Publication Date:
2013-04-04
Description:
[1] To assess horizontal and vertical transports of methane (CH 4 ) concentrations at different heights within the troposphere, we analyzed simulations by12 chemistry-transport models (CTMs) that participated in the TransCom-CH 4 intercomparison experiment. Model results are compared with aircraft measurements at 13 sites in Amazon/Brazil, Mongolia, Pacific Ocean, Siberia/Russia and United States during the period of 2001 to 2007.The simulations generally show good agreement with observations for seasonal cycles and vertical gradients. The correlation coefficients of the daily averaged model and observed CH 4 time series for the analyzed years are generally larger than 0.5, and the observed seasonal cycle amplitudes are simulated well at most sites, considering the between-model variances. However, larger deviations show up below 2 km for the model-observation differences in vertical profiles at some locations, e.g., at Santarem, Brazil, and in the upper troposphere, e.g., at Surgut, Russia. Vertical gradients and concentrations are underestimated at Southern Great Planes, United States and Santarem, and overestimated at Surgut. Systematic over- and under-estimation of vertical gradient are mainly attributed to inaccurate emission and only partly to the transport uncertainties. However, large differences in model simulations are found over the regions/seasons of strong convection, which is poorly represented in the models. Overall, the zonal and latitudinal variations in CH 4 are controlled by surface emissions below 2.5 km, and transport patterns in the middle and upper troposphere. We show that the models with larger vertical gradients, coupled with slower horizontal transport, exhibit greaterCH 4 interhemisphericgradients in the lower troposphere. These findings have significant implications for the future development of more accurate CTMs with the possibility of reducing biases in estimated surface fluxes by inverse modeling.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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