Coherent Momentum Exchange above and within a Scots Pine Forest

Abstract

Biorthogonal decomposition (BOD) is used to detect and study synchronous coherent structures occurring at multiple levels in the vertical momentum flux (u 1 w 1 ) within and above a planted Scots pine forest during a 12-week continuous measurement period. In this study, the presented method allowed for the simultaneous detection and quantification of the number of coherent structures (N), their duration (D) and separation (S) at five measurement heights (z 1 -z 5 ) covering the range z 1 /h = 0.11 to z 5 /h = 1.67, with h being the mean stand height at the measurement site. Results presented for five different exchange regimes (C1-C5) and for four different atmospheric stability conditions (stable, transition to stable, near-neutral, forced convection) demonstrate that during the measurement period, above-canopy momentum flux was only to a limited extent involved in the evolution of spatiotemporal momentum flux patterns found within the below-canopy space. Fully-coupled turbulent momentum exchange over the investigated height range occurred during 19% of all analyzed half-hourly datasets. Across the analyzed exchange regimes, the median contribution of strong sweeps and ejections to total momentum transfer above the canopy varied between 30% and 39% while covering 28%-32% of the time. In the below-canopy space, the contribution of coherent structures varied between 19% and 21% while covering the same amount of time. This suggests that momentum transfer through synchronous coherent structures is very efficient above the forest canopy, but attenuated in the below-canopy space. Since the majority of the presented results agrees well with the results from previous studies that analyzed coherent structures at single levels, the BOD is a promising tool for the consistent investigation of synchronous coherent structures at multiple measurement heights.

Assessment of the Response of a Scots Pine Tree to Effective Wind Loading

“…The airflow data from this height was chosen because it allows a close relationship to the dynamics of the tree reactions to be established and the effective wind load components to be estimated [36,37]. The dominant above-canopy wind direction sectors at the research site are south and southwest [47].…”

Analysis of Air Pressure Fluctuations and Topsoil Gas Concentrations within a Scots Pine Forest

“…The half-hourly time series of wind vector data were despiked and a double rotation was applied [50]. In addition, the data of heights -were rotated in the mean wind direction at , ensuring that for the time series of all heights the same coordinate system was used.…”

“…The canopytop stability parameter = / was calculated for every 30 min interval [50]. All available halfhourly data sets were then assigned to the six stability classes listed in Table 2.…”

“…Five exchange regimes (C1 − C5) were defined based on the correlation of coherent structures derived from momentum flux analysis over all measurement heights [50]. Exchange regime C1 is associated with uncoupled momentum flux above and below the forest canopy, while exchange regime C5 is associated with fully coupled momentum flux from to .…”

“…That means, during times specified as C1, no momentum is transferred from downwards. On the other hand, during times specified as C5, momentum is transferred from down to the forest floor at [50].…”