The isotopic composition of precipitation from a winter storm - a case study with the limited-area model COSMOiso

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 (H2O)-O-18 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 delta O-18 observations. COSMOiso very accurately reproduces the statistical distribution of delta O-18 in precipitation, and also the synopticscale 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 delta O-18 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.