Abstract
Abstract. Stable water isotopes are valuable tracers of the atmospheric water cycle, and potentially provide useful information also on weather-related processes. In order to further explore this potential, the water isotopes H218O and HDO are incorporated into the limited-area model COSMO. In a first case study, the new COSMOiso model is used for simulating a winter storm event in January 1986 over the eastern United States associated with intense frontal precipitation. The modelled isotope ratios in precipitation and water vapour are compared to spatially distributed δ18O observations. COSMOiso very accurately reproduces the statistical distribution of δ18O in precipitation, and also the synoptic-scale spatial pattern and temporal evolution agree well with the measurements. Perpendicular to the front that triggers most of the rainfall during the event, the model simulates a gradient in the isotopic composition of the precipitation, with high δ18O values in the warm air and lower values in the cold sector behind the front. This spatial pattern is created through an interplay of large scale air mass advection, removal of heavy isotopes by precipitation at the front and microphysical interactions between rain drops and water vapour beneath the cloud base. This investigation illustrates the usefulness of high resolution, event-based model simulations for understanding the complex processes that cause synoptic-scale variability of the isotopic composition of atmospheric waters. In future research, this will be particularly beneficial in combination with laser spectrometric isotope observations with high temporal resolution.
Highlights
Stable water isotopes are useful tracers of processes in the global water cycle and are widely applied for, e.g., hydrological and paleo-climatological studies (Gat, 1996)
In order to be able to reasonably simulate isotopic variations in atmospheric waters in comparison with event-based observations, first of all the meteorological conditions simulated by COSMOiso should be realistic
These data are more depleted than the simulated δ18O values in snow. This difference may well be due to the spatially very limited observational coverage. These results show that COSMOiso very accurately reproduces the statistical distribution of δ18O in precipitation during the winter storm in January 1986
Summary
Stable water isotopes are useful tracers of processes in the global water cycle and are widely applied for, e.g., hydrological and paleo-climatological studies (Gat, 1996). Hourly to daily time scales, the isotopic composition of atmospheric waters and precipitation is subject to strong variability (e.g., Rindsberger et al, 1990; Wen et al, 2010) and potentially provides valuable information on moisture sources, water transport and cloud microphysics (Lawrence et al, 1982; Smith, 1992; Gedzelman and Arnold, 1994; Pfahl and Wernli, 2008) This potential has not yet been fully explored, mostly owing to the complexity of the involved dynamical and microphysical processes and the sparsity of isotope observations with high temporal resolution. Yoshimura et al (2010) simulated the isotopic content of precipitation from an atmospheric river event at the US west coast with the model IsoRSM and compared the results from this case study to observations by Coplen et al (2008).
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