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Mean Flow Near Edges and Within Cavities Situated Inside Dense Canopies

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Abstract

A streamfunction-vorticity formulation is used to explore the extent to which turbulent and turbulently inviscid solutions to the mean momentum balance explain the mean flow across forest edges and within cavities situated inside dense forested canopies. The turbulent solution is based on the mean momentum balance where first-order closure principles are used to model turbulent stresses. The turbulently inviscid solution retains all the key terms in the mean momentum balance but for the turbulent stress gradients. Both exit and entry versions of the forest edge problem are explored. The turbulent solution is found to describe sufficiently the bulk spatial patterns of the mean flow near the edge including signatures of different length scales reported in canopy transition studies. Next, the ‘clearing inside canopy’ or the so-called ‘cavity’ problem is solved for the inviscid and turbulent solutions and then compared against flume experiments. The inviscid solution describes the bulk flow dynamics in much of the zones within the cavity. In particular, the solution can capture the correct position of the bulk recirculation zone within the cavity, although with a weaker magnitude. The inviscid solution cannot capture the large vertical heterogeneity in the mean velocity above the canopy, as expected. These features are better captured via the first-order closure representation of the turbulent solution. Given the ability of this vorticity formulation to capture the mean pressure variations and the mean advective acceleration terms, it is ideal for exploring the distributions of scalars and roughness-induced flow adjustments on complex topography.

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Acknowledgments

Banerjee and Katul acknowledge the support via cooperative agreement (09-ca-11330140-059) from the United States Forest Service (USFS). Katul also acknowledges the support from the National Science Foundation (NSF) (Grants EAR-1013339, AGS-110227, CBET-103347), the US Department of Agriculture (2011-67003-30222), the Binational Agricultural Research and Development (BARD) Fund (Research Grant Award No. IS-4374-11C), the U.S. Department of Energy through the Office of Biological and Environmental Research (BER) Terrestrial Carbon Processes (TCP) program (DE-SC000697) and the Fulbright Italy distinguished scholars program.

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Correspondence to Tirtha Banerjee.

Appendix: Numerical Details of the Problems

Appendix: Numerical Details of the Problems

For solving the systems of equation described in Sect. 2.1, the following algorithm has been used.

  1. 1.

    Initiate \({U}\) and \({W}\) fields. For the \({U}\) field, an exponential profile is used inside the canopy and a logarithmic profile is used above the canopy. \(W\) is set to zero uniformly.

  2. 2.

    Initiate the vorticity \({\omega }\) field using the assumed \({U}\) and \({W}\) fields using Eq. 16.

  3. 3.

    Find the vorticity at the next timestep using Eq. 12.

  4. 4.

    Using the new vorticity, solve the Poisson equation (Eq. 19) with boundary conditions to obtain the streamfunction \({\psi }\) at the new timestep.

  5. 5.

    Compute \({U}\) and \({W}\) from \({\psi }\) using Eqs. 17 and 18.

  6. 6.

    Repeat the steps with the new \({U}\) and \({W}\) until the differences between successive iterations in vorticity falls below a pre-set tolerance value (usually \(5\times 10^{-3}\)).

  7. 7.

    Using the converged vorticity, determine the final updated \({\psi }, {U}\) and \({W}\).

  8. 8.

    Solve the pressure Poisson Eq. 9 to determine \({p}(x,z)\) using boundary conditions defined afterwards.

The numerical details are listed in Table 4 where \(h, x, z, \text{ d }x, \text{ d }z\) indicate canopy height, horizontal and vertical domain sizes, horizontal and vertical grid spacings, respectively. The grid parameters are normalized by canopy height \({h}\). \({C_\mathrm{d}}\) and LAI indicate canopy drag coefficient and the canopy LAI respectively.

Table 4 Numerical details of the problems solved

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Banerjee, T., Katul, G., Fontan, S. et al. Mean Flow Near Edges and Within Cavities Situated Inside Dense Canopies. Boundary-Layer Meteorol 149, 19–41 (2013). https://doi.org/10.1007/s10546-013-9826-x

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