Turbulent exchange above a pine forest,I: Fluxes and gradients |
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| Authors: | Ulf Högström Hans Bergström Ann-Sofi Smedman Sven Halldin Anders Lindroth |
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| Institution: | (1) Department of Meteorology, Box 516, S-751 20 Uppsala, Sweden;(2) Section of Hydrology, V. »gatan 24, S-752 20 Uppsala, Sweden;(3) Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden |
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| Abstract: | Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fluxes have been carried out over a sparse pine forest at Jädra»s in Sweden. In order to ascertain that correct gradient estimates were obtained, two independent measuring systems were employed: one system with sensors at 10 fixed levels on a 51 m tower and another with reversing sensors for temperature and humidity, covering the height interval 23 to 32 m. Turbulent fluxes were measured at three levels simultaneously. Data from three field campaigns: in June 1985, June 1987 and September 1987 have been analyzed. The momentum flux is found on the average to be virtually constant from tree top level, at 20 to 50 m. The average fluxes of sensible and latent heat are not so well behaved. The ratio of the non-dimensional gradients of wind and temperature to their corresponding values under  ideal conditions (low vegetation) are both found to be small immediately above the canopy (about 0.3 for temperature and 0.4 for wind). With increasing height, the ratios increase, but the values vary substantially with wind direction. The ratios are not found to vary systematically with stability (unstable stratification only studied). The ratio of the non-dimensional humidity gradient to the corresponding non-dimensional potential temperature gradient (equivalent to k
h
/k
w
) is found to be unity for (z – d)/L
v
less than about –0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large. |
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