Boundary layer decoupling and water vapor isotopic composition at the ARM Eastern North Atlantic site

Abstract:

The stable isotopic composition of water vapor in the marine boundary
layer (MBL) has typically been interpreted in terms of mixing between
the flux of evaporated water vapor from the ocean surface and dry,
isotopically-depleted air from the cloudy layer or the free
troposphere. In principle, the analysis of water vapor isotopes from
the MBL can provide quantitative constraints on some of the key mixing
processes that govern the humidity budget of the MBL, but the suite of
processes involved remains poorly understood. In-situ measurements of
water vapor isotopic composition were collected at the ARM Eastern
North Atlantic site on Graciosa Island, Azores, between March 2018 and
March 2019. These measurements are merged with metrics of boundary
layer decoupling derived from soundings. The results show that
boundary layer decoupling exerts a first-order control on the joint
distribution of mixing ratio and water vapor isotopic composition:
when the MBL is strongly decoupled, the isotopic composition of the
water vapor at ENA is very close to the estimated flux from the ocean
surface, with little drying or isotopic depletion. In contrast, when
the MBL is only weakly decoupled, the joint distribution reflects a
high degree of mixing between the oceanic flux and a dry,
isotopically-depleted water vapor source from the cloudy layer or the
lower free troposphere. These results provide a new framework for
using MBL isotopic measurements to constrain the relationships between
boundary layer decoupling, mixing, and the humidity budget of the MBL.