Trace Gas Transport By Mountain Circulations

We investigated trace gas transport by mountain circulations based on aircraft data collected during the Airborne Carbon in the Mountains (ACME)-04 experiment. We found that the mountain circulation that consists of the upslope flow, the return flow, and the descending flow can effectively transport CO2 and polluted air accumulated in the nighttime boundary layer to the atmosphere above and in the lee side of the mountain ridge top. The return flow is embedded in an elevated mixed layer and superimposed on the synoptic flow above the mountain ridge top. Both the elevated mixed layer and the descending flow create a stable lid, which makes the entrainment of the CO2 into the lee-side convective mixing layer difficult. We hypothesize that the net effect of the mountain circulation on the regional carbon budget depends on the duration and strength of the mountain circulation on transporting relatively high respired CO2 in the early morning and relatively low photosynthesized CO2 later into the day. This observation study suggests that the ecosystem-atmosphere carbon exchange occurs in a much deeper layer over mountainous terrain than over flat terrain. The long-term influence of the mountain circulation can be significant on the regional and global carbon budget.