Wind fields in heterogeneous conifer canopies: parameterisation of momentum absorption using high-resolution 3D vegetation scans

Abstract

Applications of flow models to tall plant canopies are limited, amongst other factors, by the lack of detailed information on vegetation structure. A method is presented to record 3D vegetation structure and make this information applicable to the derivation of turbulence parameters suitable for flow models. The relationship between wind speed, drag coefficient (C D ) and plant area density (PAD) was experimentally investigated in a mixed conifer forest in the lower part of the Eastern Ore Mountains. Essential information was gathered by collecting multi-level high-frequency wind velocity measurements and a dense 3D representation of the forest was obtained from terrestrial laser scanner data. Wind speed dependence or streamlining was observed for most of the wind directions. Edge effects, i.e. the influence of the here not regarded pressure gradient and the advective terms of the momentum equation, are assumed to cause this heterogeneity. Contrary to the hypothetic shelter effect, which would reduce the drag on sheltered plant parts, the calculated profiles of drag coefficients revealed an increasing C D with PAD (i.e. a dependence on canopy and plant structure).

Estimating the Instantaneous Drag–Wind Relationship for a Horizontally Homogeneous Canopy

“…Although single-point measurements are not always representative of spatially-averaged metrics of the flow, field data suggest that the dependence of C d mod on U is clearer and less scattered than the dependence of C d on u (Cescatti and Marcolla 2004;Queck et al 2012). The dependence of local mean drag coefficients on local velocity scales reflects the dependence of the instantaneous local drag coefficient, C d , on the instantaneous velocity.…”

“…Deriving analytical models for the mean and instantaneous local drag coefficients from first principles is not trivial. Traditionally, vertically distributed single-point measurements of instantaneous velocity time series were used to investigate the mean drag-wind relationship (e.g., Cescatti and Marcolla 2004;Queck et al 2012). This traditional approach assumes that data are representative of spatially-averaged metrics of statistically stationary flow within and above a horizontally homogeneous canopy (Pinard and Wilson 2001).…”

“…An uncertainty of 5 % in the estimated u w can induce an error of 100 % in estimating its vertical gradient that balances the streamwise mean pressure gradient. In field studies for terrestrial canopies, the streamwise mean pressure gradient in (7) is usually neglected (e.g., Pinard and Wilson 2001;Cescatti and Marcolla 2004;Queck et al 2012). …”

“…While a constant drag coefficient model has been widely used, in reality the local drag coefficients tend to decrease with increasing local velocity due to the effects of viscous drag (Raupach and Thom 1981;Poggi et al 2004b) and reconfiguration of flexible canopy elements (Alben et al 2002;Gosselin et al 2010;Luhar and Nepf 2011). For terrestrial and aquatic canopies, this dependence is commonly observed to follow a power law of the local characteristic velocity scales with an exponent between zero and −4/3 (Vogel 1984;Gaylord et al 1994;Harder et al 2004;Albayrak et al 2012;Queck et al 2012;Pan et al 2014a). For natural canopies in which simple bending is observed (e.g., seagrasses and wheat), both measurement and theory suggest a power-law exponent between −2/3 and −1 (Pan et al 2014b).…”

“…Even if these conditions are satisfied, two major issues remain in the analysis of observational data. Firstly, the streamwise mean pressure gradient is typically neglected for terrestrial canopies, mostly due to the difficulty in estimating it from measurements (e.g., Cescatti and Marcolla 2004;Queck et al 2012). Secondly, despite the expected dependence of the mean local drag coefficients on the local velocity scales, no standard approach has been proposed to estimate the instantaneous drag coefficients that correlate with instantaneous velocities.…”