Abstract
Abstract. Stable water isotopes are naturally available tracers of moisture in the atmosphere. Due to isotopic fractionation, they record information about condensation and evaporation processes during the transport of air parcels, and therefore present a valuable means for studying the global water cycle. However, the meteorological processes driving isotopic variations are complex and not very well understood so far, in particular on short (hourly to daily) timescales. This study presents a Lagrangian method for attributing the isotopic composition of air parcels to meteorological processes, which provides new insight into the isotopic history of air parcels. It is based on the temporal evolution of the isotope ratios, the humidity, the temperature, and the location of the air parcels. Here these values are extracted along 7-day backward trajectories started every 6 hours from near the surface in a 30-year regional climate simulation over Europe with the isotope-enabled version of the model of the Consortium for Small-Scale Modelling (COSMOiso). The COSMOiso simulation has a horizontal resolution of 0.25∘ and is driven at the lateral boundaries by a T106 global climate simulation with the isotope-enabled version of the European Centre Hamburg model (ECHAMwiso). Both simulations are validated against measurements from the Global Network of Isotopes in Precipitation (GNIP), which shows that nesting COSMOiso within ECHAMwiso improves the representation of δ2H and deuterium excess in monthly accumulated precipitation. The method considers all isotopic changes that occur inside the COSMOiso model domain, which, on average, correspond to more than half of the mean and variability in both δ2H and deuterium excess at the air parcels' arrival points. Along every trajectory, the variations in the isotope values are quantitatively decomposed into eight process categories (evaporation from the ocean, evapotranspiration from land, mixing with moister air, mixing with drier air, liquid cloud formation, mixed phase cloud formation, ice cloud formation, and no process). The results show that for air parcels arriving over the ocean, evaporation from the ocean is the primary factor controlling δ2H and deuterium excess. Over land, evapotranspiration from land and mixing with moister air are similarly important. Liquid and mixed phase cloud formation contribute to the variability of δ2H and deuterium excess, especially over continental Europe. In summary, the presented method helps to better understand the linkage between the meteorological history of air parcels and their isotopic composition, and may support the interpretation of stable water isotope measurements in future.
Highlights
Stable water isotopes (H126O, HD16O, and H128O) experience fractionation during phase transitions, meaning that they become enriched in one phase and depleted in the other
We have presented a Lagrangian process attribution of isotopic variations in water vapour, which follows air parcel trajectories and assigns all isotopic changes during transport to a meteorological process
We quantified the imprint of the processes on the isotopic composition of nearsurface water vapour in a 30-year climatological simulation using the regional isotope-enabled model COSMOiso with lateral boundary conditions from ECHAMwiso
Summary
Stable water isotopes (H126O, HD16O, and H128O) experience fractionation during phase transitions, meaning that they become enriched in one phase and depleted in the other. In this way they can record information about evaporation and condensation processes during the transport of air parcels. Since the strength of fractionation depends on meteorological conditions (e.g. temperature, relative humidity and wind speed), stable water isotopes have become useful tracers of the global water cycle. Low δ2H or δ18O values in atmospheric water vapour (where the δ notation describes the concentrations of the heavy isotopes relative to Vienna Standard Mean Ocean Water – VSMOW) indicate low temperatures and strong rainout of air parcels
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