Sub-Project Field Measurements
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[Objectives] [Measurements] [References] [Pictures] [Setup]
Responsible Department
Objectives
- Recording of the turbulent flow and analysing of the fluxes (momentum, energy, and mass) along a transect through a heterogeneous forest stand with high spatial (horizontal ca. 50 m, vertical 10 m) and temporal (20 Hz) resolution
- Analysis of the turbulent structure and classification of different modes
- Comprehensive inventories of all the trees using geodetic methods and determination of structure parameters of the canopy measured by 3D-Laser technique
- Derivation of model parameters (drag coefficients, mixing length) from vegetation structure and turbulence measurements
- Setup of a data basis for the validation of model results as reference
Introduction to the Measurements
Vegetation Mapping
During the first phase of the project the structure of the canopy of the Fluxnet site Tharandter Wald (Grünwald & Bernhofer, 2007, Prescher et al 2010) was recorded with great attention to detail (Queck et al, 2012). The results of this recordings served as a basis for the creation of the physical model used in the wind tunnel. Beside of the detailed forest assessment and single tree recordings (position, height, breast height diameter and crown area) a 3D-laser scanning was conducted (see SP Vegetation Structure). Hereby the canopy was scanned from more over 20 different positions including positions from the top of the towers. Thus not only the lower portion of the canopy could be scanned but also the topside. You will find an expanded description on the web-site forest assessment.
Wind- and Temperature Measurements
Period: 01.05.2008 – 01.5.2009,
See also the detailed description on page ==> Setup
Based on the experience of campaigns designed to measure the
advection of energy and trace gases (Feigenwinter et al., 2004,
Moderow et al., 2007, Feigenwinter et al. 2008), the existing
infra structure of the site was extended by 3 additional towers
(40m, 40m, 30m) and five ground stations. Thus we were able to
record wind profiles on four positions on a transect over the
forest clearing “Wildacker”, which were supported by the ground
stations in a height of 2 m (see web site Setup Fig. 2).
Applying 25 ultra sonic anemometers (sonics) we recorded the
turbulent flow around the forest clearing on 32 positions. A
network of 19 sensors was operated permanently (heights 2 m, 10
m, 20 m 30 m and 40 m), Additionally, 13 positions were
temporarily operated to fill in gaps in between the main
positions.
The raw data were recorded with a frequency of 20 Hz to cover
the relevant spectral range and to allow a later spectral
analysis of the momentum and energy exchange (Spank und
Bernhofer, 2008).
As a further extension of the setup two thermo-couple profiles
(type: E, Chromel-Konstantan, thickness: 12.5 µm) were
installed, one at tower on the clearing and the other one at
the Fluxnet tower. Selected results are available on the
web-site results of the wind
measurements and in the Data Base .
Source Area Experiment "Black-Days"
Period: 26.9.08 13:00 - 30.9.08 13:00
During a special observation period, an artificial modification
of the grassland on the clearing was conducted. 2000 m² of the
clearing were covered by a black foil. Thus almost the complete
net radiation was converted into the sensible heat flux. The
radiation balance was measured on the clearing and over the
forest. Additionally a thermal camera was mounted on the tower
to survey the surface temperature on the clearing.
References
Feigenwinter C, Bernhofer C, Eichelmann U, Heinesch B, Hertel M, Janous D, Kolle O, Lagergren F, Lindroth A, Minerbi S, Moderow U, Mölder M, Montagnani L, Queck R, Rebmann C, Vestin P, Yernaux M, Zeri M, Ziegler W, Aubinet M (2008) Comparison of horizontal and vertical advective CO2 fluxes at three forest sites. Agric For Meteorol 148:12–24 Feigenwinter C, Bernhofer C, Vogt R (2004) The influence of advection on the short term CO2-budget in and above a forest canopy. Bound-Layer Meteorol 113:201–224Grünwald T, Bernhofer C (2007) A decade of carbon, water and energy flux measurements of an old spruce forest at the Anchor Station Tharandt. Tellus B 59:387–396 Moderow U, Feigenwinter C, Bernhofer C (2007) Estimating the components of the sensible heat budget of a tall forest canopy in complex terrain. Boundary-Layer Meteorology 123:99–120 Prescher A-K, Grünwald T, Bernhofer C (2010) Land use regulates carbon budgets in eastern Germany: From NEE to NBP. Agricultural and Forest Meteorology 150:1016–1025
Queck R, Bienert A, Maas H-G, Harmansa S, Goldberg V, Bernhofer C (2012) Wind fields in heterogeneous conifer canopies: parameterisation of momentum absorption using high-resolution 3D vegetation scans. Eur J Forest Res 131:165–176 Spank U, Bernhofer C (2008) Another Simple Method of Spectral Correction to Obtain Robust Eddy-Covariance Results. Bound-Layer Meteorol 128:403–422
Pictures:
Setup of the field measurements (March/April 2008, all fotos © Chair of Meteorology)