The dependence of canopy layer turbulence on within-canopy thermal stratification

Abstract

Transport properties near the Earth's surface are strongly influenced by the thermal stratification of the atmosphere. Until now, no distinction has been made between thermal stability parameters within and above a plant canopy, and it has been usual to classify canopy transport processes in terms of above-canopy stability parameters only. The question arises, however, whether such parameters adequately describe within-canopy properties because it is often the case that thermal stratification differs considerably between air layers above and below the top of the canopy. In the present study, two within-canopy thermal stratification parameters have been defined and tested to determine whether they yield additional information about canopy turbulence. It appears that a within-canopy bulk Richardson number provides useful information under low-wind nocturnal conditions. Strongly unstable conditions inside dense canopies commonly occur at night when the air layers above the canopy are very stable, resulting in a deeoupling between the above-and within-canopy regions. A local withincanopy Obukhov length proved to be less useful, perhaps because the sensible heat flux within the canopy was nearly always directed upwards, regardless of the temperature gradient. A penetration length scale, defined for daytime conditions only, was of the order of the height of the canopy. This suggests that the height of the canopy is a suitable length scale for within-canopy processes.

Nighttime exchange processes near the soil surface of a maize canopy

“…4 for the two selected days both turbulent velocity scales have been plotted along with the Obukhov stability length scale. This result demonstrates that during calm nights the within-canopy velocity scale, w, , dominates, hence that this velocity scale determines the within-canopy turbulence (Jacobs et al, 1992. During the convective period it is important to know how the available energy at the soil surface, R,(Oh) -G(Oh), which equals the sensible heat and latent heat at the soil surface, H(Oh) + LE(Oh), behaves in comparison to the sum of the convective fluxes,…”

“…During daytime there usually exists a strong coupling between the above-canopy weather regime and the within-crop state. Then the above-canopy exchange mechanism dominates the within-canopy exchange mechanism (Goudriaan, 1989;Jacobs et al, 1992Raupach, 1988;Seginer et al, 1976;Shaw et al, 1989). During nighttime, however, when the above-canopy wind regime drops, only a weak coupling between both mechanisms by downdraught penetration occurs.…”

“…In order to make an assessment of the sensible heat flux at the soil surface the following procedure is suggested for calm nighttime conditions. During calm nights when the wind drops, a free convection state can develop at the canopy floor (Leclerc et al, 1991;Jacobs et al, 1992). Free convection will dominate the exchange mechanism if the dimensionless Rayleigh number, Ra, exceeds the criterion (Monteith, 1980;Gates, 1980;Kreith and Bohn, 1986; …”

Vertical and horizontal distribution of wind speed and air temperature in a dense vegetation canopy

“…Then, within the canopy, a free convection state occurs in which free convection cells are generated by the relatively warm canopy floor (Jacobs et al, 1992). By radiative cooling, the air at the upper part of the vegetation is stabilized and thus the unstable lower vegetation layer is capped and thereby decoupled from the above-canopy region.…”

“…Jacobs et al / Journal of Hydrology 166 (1995) 313-326 315 where u(z) and T(z) are the mean wind speed and mean air temperature, respectively, at height z, d is displacement height, z0 is roughness length, u* is the friction velocity, 7" is the scaling temperature defined by T* = -w'T'/u*, t~ --0.4 is Von Khrm~in's constant, L is Obukhov's stability length scale, and ~m and ~b h are stratification correction functions for momentum and heat, respectively. During daytime, the within-canopy processes are dominated by the large eddy exchange mechanism (Finnigan and Raupach, 1987;Jacobs et al, 1992). Under these conditions, an appropriate within-canopy scaling velocity and scaling temperature may equal the above-canopy friction velocity, u*, and scaling temperature, T ~, respectively.…”