Stability Dependence of Canopy Flows over a Flat Larch Forest |
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| Authors: | N Kobayashi T Hiyama |
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| Institution: | (1) Atmospheric Boundary Layer Group, I & O T Division, Indian Institute of Tropical Meteorology, Dr Homi Bhabha Road, Pune, 411 008, India;(2) F R Division, Indian Institute of Tropical Meteorology, Dr Homi Bhabha Road, Pune, 411 008, India;(3) P M & A Division, Indian Institute of Tropical Meteorology, Dr Homi Bhabha Road, Pune, 411 008, India |
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| Abstract: | The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch
forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two
heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that
of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly
stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the
RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra.
This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence.
The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind
spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant
eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised
at nearly half its value under stable conditions. |
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