Simulation of stable water isotopic composition in the atmosphere using an isotopic Atmospheric Water Balance Model



In this article, we present a simplified isotopic Atmospheric Water Balance Model (iAWBM), to simulate spatial distribution and temporal variations of δ18O in atmospheric vapour and in precipitation, and the correlations of δ18O in precipitation with precipitation amount and with temperature. This method overcomes deficiencies of the Global Network of Isotopes in Precipitation (GNIP) due to its limitation in space and time resolution and those of the isotope-enabled General Circulation Models (iGCMs) due to its complexity. The simulated results reproduce well the latitude effect, continent effect and seasonality of δ18O in global precipitation. The simulation suggests that the precipitation amount effect occurs mainly in the low-mid latitude oceans and monsoon areas, and the simulated temperature effect occurs mainly in the mid-high latitude continents. However, the simulation results also indicate some temperature effect in some low-latitude areas where the precipitation amount effect is strong. The modelled precipitation isotopic composition is compared to 3-year observations in Changsha, China (2010–2012). The simulations reproduce the observed seasonal variations of δ18O in precipitation. The basic characteristics that stable isotopes in precipitation are depleted during the rainy season and enriched during the dry season, and the observed amount effect is well simulated under daily time scale. The simulated weighted average δ18O (−6.58‰) in precipitation during the prevailing winter monsoon season (October–March) is almost the same as the observed δ18O in the same period (−6.56‰), but the simulated δ18O (−9.58‰) during the prevailing summer monsoon season (April–September) is lower than the observed δ18O (−7.66‰). The simulation result also shows temporal variations of vapour isotopic composition in the atmosphere, and the proportional contributions from horizontal vapour exchange, precipitation and evaporation.