Abstract
Abstract. We evaluate the isotopic composition of water vapor and precipitation simulated by the LMDZ (Laboratoire de Météorologie Dynamique-Zoom) GCM (General Circulation Model) over Siberia using several data sets: TES (Tropospheric Emission Spectrometer) and GOSAT (Greenhouse gases Observing SATellite) satellite observations of tropospheric water vapor, GNIP (Global Network for Isotopes in Precipitation) and SNIP (Siberian Network for Isotopes in Precipitation) precipitation networks, and daily, in situ measurements of water vapor and precipitation at the Kourovka site in Western Siberia. LMDZ captures the spatial, seasonal and daily variations reasonably well, but it underestimates humidity (q) in summer and overestimates δD in the vapor and precipitation in all seasons. The performance of LMDZ is put in the context of other isotopic models from the SWING2 (Stable Water Intercomparison Group phase 2) models. There is significant spread among models in the simulation of δD, and of the δD-q relationship. This confirms that δD brings additional information compared to q only. We specifically investigate the added value of water isotopic measurements to interpret the warm and dry bias featured by most GCMs over mid and high latitude continents in summer. The analysis of the slopes in δD-q diagrams and of processes controlling δD and q variations suggests that the cause of the dry bias could be either a problem in the large-scale advection transporting too much dry and warm air from the south, or too strong boundary-layer mixing. However, δD-q diagrams using the available data do not tell the full story. Additional measurements would be needed, or a more sophisticated theoretical framework would need to be developed.
Highlights
General circulation models (GCMs) have persistent and systematic biases in the representation of modern climate and its associated water cycle (Meehl et al, 2007)
Isotope ratios are commonly reported relative to a standard in δ-notation: δ = (Rsample / Rstandard − 1) · 1000, where the standard used is Vienna Standard Mean Ocean Water (VSMOW), δ represents either δD or δ18O expressed in ‰, and R is the ratio of HDO or of H128O to H2O
The measurements of atmospheric surface water vapor isotopic composition are performed by the Picarro isotopic analyzer L2130-i based on wavelength-scanned cavity ring down spectroscopy (WS-CRDS)
Summary
General circulation models (GCMs) have persistent and systematic biases in the representation of modern climate and its associated water cycle (Meehl et al, 2007) One example of these biases is the systematic warm and dry bias over mid latitude continental regions in summer, which is evident especially in the Great Plains of the United States and in Central and Eastern Europe (Kittel et al, 1997; Cattiaux et al, 2013). We exploit a new set of continuous isotopic measurements in low-level atmospheric water vapor and precipitation at Kourovka (70 km northwest from Ekaterinburg) near the western boundary of Western Siberia This data set is complemented by satellite measurements (Worden et al, 2007; Frankenberg et al, 2013) and compared with existing isotopic GCMs. In particular, using the LMDZ (Laboratoire de Météorologie Dynamique-Zoom) GCM (Risi et al, 2010c), processes controlling water isotopic composition are investigated and sources of model–data mismatches are discussed.
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