Review Article A Review of Water Isotopes in Atmospheric General Circulation Models: Recent Advances and Future Prospects (original) (raw)
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Stable water isotopes in the coupled atmosphere–land surface model ECHAM5-JSBACH
Geoscientific Model Development, 2013
In this study we present first results of a new model development, ECHAM5-JSBACHwiso, where we have incorporated the stable water isotopes H 2 18 O and HDO as tracers in the hydrological cycle of the coupled atmosphere-land surface model ECHAM5-JSBACH. The ECHAM5-JSBACH-wiso model was run under present-day climate con-5 ditions at two different resolutions (T31L19, T63L31). A comparison between ECHAM5-JSBACH-wiso and ECHAM5-wiso shows that the coupling has a strong impact on the simulated temperature and soil wetness. Caused by these changes of temperature and the hydrological cycle, the δ 18 O in precipitation also shows variations from −4.5 ‰ up to 4.5 ‰. One of the clearest anomalies is shown over North-East Asia where, depend-10 ing on an increase of temperature, the δ 18 O in precipitation increases as well. In order to analyze the sensitivity of the fractionation processes over land, we compare a set of simulations with various implementations of water isotope fractionation processes over the land surface. The simulations allow us to distinguish between no fractionation, fractionation included in the evaporation flux (from bare soil) and also fractionation included 15 in both evaporation and transpiration (from water transport through plants) fluxes. The simulated δ 18 O and δD in precipitation of these setups generally fit well with the observations and the best agreement between observation and simulation is given in the case where no fractionation over land surface is assumed.
Journal of Geophysical Research, 2007
1] We have incorporated the cycling of water isotopes into the NCAR atmospheric general circulation model, CAM2. Isotope dynamics mostly follow those of previous isotope GCMs, with fractionation being produced by evaporation at the surface and by cloud processes. A new feature that we have added is the direct estimation of the degree of isotopic equilibration between vapor and raindrops as a function of temperature and rain rate. The model yields a reasonable global pattern of water isotopes in precipitation, but detailed comparison with observations is limited by known inaccuracies in precipitation and temperatures yielded by CAM2. We use the results to evaluate the fundamental controls on water isotopic composition in precipitation. We emphasize that, over much of the surface of the Earth, the concept of Rayleigh distillation is inadequate to understand the large-scale geographic distribution of water isotopes in precipitation, because the effects of surface fluxes are more important than those of distillation, in particular at low and midlatitudes and over the oceans. In oceanic regions the balance between precipitation and evaporation (P À E), which reflects the large-scale atmospheric circulation, is the primary determinant of the isotopic composition of precipitation and vapor. Variations of P À E at low latitudes over the oceans produce about 7% variation of precipitation d 18 O that is independent of temperature variation. Where P > E, the convergence of atmospheric vapor derived from various sources leads to low values and a particularly wide range in d ppt . In the tropical and subtropical troposphere the vertical decrease of d 18 O in vapor is different from the values expected from Rayleigh distillation because of entrainment, convective mixing, detrainment, evaporation of detrained water, and subsidence of low-d 18 O highaltitude air. The low d 18 O of atmospheric vapor over the oceans at high (ca. 55°) latitudes produces, as a result of kinetic effects associated with evaporation, a zone of heavy (high-d 18 O) evaporation from the oceans. This effect may account for the low d 18 O of some high-latitude ocean surface waters and also helps attenuate the effects of global temperature changes on the isotopic composition of polar precipitation.
Stable water isotope modelling for weather and climate studies
2005
Water isotopes have proved of immense value in environmental analysis. Oxygen has eleven isotopes (12O to 22O) and eight of these are radioactive with half-lives varying from 122 seconds to less than 10-15 seconds. The three stable isotopes of oxygen are 16O, ...
Journal of Geophysical Research, 1998
Results are presented of a global simulation of the stable water isotopes H•80 and HD160 as implemented in the hydrological cycle of the ECHAM atmospheric general circulation model. The ECHAM model was run under present-day climate conditions at two spatial resolutions (T42,T21), and the simulation results are compared with observations. The high-resolution model (T42) more realistically reproduced the observations, thus demonstrating that an improved representation of advection and orography is critical when modeling the global isotopic water cycle.
Iso-MATSIRO, a land surface model that incorporates stable water isotopes
Global and Planetary Change, 2006
This paper describes Iso-MATSIRO, a land surface model that includes stable water isotopes and simulates physically reasonable isotopic fluxes and reservoirs at the ground. The model calculates kinetic and equilibrium fractionation of HDO and H 2 18 O between ice, liquid, and vapor phases and separately considers soil surface evaporation, vegetation transpiration, evaporation from the canopy-intercepted reservoir, and snow sublimation. New aspects are consideration of subsurface advective flow and the use of canopy resolved leaf water. One-dimensional simulations with modeled meteorological forcings showed reasonable features in the annual isotopic budget, seasonal variations of δ 18 O in soil moisture, diurnal variations of leaf water with some enrichment, and δ-diagram of representative surface reservoirs and fluxes. Kinetic fractionation, however, should be carefully parameterized. A subsequent, independent global simulation used Iso-MATSIRO coupled with an atmospheric isotope circulation model for a half year in 1998. Simulated precipitation δ 18 O was closer to observations than in a previous study, confirming the physical treatments of isotopes in the land surface processes, and indicating their large impact on precipitation isotopes over middle to high latitudinal continents.
