TY - JOUR

T1 - Spatial Variability of Scalar Concentrations and Fluxes Downstream of a Clearing-to-Forest Transition

T2 - A Large-Eddy Simulation Study

AU - Kanani-Sühring, Farah

AU - Raasch, Siegfried

PY - 2015/4

Y1 - 2015/4

N2 - The aim of this large-eddy simulation study is to improve the yet scarce understanding of the scalar (e.g. CO2) transport in forest-edge flows. In order to contribute to a basic knowledge on the scalar transport, we focus on idealized neutral flows across a clearing-to-forest transition, with a passive scalar released from a horizontally homogeneous source at the clearing and at the forest floor. Corresponding to previous studies, we found pronounced peaks in scalar concentration and flux downstream of the forest edge, where the flux peak significantly exceeded the given surface flux. We investigated for the first time those transport mechanisms that steer the scalar accumulation, by analyzing the terms in the scalar transport equation. The analysis reveals that the accumulation is accomplished by the streamwise convergence of the mean and turbulent transport. In order to investigate how the concentration and flux enhancement behaves under different conditions, we performed a series of simulations with varying forest density and wind speed. We demonstrate a strong dependence of the peak location and magnitude on forest density. With increasing density, concentration and flux peaks were found closer to the forest edge and peak values increased significantly. Decreasing the wind speed caused an increase of the concentration peak, while the flux peak remained unaffected. For an adequate interpretation and design of micrometeorological measurements near forest edges, it is necessary to identify the regions (fetches) with enhanced concentrations and fluxes. We therefore analyzed different approaches for a proper fetch estimation.

AB - The aim of this large-eddy simulation study is to improve the yet scarce understanding of the scalar (e.g. CO2) transport in forest-edge flows. In order to contribute to a basic knowledge on the scalar transport, we focus on idealized neutral flows across a clearing-to-forest transition, with a passive scalar released from a horizontally homogeneous source at the clearing and at the forest floor. Corresponding to previous studies, we found pronounced peaks in scalar concentration and flux downstream of the forest edge, where the flux peak significantly exceeded the given surface flux. We investigated for the first time those transport mechanisms that steer the scalar accumulation, by analyzing the terms in the scalar transport equation. The analysis reveals that the accumulation is accomplished by the streamwise convergence of the mean and turbulent transport. In order to investigate how the concentration and flux enhancement behaves under different conditions, we performed a series of simulations with varying forest density and wind speed. We demonstrate a strong dependence of the peak location and magnitude on forest density. With increasing density, concentration and flux peaks were found closer to the forest edge and peak values increased significantly. Decreasing the wind speed caused an increase of the concentration peak, while the flux peak remained unaffected. For an adequate interpretation and design of micrometeorological measurements near forest edges, it is necessary to identify the regions (fetches) with enhanced concentrations and fluxes. We therefore analyzed different approaches for a proper fetch estimation.

KW - Large-eddy simulation

KW - Enhanced scalar fluxes

KW - Fetch determination

KW - Forest-edge flow

KW - Micrometeorological measurements

KW - Scalar concentration peak

UR - http://www.scopus.com/inward/record.url?scp=84919904057&partnerID=8YFLogxK

U2 - 10.1007/s10546-014-9986-3

DO - 10.1007/s10546-014-9986-3

M3 - Article

AN - SCOPUS:84919904057

VL - 155

SP - 1

EP - 27

JO - Boundary-Layer Meteorology

JF - Boundary-Layer Meteorology

SN - 0006-8314

IS - 1

ER -