LAUSR

We propose a framework using water vapor isotopes to study mixing processes in the marine boundary layer (MBL) during quiescent conditions, where we expect evaporation to contribute to the moisture budget. This framework complements the existing models, by taking into account the changing isotopic composition of the evaporation flux (δe), both directly in response to the mixing and indirectly in response to mixing and surface conditions through variations in MBL humidity. The robustness of the model is demonstrated using measurements from the North Atlantic Ocean. This shows the importance of considering the δe variability simultaneous to the mixing of the lower free troposphere to the MBL, to simulate the MBL water vapor, whereas a mixing model using a constant δe fails to reproduce the data. The sensitivity of isotope observations to evaporation and shallow mixing further demonstrates how these observations can constrain uncertainties associated with these key processes for climate feedback predictions.