Lagrangian simulations of stable isotopes in water vapor: An evaluation of nonequilibrium fractionation in the Craig-Gordon model

The Craig-Gordon model is the basis for the parameterization of water isotope fractionation during evaporation from the ocean in many atmospheric isotope models. Its exact formulation (e.g., with respect to the nonequilibrium fractionation factor k) is mainly based on theoretical considerations and not very well constrained by observations. This study addresses this issue by combining a recently developed Lagrangian moisture source analysis with a Craig-Gordon fractionation parameterization for the identified evaporation events in order to model isotope ratios in water vapor. This technique is applied to 45 measurement days of isotopes in water vapor at Rehovot (Israel) during the years 2001-2006. A comparison of the simulated deuterium excess with the measurements reveals that a much better agreement can be achieved using a wind speed independent formulation of k instead of the classical parameterization introduced by Merlivat and Jouzel, which is widely applied in isotope GCMs. The numerical values of k that lead to the best agreement of simulated and observed deuterium excess (k(H218O) = 0.9925, k(HDO) = 0.9961) can be shown to be consistent with data from other studies. In future research, it should be tested if this empirically derived, wind speed independent parameterization also leads to an improvement of the performance of more complex models, including GCMs, in modeling deuterium excess.