Carbon Dioxide Transport over Complex Terrain

A pilot experiment was conducted at the Niwot Ridge AmeriFlux tower site to investigate CO2 transport over complex terrain. During the pilot experiment, 4 additional towers were installed in addition to 6 existing towers at the site. We found that spatial distribution of CO2 was dominated by drainage flow. The drainage flow was sensitive to local stability. The main drainage flow followed the main slope at the site, and dominated the flow pattern in the early evening and morning. As a result, the CO2 concentration was high down the main slope. The main drainage flow and its associated CO2 spatial distribution survived even under relatively strong wind gust events, while such mixing events might wipe out any spatial and vertical CO2 gradients over relatively flat terrain. Under very stable and weak wind conditions during the mid-night, a local creek that is embedded on the main slope contributed to the local flow pattern. As a result, the CO2 concentration was high along the creek side. Although the depth of the along-creek-high-CO2 was less than 6 m, since the CO2 increases with decreasing height exponentially, the contribution of the local drainage flow is significant. Based on the data collected since 1999 at the site, the largest nighttime respiration and low wind occurred in June and July. The CO2 transport by the drainage flow is expected to be stronger in June and July than that we observed during Sept. 2002.