a Three-Dimensional Global Dynamical and Chemical Model of Methane

A three-dimensional global chemical and dynamical methane model has been generated which includes horizontal and vertical advection and diffusion, chemical reactions and surface release terms. The input winds and sinks vary with the seasons and a lone seasonal variation in the source is introduced to simulate rice paddy release. The calculated results are compared with measured methane horizontal distributions, vertical profiles, latitudinal distributions and zonal cross sections. The calculated surface horizontal distributions are consistent with methane measurement at 23 stations from about 76^ circN to the south pole (Steele, et al., 1987). The calculated surface methane shows a maximum around Asia in the rice harvest season (about 18 percent higher than in the adjacent areas). However, there are essentially no measured methane values in this area to compare with these calculations. In the lower troposphere of the Northern Hemisphere, methane exhibits a seasonal cycle which is mainly driven by the seasonal cycles of the die releases, the chemical sinks and transport. There are two rice production seasons which produced, according to the calculations, an increase in the methane transport to the northeast of the rice production areas in eastern Asia. This may offer an explanation for the methane maximums observed by Khalil and Rasmussen (1983) along the northwest coast of the United States about a month after the Asian rice harvests. In the lower troposphere of the Southern Hemisphere, the methane seasonal cycle exhibits a maximum in September and a maximum in March which are mainly driven by the seasonal cycles of the chemical sinks. However, in the upper troposphere, the methane cycle is mainly influenced by transport. In the stratosphere, methane flows readily between the Northern Hemisphere and the Southern Hemisphere. The calculated methane has a maximum around 20^ circN in July and 20^circ S in January. Methane measured by satellite (Nimbus 7) shows similar spatial and temporal features.