|Review of ‘The ability of atmospheric data to resolve differences in wetland methane estimates over North America’ by S. M. Miller et al. November 2015|
This revised manuscript presents an evaluation of a set of North American methane flux estimates from the global comparison project named WETCHIMP (Wetland and Wetland CH4 Inter-comparison of Models Project). This revision has addressed many questions and comments from the initial reviews. Currently the manuscript contains a more systematic analysis of all the WETCHIMP models using synthesis data Bayesian Information Criterion (BIC) experiments, atmospheric observations BIC experiments and WETCHIMP models comparison to atmospheric observations and inversion results. The additional explanations and clarifications have made the manuscript more understandable. However, some details are still missing and some questions remain (see specific comments below). Therefore this manuscript requires some minor revisions before it can be accepted for publication.
The results in the revised manuscript have changed substantially from the initial manuscript, specifically the results in Figs 3 and 4, as well as Table 1. This is an indication of the uncertainty in the experimental results, which depends strongly on several factors, including the number of WETCHIMP models used, the scaling or non-scaling of the EDGAR emissions, the scaling or non-scaling of HBL fluxes, and so on. Therefore the uncertainties and the lack of robustness in the analysis results need to be explained and discussed more clearly in the abstract, discussion and conclusion.
Page 2, lines 8-9: ‘we then use real data to analyze the magnitude, seasonality, and spatial distribution of each model estimate’, change ‘real data’ to ‘real data and inversion model results’ as some comparison used inversion results (seasonal cycle derived from real data through inversion analysis) rather than directly to ‘real data’. Inversion model results with potentially large uncertainties should not be confused with ‘real data’ or ‘atmospheric data’. The distinction between ‘real data’ and ‘inversion model results’ has to be clearly noted throughout the manuscript. In fact, the usage of inversion model results in addition to real data should be stated in the title of the manuscript.
Page 2, lines 10-11: ‘Many models predict a seasonality that is narrower than implied by atmospheric CH4 data’, change to ‘Many models predict a seasonality that is narrower than implied by inversion model results’
Page 3, lines 16-17: ‘The present study compares the WETCHIMP CH4 flux estimates against atmospheric CH4 data from 2007–2008 through two sets of analyses.’ Change ‘against atmospheric CH4 data’ to ‘against atmospheric CH4 data and inversion model results’.
Page 4, lines 24-25: ‘Based on these synthetic experiments, we conduct a second set of analyses using real atmospheric data.’ Change to ‘Based on these synthetic experiments, we conduct a second set of analyses using real atmospheric data and inversion model results.’
Page 6, lines 11-12: ‘H(nxm)’, define m.
Page 7, lines 18-19: ‘As a result, the model selection framework cannot scale other variables in X to reproduce the atmospheric CH4 signal from wetlands’ what are the ‘other variables’ referring to?
Page 7, lines 22-24: ‘For consistency among the synthetic datasets, we scale the annual HBL CH4 budget in each WETCHIMP model to match the overall magnitude estimated by several top-down studies’. Are the fluxes outside the HBL region scaled the same way? If the fluxes elsewhere are scaled differently than the HBL region, then each WETCHIMP model has been altered and the experiments are not evaluating the ‘original’ WETCHIMP models. How do the results vary with and without scaling HBL as mentioned?
Page 8, lines 3-5: ‘the intercept for each month is represented by a vector of ones in the matrix X, and this intercept is always included within X.’ The word ‘always’ is confusing. Is the ‘intercept’ included in ‘section 2.3 Real data experiments’ only or in ‘section 2.2 Synthetic data experiment’ also (then it should be mentioned in 2.2)?
Page 10, lines 4-5: ‘We run several additional test scenarios to explore why the synthetic observations may not always be able to detect wetland CH4 fluxes.’ How many ‘repeats’ were done in these test scenarios? How do the BIC test results vary with the number of repeats?
Page 12, line 22: ‘4 Results and discussion: comparisons with atmospheric data’ change to ‘4 Results and discussion: comparisons with atmospheric data and inversion model results’
Page 13, lines 19-20: ‘LPJ-Bern and LPJ-WHyMe also use land cover maps and/or land surveys to estimate wetland’ This is suggested as a reason why LPJ-Bern was ‘selected’ by the BIC experiment. Explain why LPJ-WhyMe was not selected and what other factors could be active.
Page 13, line 14: ‘all six WETCHIMP models’ change six to seven.