This dataset was collected during one of the ARCFLO (Advanced Resolution Canopy FLOw) experiment series’ field campaigns. This field campaign was carried out in WS1 of the HJ Andrews Experimental forest during July-September 2012 by the biomicrometeorology group, PI Christoph Thomas. The ARCFLO experiment series spanned a wide range of topographic conditions (flat, sloped, mountainous) and canopy architectures (grassland, orchard, open forest, dense forest) and was carried out between 2011 and 2014. It was funded through the NSF Career Award in Physical and Dynamical Meteorology to PI Christoph Thomas. The main goal of this project was to develop a novel improved framework to describe the airflow and its transport under weak-wind conditions for a continuous variation of overstory density and stratification. The objective is to i) identify forcing mechanisms of submeso motions, ii) evaluate the impact of plant canopies of different overstory density on the wind, temperature, and humidity fields, and iii) improve predictors for mixing in plant canopies that incorporate the important physical mechanisms. Observations were be made with a unique combination of new and standard techniques including optical fiber measurement of temperature structure, acoustic remote sensing, ultrasonic anemometers, and laser-illuminated flow visualizations.
Christoph K Thomas, John McGinity, Laura Kingzett, Stephen Drake
Air exchange between forests and the lower atmosphere plays an important role for the transport of heat, moisture, momentum and other trace gases between the ground surface and the atmosphere, thereby directly impacting human life and the environment. Much remains to be learned about the mechanisms of the air exchange within the canopy layer, and its interaction with the deeper atmospheric boundary layer. The generally weak subcanopy winds and the mechanical barrier of the overstory render conceptual frameworks, such as commonly applied similarity theories, inadequate. The common generation of turbulence by shear on a variety of time scales, poor exchange between the subcanopy and above-canopy air, and short-circuiting of the energy cascade are not included in similarity theory that forms the basis for turbulent fluxes in models. Moreover, always-present background ‘submeso’ motions of spatial scales from tens of meters to several kilometers become important and lead to unpredictable sudden wind direction changes, intermittent mixing, and non-equilibrium turbulence. No current physical concept describes the nature of these motions. Cases of weak airflow in concert with limited vertical mixing also maintain high concentrations of contaminants near the surface, determined by poorly predicted within-canopy transport.