Abstract
Abstract. An important current problem in micrometeorology is the characterization of turbulence in the roughness sublayer (RSL), where most of the measurements above tall forests are made. There, scalar turbulent fluctuations display significant departures from the predictions of Monin–Obukhov similarity theory (MOST). In this work, we analyze turbulence data of virtual temperature, carbon dioxide, and water vapor in the RSL above an Amazonian forest (with a canopy height of 40 m), measured at 39.4 and 81.6 m above the ground under unstable conditions. We found that dimensionless statistics related to the rate of dissipation of turbulence kinetic energy (TKE) and the scalar variance display significant departures from MOST as expected, whereas the vertical velocity variance follows MOST much more closely. Much better agreement between the dimensionless statistics with the Obukhov similarity variable, however, was found for the subset of measurements made at a low zenith angle Z, in the range 0° < |Z| < 20°. We conjecture that this improvement is due to the relationship between sunlight incidence and the “activation–deactivation” of scalar sinks and sources vertically distributed in the forest. Finally, we evaluated the relaxation coefficient of relaxed eddy accumulation: it is also affected by zenith angle, with considerable improvement in the range 0° < |Z| < 20°, and its values fall within the range reported in the literature for the unstable surface layer. In general, our results indicate the possibility of better stability-derived flux estimates for low zenith angle ranges.
Highlights
In the atmospheric surface layer above the roughness sublayer (RSL) height z∗, flux estimates based on mean concentration measurements are made with the help of Monin–Obukhov similarity theory (MOST) and the corresponding similarity functions
We found that dimensionless statistics related to the rate of dissipation of turbulence kinetic energy (TKE) and the scalar variance display significant departures from Monin– Obukhov similarity theory (MOST) as expected, whereas the vertical velocity variance follows MOST much more closely
We evaluated the relaxation coefficient of relaxed eddy accumulation: it is affected by zenith angle, with considerable improvement in the range 0◦ < |Z| < 20◦, and its values fall within the range reported in the literature for the unstable surface layer
Summary
In the atmospheric surface layer above the roughness sublayer (RSL) height z∗ (approximately 3 times the height of the roughness obstacles, h – Cellier and Brunet, 1992), flux estimates based on mean concentration measurements are made with the help of Monin–Obukhov similarity theory (MOST) and the corresponding similarity functions. Cellier and Brunet (1992) propose a dimensionless factor γ to account for the increase in turbulent diffusivity Following this suggestion, Schween et al (1997), using data measured over a 12 m tall oak and pine tree forest, found γθ = 2.2. Other attempts to organize roughness sublayer data include the use of z/z∗ by Cellier (1986) and the Mölder et al (1999) proposal of a function (z/z∗)n multiplying the dimensionless gradients: Mölder et al (1999) found n = 1 for scalars and n = 0.6 for momentum; they claim that the use of this factor produces acceptable results. Typical of micrometeorological studies, this somewhat low number of usable runs is bound to limit, for example, the percentage of reliable fluxes that can be retrieved in long-term studies Parallel efforts will be required to increase data availability and representativeness
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