Turbulent passive scalar transport at the canopy interface: the role of sweeps and ejections

We present an experimental study of the vertical transport of a passive scalar, released from a localised source at the top of a two-height canopy. The study is based on simultaneous velocity and concentration measurements under neutrally stable conditions in an environmental wind tunnel. By analysing turbulent scalar diffusivity directly from our data, we identify regions of counter-gradient flux in the roughness sublayer above the canopy. Here, turbulent flux opposes the local mean concentration gradient. Quadrant analysis shows that sweep events ( w prime less than 0 comma u prime greater than 0 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msup> <mml:mi>w</mml:mi> <mml:mo>′</mml:mo> </mml:msup> <mml:mo>&lt;</mml:mo> <mml:mn>0</mml:mn> <mml:mo>,</mml:mo> <mml:msup> <mml:mi>u</mml:mi> <mml:mo>′</mml:mo> </mml:msup> <mml:mo>&gt;</mml:mo> <mml:mn>0</mml:mn> </mml:math> $w' \lt 0, u' \gt 0$ ) entrain low-concentration air from above into the canopy. Ejection events ( w prime greater than 0 comma u prime less than 0 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msup> <mml:mi>w</mml:mi> <mml:mo>′</mml:mo> </mml:msup> <mml:mo>&gt;</mml:mo> <mml:mn>0</mml:mn> <mml:mo>,</mml:mo> <mml:msup> <mml:mi>u</mml:mi> <mml:mo>′</mml:mo> </mml:msup> <mml:mo>&lt;</mml:mo> <mml:mn>0</mml:mn> </mml:math> $w' \gt 0, u' \lt 0$ ) mostly contribute to down-gradient scalar transport. We developed a semi-analytical model using an orthogonal series expansion of the three-dimensional joint probability density function. This model quantifies the imbalance between sweep and ejection contributions to vertical turbulent scalar flux. The results explain why gradient diffusion based models break down in canopy flows with an elevated source and highlight the importance of coherent motions in turbulent scalar exchange at the canopy–atmosphere interface. In addition, the measured concentration statistics were examined and showed deviations from classical models, particularly at high concentration values.