The TurbEFA Field Experiment—Measuring the Influence of a Forest Clearing on the Turbulent Wind Field

Abstract

Forest ecosystems play an important role in the interaction between the land surface and the atmosphere. Measurements and modelling efforts have revealed significant uncertainties in state-of-the-art flux assessments due to spatial inhomogeneities in the airflow and land surface. Here, a field experiment is used to describe the turbulent flow across a typical Central European forest clearing. A three-dimensional model of the inhomogeneous forest stand was developed using an innovative approach based on terrestrial laser-scanner technology. The comparison of the wind statistics of two measurement campaigns (5 and 12 months long) showed the spatial and temporal representativeness of the ultrasonic anemometer measurements within the canopy. An improved method for the correction of the vertical velocity enables the distinction between the instrumental offsets and the vertical winds due to the inclination of the instrument. Despite a 13 % fraction of deciduous plants within the otherwise evergreen canopy, the effects of phenological seasons on the velocity profiles were small. The data classified according to the wind speed revealed the intermittent nature of recirculating air in the clearing. Furthermore, the development of sub-canopy wind-speed maxima is explained by considering the velocity moments and the momentum equation (including measurements of the local pressure gradient). Clearings deflect the flow downward and feed the sub-canopy flow, i.e., advective fluxes, according to wind speed and, likely, clearing size, whereas local pressure gradients play an important role in the development of sub-canopy flow. The presented dataset is freely available at the project homepage.

Turbulent transport of energy across a forest and a semiarid shrubland

“…Several studies have attempted to study the nature of turbulence across a roughness transition such as a grassland and a forest canopy by means of experimental and numerical methods (Li et al, 1990;Peltola, 1996;Irvine et al, 1997;Belcher et al, 2003;Yang et al, 2006;Cassiani et al, 2008;Detto et al, 2008;Dupont and Brunet, 2009;Dalpe and Masson, 2009;Fesquet et al, 2009;Gavrilov et al, 2010Gavrilov et al, , 2011Huang et al, 2011;Rominger and Nepf, 2011;Schlegel et al, 2012;Banerjee et al, 2013;Chatziefstratiou et al, 2014;Markfort et al, 2014;Kanani-Sühring and Raasch, 2015;Queck et al, 2016;Kröniger et al, 2017) and documented several length scales associated with the roughness transitions, recirculation zones and the nature of the turbulent momentum budget. However, all of these studies are concerned with the flow adjustment in the immediate vicinity of the roughness transition (edges or gaps).…”

Review article: A European perspective on wind and storm damage – from the meteorological background to index-based approaches to assess impacts

“…maximum gradient of wind speed) is weaker in heterogeneous compared to homogeneous canopies and that it occurs deeper within the canopy; i.e. the displacement height is lower (Cionco, 1972;Belcher et al, 2012;Queck et al, 2016). Furthermore, homogeneous forests are more vulnerable than naturally uneven aged woods (Everham and Brokaw, 1996;Mitchell, 2013).…”

Realistic Forests and the Modeling of Forest‐Atmosphere Exchange

“…TKE and Reynolds stresses decay faster behind closed edges and strong cross flows may develop. Away from the edges, large gaps and clearings deflect the flow downwards, creating advective fluxes within the forest air space (Queck et al, 2016). Boudreault et al (2017) use TLS measurements (outlined in Boudreault et al [2015]) and LES to model air flow across a forest edge.…”