Using Stable Water Isotopes to Evaluate Basin-Scale Simulations of Surface Water Budgets
Journal of Hydrometeorology, 2004
Two rare but naturally occurring isotopes of water, 1 H 2 18 O and 1 H 2 H 16 O, are becoming of practical use in diagnosis of climate and earth system model performance. Their value as tracers and validation tools in hydrological subsystems derives from the systematic and different (from each other and from the most abundant water isotope: 1 H 1 H 16 O) paths and residence times they exhibit as a result of phase change, chemical exchange, and diffusive differentiation. Applications of the simulation of stable isotopic behavior to resolving uncertainty in global climate or earth system models, including river isotopic characterization of basin changes and plantrespired oxygen isotope ''tagging,'' are limited until more basic criteria such as conservation, current mean climate, and capture of observed variability are demonstrated. Here the authors assess the simulation of isotopic fluxes in basin-scale hydrology, focusing on the representation of land surfaces in numerical models as the current mechanism for incorporating water isotopes. They find that surface water budgets are still rather poorly simulated and inadequately constrained at the scale of large basins, yet surface energy partition can be apparently well captured by models with inadequate land surface parameterization. Despite this, simulations of fluxes and reservoirs of the isotopes H 2 18 O and 1 H 2 H 16 O are demonstrated here to have diagnostic utility in evaluating surface energy and water budgets. The hypothesis that aspects of basin water budgets and fluxes are explained and improved by isotopic investigation is demonstrated.
Modelling of hydrogen and oxygen isotope compositions for local precipitation
Tellus B, 2005
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 (δ 2 H) and oxygen (δ 18 O) 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.
Simulation of stable water isotope variations by the GENESIS GCM for modern conditions
Journal of Geophysical Research, 2002
Incorporating stable isotope physics in a general circulation model (GCM) provides a promising means to study isotopic variability in precipitation, including the processes that cause isotopic variability in paleoclimatic archives such as ice cores. This paper describes the implementation and validation of stable isotope tracers in the GENESIS 2.0 GCM. The model reproduces the main features of present-day isotopic fields and the characteristic large-scale isotope-climate relationships. Global d 18 OdD , temperature-d 18 O, and precipitation-d 18 O relationships are well simulated, and the modeled regional patterns associated with continental vapor recycling over Europe and vertical gradients agree well with observations. In GENESIS a more sophisticated parameterization of interactions between precipitation and atmospheric vapor contributes to a better simulation of isotopic variations in dry climates. The standard model underestimates the global mean deuterium excess (dD-8d 18 O) in precipitation, although a heuristic sensitivity test suggests this may be remedied by accounting for nonneutral stratification in isotopic evaporative fractionation over ocean. Errors in simulated isotopic fields are analyzed to determine whether they are caused by local climatic biases in the GCM or by inaccurate parameterizations of isotope physics. Using the results of sensitivity experiments and comparisons with other isotopic GCM results, we identify key isotopic and climatic processes at the origin of the main errors and suggest additional studies to improve isotope simulations.
Water isotope expressions of intrinsic and forced variability in a coupled ocean-atmosphere model
Journal of Geophysical Research, 2007
1] Water isotopes provide a clear record of past climate variability but establishing their precise relationship to local or regional climate changes is the key to quantitative interpretations. We have incorporated water isotope tracers within the complete hydrological cycle of Goddard Institute for Space Studies coupled ocean-atmosphere model (ModelE) in order to assess these relationships. Using multicentennial simulations of the modern (preindustrial) and mid-Holocene (6 kyr BP) climate, we examine the internal variability and the forced response to orbital and greenhouse gas forcing. Modelled isotopic anomalies clearly reflect climatic changes and, particularly in the tropics, are more regionally coherent than the precipitation anomalies. Matches to observations at the mid-Holocene and over the instrumental period are good. We calculate water isotope-climate relationships for many patterns of intrinsic and for forced variability relevant to the Holocene, and we show that in general, calibrations depend on the nature of the climate change. Specifically, we examine relationships between isotopes in precipitation and local temperatures and precipitation amounts in the principal ice coring regions (Greenland, Antarctica, and the tropical Andes) and the seawater isotopesalinity gradients in the ocean. We suggest that isotope-based climate reconstructions based on spatial patterns and nonlocal calibrations will be more robust than interpretations based on local relationships.