Constraining influences on marine boundary layer humidity in the tropical Atlantic using shipborne stable water isotopic measurements

Abstract:

Shallow trade-wind cumulus clouds play a key role in the climate system by influencing radiative balance and the general circulation over large stretches of the subtropical oceans. The EUREC4A field campaign during early 2020 focused on taking comprehensive measurements of the exemplar trade-wind region east of Barbados to better understand the coupled relationships between these clouds and the surrounding ocean and atmosphere. In these marine tropical settings, constraining the exchange of moisture between the ocean surface, marine boundary layer, and free troposphere is key to understanding the humidity profile and low-cloud maintenance. This is particularly important for diagnosing the response of low-cloud feedbacks within global climate models (GCMs). However, this exchange is both difficult to measure and computationally expensive to model. The sensitivity of water vapor isotopic composition to mixing and phase change can be leveraged to yield information on the vertical sourcing and exchange of moisture.

Here water vapor stable isotopologue measurements from the Research Vessel Meteor are presented as part of the EUREC4A-iso effort. This dataset is compared with shipborne atmospheric and oceanic measurements to constrain the relative balance of oceanic moisture fluxes, boundary layer moist processes, and free tropospheric input on near-surface humidity. Sea surface temperature and meteorological data are used in a Craig-Gordon model framework to estimate isotopic composition of the oceanic flux, while Rayleigh distillation is used to estimate the range of isotopic compositions within the boundary layer and of the free troposphere. Deviations of the water vapor isotopologue measurement time series from the Craig-Gordon model estimates are compared to estimated inversion strength (EIS) determined from collocated, ship-launched radiosonde profiles to assess the balance of boundary layer and free tropospheric influence on near-surface humidity.