Modeling and measuring the nocturnal drainage flow in a high‐elevation, subalpine forest with complex terrain

Abstract

[1] The nocturnal drainage flow of air causes significant uncertainty in ecosystem CO 2 , H 2 O, and energy budgets determined with the eddy covariance measurement approach. In this study, we examined the magnitude, nature, and dynamics of the nocturnal drainage flow in a subalpine forest ecosystem with complex terrain. We used an experimental approach involving four towers, each with vertical profiling of wind speed to measure the magnitude of drainage flows and dynamics in their occurrence. We developed an analytical drainage flow model, constrained with measurements of canopy structure and SF 6 diffusion, to help us interpret the tower profile results. Model predictions were in good agreement with observed profiles of wind speed, leaf area density, and wind drag coefficient. Using theory, we showed that this one-dimensional model is reduced to the widely used exponential wind profile model under conditions where vertical leaf area density and drag coefficient are uniformly distributed. We used the model for stability analysis, which predicted the presence of a very stable layer near the height of maximum leaf area density. This stable layer acts as a flow impediment, minimizing vertical dispersion between the subcanopy air space and the atmosphere above the canopy. The prediction is consistent with the results of SF 6 diffusion observations that showed minimal vertical dispersion of nighttime, subcanopy drainage flows. The stable within-canopy air layer coincided with the height of maximum wake-to-shear production ratio. We concluded that nighttime drainage flows are restricted to a relatively shallow layer of air beneath the canopy, with little vertical mixing across a relatively long horizontal fetch. Insight into the horizontal and vertical structure of the drainage flow is crucial for understanding the magnitude and dynamics of the mean advective CO 2 flux that becomes significant during stable nighttime conditions and are typically missed during measurement of the turbulent CO 2 flux. The model and interpretation provided in this study should lead to research strategies for the measurement of these advective fluxes and their inclusion in the overall mass balance for CO 2 at this site with complex terrain.

Application of a mass consistent flow model to study the CO2 mass balance of forests

“…Other advection studies define a coordinate system with x in the direction of the mean downslope wind, and z normal to the slope (e.g., Yi et al, 2005;Heinesch et al, 2007), but shortcomings related to tilt corrections are highlighted in the literature (e.g., Vickers and Mahrt, 2006;Sun, 2007). As discussed by Sun (2007), the Cartesian coordinate system is a good choice, since it is independent of time and space, and the buoyancy force is normal to the geopotential.…”

Stably stratified canopy flow in complex terrain

“…A super-stable layer acts as a "lid" or "barrier" that separates fluid into two uncorrelated layers: (1) the lower layer between the ground and the super-stable layer, and (2) the upper layer above the super-stable layer. This canopy flow separation was verified by SF 6 diffusion observations (Yi et al, 2005) and carbon isotope experiments (Schaeffer et al, 2008). The lower layer is sometimes called a "decoupled layer" (Alekseychik et al, 2013) that is shallow, usually within the trunk space of a forest.…”

“…The characteristics of the channeled air are highly dependent on soil conditions, containing a high concentration of soil respired CO 2 and soil evaporated water vapor, and consisting of colder air cooled by radiative cooling at the ground surface (Schaeffer et al, 2008). The channeled air is sometimes termed "drainage flow", and is a common phenomenon in hilly terrains under stable atmospheric conditions, such as on calm and clear nights (Yi et al, 2005;Alekseychik et al, 2013). The drainage flow limits the accuracy of tower-based estimates of ecosystem-atmosphere exchanges of carbon, water, and energy.…”

“…The features of S-shaped wind profiles also dictate the existence of super-stable layers near levels where wind speed is maximum (or minimum) and temperature inversion (temperature increasing with height) exists, leading the Richardson number (Ri) to be extremely large or infinity (Yi et al, 2005). A super-stable layer acts as a "lid" or "barrier" that separates fluid into two uncorrelated layers: (1) the lower layer between the ground and the super-stable layer, and (2) the upper layer above the super-stable layer.…”

“…S-shaped wind profiles have been observed within forest canopies in numerous studies (Baldocchi and Meyers, 1988;Bergen, 1971;Fons, 1940;Lalic and Mihailovic, 2002;Landsberg and James, 1971;Lemon et al, 1970;Meyers and Paw U, 1986;Oliver, 1971;Shaw, 1977;Turnipseed et al, 2003;Yi et al, 2005;Queck and Bernhofer, 2010;Sypka and Starzak, 2013). The S-shaped profile refers to a secondary wind maximum that is often observed within the trunk space of forests and a secondary minimum wind speed in the region of greatest Published by Copernicus Publications on behalf of the European Geosciences Union.…”