Modelling of hydrogen and oxygen isotope compositions for local precipitation

Stable isotope compositions of hydrogen and oxygen for continental condensates are determined foremost by equilibrium and kinetic isotope fractionation during evaporation at the oceanic source regions. Subsequently they are modified by a series of in-cloud processes, which include condensation and possible admixture of vapour from evaporation and transpiration over the continents. The effects of vapour admixture from evaporation and transpiration on the isotope compositions of hydrogen (δ2H) and oxygen (δ18O) for local precipitation are discussed in this paper. Using a modified Rayleigh fractionation model, the effects are described for both constant and stepwise evaporation and transpiration fluxes. Further, deuterium (d)-excess values are employed to estimate the evaporation ratio and the slopes of δ–T polynomial regression curves are used to estimate the transpiration ratio. Finally, the influence of sea surface temperature on isotope compositions in condensates is modelled and its effect on d-excess and regression equations is examined.