Scalar turbulent behavior in the roughness sublayer of an Amazonian forest

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 81.6m 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 degrees < vertical bar Z vertical bar < 20 degrees. 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 degrees < vertical bar Z vertical bar < 20 degrees, 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

The Relaxed Eddy Accumulation Method Over the Amazon Forest: The Importance of Flux Strength on Individual and Aggregated Flux Estimates

“…They are also clearly different in stable (mean of 0.6049) and unstable (mean of 0.5564) conditions. While analyzing the REA method at the same site with a different dataset, Zahn et al (2016b) obtained similar values for β s under unstable conditions, namely a mean of 0.5287 between the medians of β θ , β q and β c at 39.4 m and 0.5847 at 81.6 m. Thus, it appears that β s increases slightly with height in the RSL over the canopy in unstable conditions.…”

“…Since the "variance method" has shown to be a good indicator of the breakdown of MOST in the RSL (Dias et al 2009;Zahn et al 2016b;Chor et al 2017;Dias-Júnior et al 2019), and even in a classical inertial sublayer (Zahn et al 2023), we test its standard form…”

“…In the roughness sublayer (RSL) over a forest there is strong evidence of large departures of scalar behavior from MOST (Dias et al 2009;Zahn et al 2016b;Chor et al 2017). Zahn et al (2016b) found large departures for all 3 scalars (temperature, H 2 O and CO 2 ) measured at the ATTO (Amazon Tall Tower Observatory) site in Central Amazonia (see description below), but noted that scalar similarity improved significantly for small zenith angles.…”

Improving Wind Speed Forecasting for Urban Air Mobility Using Coupled Simulations

“…Barnes Method. Because Barnes scheme was initially designed for horizontal filters, we are using the generalized 3D Barnes scheme developed by Lu and Broning [40] as defined by (5) where L h and L v are the horizontal and vertical length scale parameters:…”

“…e RSL is the layer near and within the surface roughness elements (e.g., trees and buildings) where routine UAS operations will take place, and urban corridors and vertiports are installed. However, the MOST is not applicable within the RSL [3][4][5]. Moreover, the RSL has not been explicitly considered in global and mesoscale atmospheric models because the PBL is coarsely resolved.…”