Comparison of an isotopic atmospheric general circulation model with new quasi-global satellite measurements of water vapor isotopologues

Abstract

[1] We performed an intensive comparison of an isotope-incorporated atmospheric general circulation model with vapor isotopologue ratio observation data by two quasi-global satellite sensors in preparation for data assimilation of water isotope ratios. A global Isotope-incorporated Global Spectral Model simulation nudged toward the reanalysis wind field, atmospheric total column data from Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) on Envisat, and midtropospheric (800 to 500 hPa) data from Tropospheric Emission Spectrometer (TES) on Aura were used. For the mean climatological dD of both the total atmospheric column and the midtroposphere layer, the model reproduced their geographical variabilities quite well. There is, however, some degree of underestimation of the latitudinal gradient (higher dD in the tropics and lower dD in midlatitudes) compared to the SCIAMACHY data, whereas there is generally less disagreement except lower dD over the Maritime Continent compared to the TES data. It was also found that the two satellite products have different relationships between water vapor amount and isotopic composition. Particularly, atmospheric column mean dD, which is dominated by lower-tropospheric vapor, closely follows the fractionation pattern of a typical Rayleigh-type "rain out" process, whereas in the midtroposphere the relationship between isotopic composition and vapor amount is affected by a "mixing" process. This feature is not reproduced by the model, where the relationships between dD and the vapor are similar to each other for the atmospheric column and midtroposphere. Comparing on a shorter time scale, it becomes clear that the data situation for future data assimilation for total column dD is most favorable for tropical and subtropical desert areas (i.e., Sahel, southern Africa, mideastern Asia, Gobi, Australia, and the southwest United States), whereas the available midtropospheric dD observations cover wider regions, particularly over tropical to subtropical oceans.

Interannual variability of isotopic composition in water vapor over western Africa and its relationship to ENSO

“…IsoGSM simulates this spatial pattern qualitatively well. Although the average is negatively biased (about 20 ‰) (Yoshimura et al, 2011) and the latitudinal gradient is weaker in IsoGSM, bias and overestimated gradient are found in SCIAMACHY when compared with ground-based Fourier-transform spectrometers (Scheepmaker et al, 2014). Accordingly we cannot conclude whether such differences from the satellite are indeed problematic or not at this stage.…”

“…Note that the range is widely different between them (−300-0 ‰ for SCIAMACHY; −190-90 ‰ for IsoGSM). This may be because IsoGSM misses the enrichment in boreal summer over tropical Africa, as suggested in previous studies (Frankenberg et al, 2009;Yoshimura et al, 2011). The bias in the mean field (Risi et al, 2010;Werner et al, 2011;Lee et al, 2012) and the underestimated seasonality (Risi et al, 2010) are also common in other GCMs.…”

“…More details of the model configurations have been described previously (Yoshimura et al, 2008). The general reproducibility of the model for daily to interannual timescales is well evaluated by comparing it with the precipitation isotope ratio (Yoshimura et al, 2008) and the vapor isotopologue ratio from satellite measurements (Yoshimura et al, 2011), and it showed sufficiently accurate results for various process studies (e.g., Berkelhammer et al, 2012;Liu et al, 2013Liu et al, , 2014. In addition to the standard experiment (Std) mentioned above, we carried out two sensitivity experiments.…”

“…The first of these experiments examined the sensitivity of the results to the "equilibrium fraction ε", which is the degree to which falling rain droplets equilibrate with the surroundings. Risi et al (2010) reported the importance of reevaporation for δ 18 O v over western Africa, and Yoshimura et al (2011) found an improved simulation result with the changed parameter. Following Yoshimura et al (2011), we set this value to 10 %, while in the standard simulation it was set to 45 %.…”

“…Risi et al (2010) reported the importance of reevaporation for δ 18 O v over western Africa, and Yoshimura et al (2011) found an improved simulation result with the changed parameter. Following Yoshimura et al (2011), we set this value to 10 %, while in the standard simulation it was set to 45 %. The other sensitivity experiment was intended to estimate the contributions to interannual variability in δ 18 O v of the distillation effect during transportation from the source regions.…”