Constraining regional carbon fluxes in complex terrain: The Airborne Carbon in the Mountains Experiment 2007

Land-atmosphere exchange of CO2 in complex terrain is poorly understood, especially at regional scales. The significant amounts of forest found in mountainous terrain may be important carbon sinks, and also highly sensitive to land use, fire, and climate change. Current estimates of regional carbon fluxes are biased toward flat terrain, which have been partially alleviated with a field experiment conducted in the central Rocky Mountains in spring-summer 2007, complementing an intial project conducted in summer 2004 The Airborne Carbon in the Mountains Experiment 2004 and 2007 (ACME04 and ACME07) collected data on airborne concentrations of CO2, CO and O2 to quantify the distribution and evolution of these gases and estimate regional flux in complex terrain. On-going continuous surface CO2 observations in this region were also used here, complemented by a variety of surface flux tower, meteorological, and inventory data and remotely sensed observations of land cover, albedo, and photosynthetic production. Nearly three-dozen research flights were conducted across the two campaigns, and paired upwind and downwind flights reveal routine signals of 1) nocturnal pooling of CO2 in forested valleys, 2) lower CO2 in afternoon downwind profiles when paired with morning flights, and 3) significant variations in boundary layer depth. The existence of valley pools, vertical wind shear, and inhomogeneous boundary layer structure requires development of top-down regional flux methodology that is different from airborne observations over flat terrain. Advanced data assimilation methods of top-down and bottom-up observations in terrain are currently in development. The initial estimates of regional flux from aircraft show some consistency with the general pattern observed at the Niwot Ridge Ameriflux tower, revealing the importance of the timing of the late summer dry period on net carbon fluxes. The SipNet ecosystem model, tuned to flux data, and driven by interpolated spatially explicit meteorology dataset and remotely sensed landcover, is being used to further refine this comparison. Finally, airborne and tower-based valley profiles of pre-vented nocturnal CO2 pools in the Fraser Experimental Forest are analyzed for how mountain pine beetle damage is affecting regional ecosystem respiration.