Cold air drainage in a forested valley: Investigating the feasibility of monitoring ecosystem metabolism

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College of Forest Resources and Environmental Science


Our objectives were to: characterize spatial and temporal variation in wind speed, direction and air temperature within a steeply sloping 96 ha forested watershed in the Oregon Cascade Mountains; assess the area contributing to advection in cold air drainage; identify appropriate conditions for sampling advected gases representative of the entire watershed; estimate ecosystem respiration from mass balance estimates. The flow dynamics could be categorized into four periods: daytime flows, evening transition, nighttime conditions (formation of a cold air pool) and morning transition. On summer days, the wind above the canopy flowed up-valley whereas the wind direction below the canopy was often down-valley. During the evening transition, the below-canopy cold air drainage became well mixed after the vertical temperature profile at the tower became isothermal, and sodar and tethersonde data revealed that a second drainage flow developed above the canopy. Between 20:00 and 24:00 h (PST), a cold air pool formed within the valley; cold air drainage continued, but wind speed decreased as the night progressed. After sunrise, CO2 in air passing the tower remained well mixed, but its concentration decreased. After the base of the watershed began to receive direct solar radiation, the in-canopy vertical CO2 concentration profile became stratified and the cold air drainage ceased or was confined below the canopy. After the cold air pool formed, the entire watershed supplied the respired CO2 that advected past the tower. During this period, the potential temperature profile within the watershed indicated a strong inversion within the watershed. Because the air passing the tower was well mixed and the cold air pool encompassed the entire watershed, it is probable that air samples collected during this period could provide the best estimate of watershed-scale respiration and carbon isotope composition of respired CO2 (Keeling plot analysis). However, sampling prior to the formation of the cold air pool may confound interpretations as the area contributing to the tower is likely changing. Mass balance calculations using an inert tracer (SF6) released below the canopy demonstrated that data from a single vertical profile could provide plausible estimates of ecosystem respiration at the watershed scale.

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Agricultural and Forest Meteorology