Respecting the Flux Control Volume: Challenges to the Leuning Mantra in Complex Terrain

The proper determination of vegetation-atmosphere fluxes of mass through micrometeorological techniques requires specification of a theoretical control volume and respect of the principle of continuity in all relevant spatial coordinates. In over six years of annual interactions with Ray Leuning at the Colorado summer course entitled Fluxes and Advanced Modeling, the 'control volume', mass balanced framework was offered as the foundation of Ray's lectures to students just beginning their intellectual pursuit of these topics. I have come to recognize this framework as the proper starting point for teaching the theoretical and observational rudiments of understanding surface-atmosphere fluxes. Over the course of Ray's lectures during these six years, and surrounded by the rugged topography of the Colorado Rocky Mountains, however, the challenge of applying this framework to natural ecosystems has been discussed frequently. In this lecture, I will honor Ray's dedication and adherence to the first-principles of biophysics, and I will discuss some of the recent efforts that have been applied to respecting these principles when studying the exchanges of carbon and water in natural ecosystems in complex terrain. I will focus on single-tower, multi-tower and aircraft-sampling efforts to construct the theoretical 'control volume' in real space and apply the principle of continuity to achieve an observational mass balance. These efforts, while admirable in their aims, continue to suffer from large uncertainties that emerge as the calculus of continuity is applied across the finite space of vegetated ecosystems. The lessons I have come to learn from Ray, as someone who has traveled a similar career path, from botany to meteorology, will be cited and elucidated.