Can Nocturnal Cold Air Drainage be Used to Monitor Ecosystem Function?

Ecosystem carbon dynamics in flat, uniform terrain are commonly studied using standard micrometeorological techniques such as eddy covariance or gradient methods. But many of the world's ecosystems are in complex topography that is inappropriate for these methods. Nocturnal cold air drainage commonly occurs in mountainous terrain. This drainage provides an opportunity to monitor ecosystem carbon dynamics because as air flows downhill through a watershed, it collects respired CO2 from the soil and vegetation. If the nocturnal drainage can be treated as a river of air flowing down a valley, sampling this air from a tower at the base of a watershed could provide an estimate of ecosystem respiration and the 12C/13C ratio. To interpret the measured CO2 and the 12C/13C ratio, the characteristics of the drainage and the footprint (source area) of air passing the tower must be understood. To explore the potential of using nocturnal cold air drainage we built a 37 m tower at the base of a deeply incised watershed of ~40 y-old Douglas-fir in the Oregon Cascades. At various heights on the tower we monitored air temperature, wind speed/direction, and the CO2 concentration and 12C/13C isotopic ratio with a combination of thermistors, sonic anemometers (2-D and 3-D) and a CO2 profile system. The temperature gradient along the axis of the watershed was monitored by 30 temperature sensors from the base to the top of the watershed. The maximum drainage windspeeds on the tower occurred near sunset and, unlike past reports of cold air drainage, this drainage was very deep (> 37 m). The drainage became well mixed when the vertical profile of potential temperature became isothermal. It remained well mixed through the night into the early morning. The drainage occurred on most summer nights and typically provided a range of CO2 (> 60 ppm) sufficient for "Keeling plot" analysis. In September 2005, we released a tracer in the watershed (SF6) to determine the varying footprint size of the tower. The footprint size and windspeed varied throughout the night, resulting in a change in the CO2 concentration at the tower. Further analysis will determine how the CO2 passing the tower is altered by entrainment of the air overlying the drainage and the change in the source area of the drainage.