Regional atmospheric modelling of the water isotope cycle

Full text: In the present document, we introduce a new module handling the water isotope tracers H218O and HDO in the atmospheric regional circulation model (A-RCM) REMO, based on previous work with the ECHAM atmospheric global circulation model (A-GCM). Preliminary results over Europe (from Moroccan coast to the Barents Sea and from the Egyptian coast to South-East Greenland) are analysed, with particular emphasis on the comparison with an isotopic precipitation climatology compiled from GNIP station measurements. The atmospheric ECHAM-4 GCM and REMO-5 RCM, both developed at the MPIM, use a similar physics schema, referred to as EC4-physics. An isotopic module was elaborated for ECHAM, accounting for isotope fractionating processes in stratiform and convective clouds, vertical diffusion, and evapo-transpiration from the surface with regard to sub-surface reservoirs. A centennial experiment was run in the T30 spectral truncature, corresponding to a horizontal resolution of 3.75 deg, i.e. ca. 450 km. This isotopic module was adapted in order to fit the REMO RCM; in its current settings, the standard resolution increased to 0.5 deg, i.e. ca. 50 km over Western and Central Europe. A rotated grid was applied, providing an equal grid cell area over the entire study domain, with its longitudes and latitudes comprised in [50 deg. E; 70 deg. W] and [30 deg. N; 75 deg. N] respectively. The general aim of this study is to enhance the knowledge of water cycle obtained with REMO e.g. in the framework of the BALTEX project, by using water isotopes as a proxy for different hydrological processes. Preliminary results of REMOiso are compared to monthly means of meteoric isotopic measurements from the Global Network for Isotopes in Precipitation (GNIP). The sensitivity of the model output to the boundary conditions is discussed, with respect to ECMWF re-/analysis or alternate ECHAM forcing. Further investigations will test the model output against events measurements at several study sites in Germany monitored by the GSF. (author)