Transport model simulation of atmospheric CH4 - implications for surface flux estimation

We have used an AGCM (atmospheric general circulation model)-based Chemistry Transport Model (ACTM) for the simulation of methane (CH4) in the lower and middle atmosphere. The model simulations are compared with measurements at hourly, daily, monthly and interannual time scales at more than 50 surface monitoring stations. From this comparison, we conclude that the recent (1990-present) trends in CH4 growth rate and seasonal cycle at most measurement sites can be fairly successfully modeled by using existing knowledge of CH4 flux trends and seasonality. Good model-observation comparison is achieved by optimizing flux amplitudes with respect to the available hydroxyl radical (OH) distribution and model transport. Detailed analysis of seasonal cycles, synoptic variations and diurnal cycles are shown to be useful for validating regional flux distribution patterns and strengths. Our results, based on two emission scenarios, suggest reduced emissions from temperate and tropical Asia region, and compensating increase in the boreal Northern Hemisphere (NH) are indicated for improved model-observation agreement. The ACTM simulated results are also compared with satellite observations (by UARS/HALOE) in the stratospheric altitudes to test the validity of model chemistry scheme and role of quasi-biennial oscillation on CH4 profiles and trends. Next we plan to use the ACTM forward transport for surface flux estimation of CH4 in a Bayesian synthesis inversion framework.