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| Titel |
Flow tilt angles near forest edges – Part 2: Lidar anemometry |
| VerfasserIn |
E. Dellwik, J. Mann, F. Bingöl |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 5 ; Nr. 7, no. 5 (2010-05-26), S.1759-1768 |
| Datensatznummer |
250004782
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| Publikation (Nr.) |
 copernicus.org/bg-7-1759-2010.pdf |
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| Zusammenfassung |
A novel way of estimating near-surface mean flow tilt angles from ground
based Doppler lidar measurements is presented. The results are compared with
traditional mast based in-situ sonic anemometry. The tilt angle assessed with
the lidar is based on 10 or 30 min mean values of the velocity field from a
conically scanning lidar. In this mode of measurement, the lidar beam is
rotated in a circle by a prism with a fixed angle to the vertical at varying
focus distances. By fitting a trigonometric function to the scans, the mean
vertical velocity can be estimated. Lidar measurements from (1) a
fetch-limited beech forest site taken at 48–175 m a.g.l. (above ground
level), (2) a reference site in flat agricultural terrain and (3) a second
reference site in complex terrain are presented.
The method to derive flow tilt angles and mean vertical velocities from lidar
has several advantages compared to sonic anemometry; there is no flow
distortion caused by the instrument itself, there are no temperature effects
and the instrument misalignment can be corrected for by assuming zero tilt
angle at high altitudes. Contrary to mast-based instruments, the lidar
measures the wind field with the exact same alignment error at a multitude of
heights.
Disadvantages with estimating vertical velocities from a lidar compared to
mast-based measurements are potentially slightly increased levels of statistical errors
due to limited sampling time, because the sampling is disjunct, and a
requirement for homogeneous flow. The estimated mean vertical velocity is
biased if the flow over the scanned circle is not homogeneous. It is
demonstrated that the error on the mean vertical velocity due to flow
inhomogeneity can be approximated by a function of the angle of the lidar
beam to the vertical and the vertical gradient of the mean vertical velocity,
whereas the error due to flow inhomogeneity on the horizontal mean wind speed
is independent of the lidar beam angle. For the presented measurements over
forest, it is evaluated that the systematic error due to the inhomogeneity of
the flow is less than 0.2°.
The results of the vertical conical scans were promising, and
yielded positive flow angles for a sector where the forest is
fetch-limited. However, more data and analysis are needed for a
complete evaluation of the lidar technique. |
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