer and depleted vapor in subsiding air. Their slope and/or extent are most strongly influenced by r_E and K_max, to a lesser extent by sea surface temperature (SST) and the fraction of subsided air in the MBL (α), and only slightly by other parameters. We show that these effects of r_E, K_max and SST on the δD vs. δ¹⁸O relationship result from their combined influence on (1) the thickness of the laminar layer, and (2) the relative amount of vapor from aloft.

The new model strengthens the inference of changes in climate from temporal and spatial variations in MBL water vapor. The model also provides initial conditions for the Rayleigh process that and governs vapor along its trajectories from their origin at the top of the MBL to precipitation sites.