Abstract
To assess horizontal and vertical transports of methane (CH4) concentrations at different heights within the troposphere, we analyzed simulations by 12 chemistry transport models (CTMs) that participated in the TransCom-CH4 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 2001-2007. The simulations generally show good agreement with observations for seasonal cycles and vertical gradients. The correlation coefficients of the daily averaged model and observed CH4 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 overestimation and underestimation of vertical gradients 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 CH4 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 greater CH4 interhemispheric gradients 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.
| Original language | English |
|---|---|
| Pages (from-to) | 3891-3904 |
| Number of pages | 14 |
| Journal | Journal of Geophysical Research: Atmospheres |
| Volume | 118 |
| Issue number | 9 |
| DOIs | |
| Publication status | Published - 16 May 2013 |
Funding
This work was supported by JSPS/MEXT KAKENHI-A (grant 22241008). A. Fraser was supported by the UK Natural Environment Research Council National Centre for Earth Observation. We acknowledge the work of J. McGregor and M. Thatcher in the development of CCAM. CCAM simulations were undertaken as part of the Australian Climate Change Science Program and used the NCI National Facility in Canberra, ACT, Australia. R. Prinn andM. Rigby were supported by NASA-AGAGE grants NNX07AE89G and NNX11AF17G to MIT. M. Rigby was also supported by a NERC Advanced Fellowship. The TOMCAT work at the University of Leeds was supported by NERC/NCEO. The research leading to the IFS results has received funding from the European Community's Seventh Framework Programme [FP7 THEME (SPA.2011.1.5-02)] under grant 283576 in the context of the MACC-II project (Monitoring Atmospheric Composition and Climate-Interim Implementation). We sincerely thank all three anonymous reviewers for critical evaluation and providing very helpful comments and suggestions for improving the article.
Keywords
- TransCom Methane
- CH4 vertical profile
- vertical
- horizontal gradient
- CHEMICAL-TRANSPORT MODEL
- ATMOSPHERIC CO2
- TROPOSPHERE
- STRATOSPHERE
- VARIABILITY
- SENSITIVITY
- VERSION
- OZONE
- FLUX
- AIR