A Multi-Scale Comparison of Modeled and Observed Seasonal Methane Emissions in Northern Wetlands

Scientists working across multiple projects compared Alaska-wide wetland CH4 emission model predictions with site to regional scale observations. In a publication just released from the journal Biogeosciences, Xiyan Xu (LBNL) and others undertook a comparison of the CH4 fluxes with eddy flux data (including those from the NGEE Arctic Barrow, AK site) that highlighted needed changes to the model’s estimate of aerenchyma area, which were implemented and tested. The model modification substantially reduced biases in CH4 emissions when compared with CarbonTracker CH4 predictions. CLM4.5 CH4 emission predictions agree well with Alaskan growing season (May-September) CarbonTracker CH4 predictions and site-level observations. However, the model underestimated CH4 emissions in the cold season (October-April). The monthly atmospheric CH4 mole fraction enhancements due to wetland emissions were also assessed using the WRF-STILT Lagrangian transport model and compared with measurements from the NASA-sponsored Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Both the tower and aircraft analyses confirm the underestimate of cold season CH4 emissions. The greatest uncertainties in predicting the seasonal CH4 cycle are from the wetland extent, cold season CH4 production, and CH4 transport processes. Predicted CH4 emissions remain uncertain, but we show here that benchmarking against observations across spatial scales can inform model structural and parameter improvements.

Reference:

Xu, X, WJ Riley, CD Koven, DP Billesbach, RY-W. Chang, R Commane, ES Euskirchen, S Hartery, Y Harazono, H Iwata, KC McDonald, CE Miller, WC Oechel, B Poulter, N Raz-Yaseef, C Sweeney, M Torn, SC Wofsy, Z Zhang, and D Zona. 2016. A multi-scale comparison of modeled and observed seasonal emissions in northern wetlands. Biogeosciences 13: 5043-5056. http://dx.doi.org/10.5194/bg-13-5043-2016

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_{Figure 1.  (Left) The methane module in ALM and CLM accurately captured growing season eddy covariance observations at the NGEE-Arctic BEO site, but substantially under-predicted cold season fluxes. (Right) Alaska-wide wetland methane emission predictions were improved by our model changes, but biases remain}