Turbulent exchange above a pine forest, I: Fluxes and gradients

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 Jiidra&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 nondimensional potential temperature gradient (equivalent to KH/KW) is found to be unity for (I -d)/L, less than about -0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large.

Exchange of reactive nitrogen compounds: concentrations and fluxes of total ammonium and total nitrate above a spruce canopy

“…pastures, meadows). However, when using this relationship close to a canopy to infer fluxes from measured vertical concentration gradients, corresponding fluxes have been found to be underestimated (Thom et al, 1975;Garratt, 1978;Högström et al, 1989). Nevertheless, the flux-gradient relationship was found to hold within the so-called roughness sublayer (Garratt, 1978(Garratt, , 1992 above forest canopies, introducing a so-called enhancement factor into the left hand term of the right hand side of Eq.…”

On the Anomalous Behaviour of Scalar Flux–Variance Similarity Functions Within the Canopy Sub-layer of a Dense Alpine Forest

“…These convective structures transport to the surface dry and cold air from the upper mixed layer (as observed in Roth and Oke 1995) or dry and warm air entrained from above the capping inversion. While much attention has thus far centered on the TKE budget and FV similarity functions for T and q (Hogstrom et al 1989;Leclerc et al 1990;Lamaud and Irvine 2006), there is growing interest in exploring FV similarity relationships for carbon dioxide concentration (C), in addition to T and q. Contemporary applications utilizing FV similarity functions for C include, (1) evaluating components of the turbulent CO 2 flux budget near the canopy top for testing higher-order turbulent closure models (Juang et al 2006), (2) developing alternative gap-filling methods for estimating annual carbon budgets (Choi et al 2004), (3) comparing turbulent transport efficiencies between CO 2 and other scalars such as T and q to inspect similarities in the transfer pathways near the ground.…”

Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

“…Flux-gradient theories have been found to overestimate scalar fluxes within the roughness sublayer, which is the region from the ground to 2 or 3 times the canopy height because the turbulent structure is influenced (mechanically and thermally) by the canopy elements (Raupach and Thom, 1981;Högström et al, 1989;Simpson et al, 1998) and tall vegetation (Garratt, 1978). That said, the theory might be less compromised than previously thought above forests even at just 1.4 times the canopy height (Simpson et al, 1998).…”

“…K can be estimated by means of a variety of aerodynamic methods derived from energy or momentum balances (Högström et al, 1989;Celier and Brunet, 1992;Simpson et al, 1998;Foken, 2006). For example, K can be determined from…”