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| Titel |
Uncertainty analysis of analytical flux footprint models |
| VerfasserIn |
Anneke van de Boer, Alexander Graf, Arnold F. Moene, Dirk Schüttemeyer |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051240
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| Zusammenfassung |
| During the last decade, a growing number of footprint models have been developed to
estimate the source area for measured surface fluxes. While a large part of recent progress is
focused on advancements in the numerical treatment of complexity, analytical or
parameterized models still are used most in practise to operationally determine whether the
source area of the measurements is representative. These models may be expected to perform
best in a flat landscape with small differences in roughness length. Ideally, the measured
fluxes themselves would also be homogeneous to fulfil all assumptions underlying analytical
models. However, spatial contrasts in at least one flux are a prerequisite not only
to the need for footprint models, but also to their validation with natural tracer
experiments.
In this study, eddy covariance flux measurements from a flat heterogeneous landscape
dominated by three land use types are presented. Three EC-stations were positioned in a
wheat, barley and sugar beet field, with 3D sonic anemometers and open-path infrared gas
analyzers at 2.5 m and 6.0 m height. A seventh equipment was installed near the
boundary between the barley and the sugar beet field. We use the models of Hsieh et
al. (2000) and Kormann and Meixner (KM, 2001) to calculate the contribution of
each land use type to each station. The Gaussian crosswind dispersion from KM
is used for both models. The results of both models, as well as of different ways
to obtain input data such as roughness length and stability, are compared to each
other.
Results show that the KM and the Hsieh model predict footprints of similar magnitude
with some minor differences in the shape and the weight of parts of the footprint. The
relative contribution of each land use type following from the combination of the
model output and a land use map is sensitive to the resolution of the land use map.
An effort to validate both models by predicting the fluxes obtained at the station
with a heterogeneous footprint from fluxes measured at stations with a sufficiently
homogeneous footprint indicates that in many cases the models may have predicted
too large footprints. We discuss these findings against the background of existing
literature evidence on footprint model validation. We also discuss implications
for any effort to derive flux estimates from multiple point measurements above a
heterogeneous landscape, a detailed example of which will be given in a companion
poster.
References
Hsieh, C.I., Katul, G., Chi, T.: 2000. An Approximate Analytical Model for Footprint
Estimation of Scalar Fluxes in Thermally Stratified Atmospheric Flows. Adv. Water Res. 23:
765-772.
Kormann, R., Meixner, F.X.: 2001. An Analytical Footprint Model for Non-Neutral Stratification.
Boundary-Layer Meteorol. 99: 207-224. |
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