Identifying key stages of radiation fog evolution using water vapor isotopes

Abstract

Fog is tied to surface energy and water budgets. However, our knowledge about the processes leading to fog evolution is still fragmentary, and their adequate representation in numerical-weather-prediction and climate models remains challenging. Water vapor isotopes are widely used to investigate Earth's water cycle dynamics and can provide process-based insights into fog evolution. Although isotopes of water vapor and droplets during radiation fog have been reported, the intra-event isotope dynamics and the mechanisms controlling water vapor isotope variability during radiation fog evolution have hardly been investigated. We use water vapor isotopes (δ2Hv, dv) combined with meteorological and eddy-covariance measurements, plus vertical profiles from a high-resolution numerical-weather-prediction model (COSMO-1), to study the processes that influence radiation fog evolution. Variability in surface-humidity and water vapor isotopes is tied to the different stages (shallow, transitional, and deep fog) of radiation fog evolution and affected by condensation, turbulent entrainment, fog droplet deposition at the surface, and evaporation processes. Strong water vapor isotopic fluctuations during radiation fog are linked to fog lifting to stratus-clouds during nighttime that lead to fog dissipation at ground level. Our results reveal highly correlated atmospheric-specific-humidity (qa) and water vapor isotopes during dew formation which is typical before the onset of radiation fog and in combination with fog; whilst during fog periods, δ2Hv and dv show larger temporal variability than qa due to their sensitivity to fog dynamics. Particularly, during fog growth, entrainment of air from the fog-top caused increasing dv, which was otherwise constant during condensation. Furthermore, δ2Hv variability during the transition into deep fog likely indicated the alternation of condensation-induced-decrease and entrainment-induced-increase of δ2Hv. Compared to the temporal isotope variability (with a magnitude of 34.6‰ for δ2Hv) associated with large-scale cold-frontal-clouds, radiation fog evolution is found to be associated with isotope variability of a similar amplitude (24.6‰ for δ2Hv).