Comparison of CMAM simulations of carbon monoxide (CO), nitrous oxide (N ₂ O), and methane (CH ₄ ) with observations from Odin/SMR, ACE-FTS, and Aura/MLS

Abstract. Simulations of CO, N2O and CH4 from a coupled chemistry-climate model (CMAM) are compared with satellite measurements from Odin Sub-Millimeter Radiometer (Odin/SMR), Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and Aura Microwave Limb Sounder (Aura/MLS). Pressure-latitude cross-sections and seasonal time series demonstrate that CMAM reproduces the observed global distributions and the polar winter time evolutions of the CO, N2O, and CH4 measurements quite well. Generally, excellent agreement with measurements is found in CO monthly zonal mean profiles in the stratosphere and mesosphere for various latitudes and seasons. The difference between the simulations and the observations are generally within 30%, which is comparable with the difference between the instruments in the upper stratosphere and mesosphere. In general, the CO measurements also show an excellent agreement between themselves although MLS retrievals are noisier than other retrievals above 10 hPa (~32 km). The measurements also show large difference in the lower stratosphere and upper troposphere. Comparisons of N2O show that CMAM results usually have a less than 15% difference to the measurements in the lower and middle stratosphere, and the observations are consistent as well. However, the standard version of CMAM has a serious low bias in the upper stratosphere. The CMAM CH4 distribution is also close to the observations in the lower stratosphere, but has a similar but smaller negative bias in the upper stratosphere. These negative biases can be reduced by introducing a vertical diffusion coefficient related to gravity wave drag. CO measurements from 2004 and 2006 show evidence of enhanced descent of air from the mesosphere into the stratosphere in the Arctic after strong stratospheric sudden warmings (SSWs). CMAM also shows strong descent of air after SSWs, but further investigation is needed. In the tropics, CMAM captures the "tape recorder" (or annual oscillation) in the lower stratosphere and the semiannual oscillations (SAO) at the stratopause and mesopause shown in MLS CO and SMR N2O observations. The inter-annual variation of the SAO at the stratopause in SMR N2O observations also shows a biennial oscillation, but CMAM cannot does not reproduce this feature. However, this study confirms that CMAM is able to simulate middle atmospheric transport processes reasonably well.

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