The ACASA (Advanced Canopy–Atmosphere–Soil Algorithm) model, with
a higher-order closure for tall vegetation, has already been
successfully tested and validated for homogeneous spruce
forests. The aim of this paper is to test the model using
a footprint-weighted tile approach for a clearing with
a heterogeneous structure of the underlying surface. The comparison
with flux data shows a good agreement with a footprint-aggregated
tile approach of the model. However, the results of a comparison
with a tile approach on the basis of the mean land use
classification of the clearing is not significantly different. It is
assumed that the footprint model is not accurate enough to separate
small-scale heterogeneities. All measured fluxes are corrected by
forcing the energy balance closure of the test data either by
maintaining the measured Bowen ratio or by the attribution of the
residual depending on the fractions of sensible and latent heat flux
to the buoyancy flux. The comparison with the model, in which the
energy balance is closed, shows that the buoyancy correction for
Bowen ratios

The comparison of modeled and measured energy and matter fluxes in
a heterogeneous landscape is still a challenge. The fluxes measured
with the eddy covariance technique are related to all surfaces on the
upwind side of the measurements, and the influence of each surface on
the measured data is given by the footprint function. The comparison
of these flux measurements with one-dimensional models can be done over
a homogeneous surface easily or over a heterogeneous surface if the
model is parameterized for all surface characteristics and averaged
according to the percentage of each surface in the footprint

A further problem is the so-called unclosed energy balance. The energy
balance with turbulence measurements

Simulating the turbulent transfer for heterogeneous landscapes
utilizing a one-dimensional SVAT model (soil–vegetation–atmosphere
transfer) represents a multifaceted challenge. For forest, coherent
structures are a typical phenomenon of turbulent exchange

This paper is based on the earlier study by

The experimental data for the initialization of the model and the
evaluation of its outputs were collected during the third intensive
observation period (IOP3) of the EGER (ExchanGE processes in
mountainous Regions) project

The experimental site (50

The measurements of IOP3 were carried out in a spruce forest next to
the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay) and in a nearby
clearing with heterogeneous low vegetation located to the south of the
FLUXNET site (Fig.

The forest consists mainly of Norway spruce (

Vegetation at the forest and the Köhlerloh clearing with their main characteristics according to

During IOP3, high-frequency turbulence measurements were conducted at
different measurement towers (Fig.

The ACASA model needs half-hourly meteorological input values as well
as mean values of soil temperature and soil moisture for
initialization. These input values (see Table

Three golden day periods (GDPs) were selected within IOP3 because of fair weather conditions and predominantly good performance of the measurement devices. The first GDP was from 26 to 29 June 2011 (corresponding to the 177th to 180th day of the year, DOY). On the last 3 days of this period, clear-sky conditions prevailed until 14:00 as did moderate westerly (26 and 27 June) or easterly (28 and 29 June) winds. The second GDP occurred from 4 to 8 July 2011 (DOY: 185 to 189), with the best weather conditions on 6 and 7 July and partly overcast sky on the rest of the days. The wind in this period was weak and blew from the west (4, 7, and 8 July) or east (5 and 6 July). Minor amounts of precipitation were measured on 4 and 8 July. For the last GDP from 14 to 17 July 2011 (DOY: 195 to 198) data are missing for the clearing at 5.5 m.

Meteorological instrumentation for the determination of the
sensible and the latent heat flux as well as the net ecosystem
exchange (NEE) for forest and clearing at different measurement
towers in distinct heights, using the eddy covariance technique
(with the frequency of 20 Hz applied as indicated), and
additionally, the most important input parameter of the model
(for this purpose the mean

ACASA

Direct and diffuse radiation can be absorbed, transmitted, or
reflected by the canopy, whereby these processes are dependent on the
leaf and branch distribution of the plants. For this purpose, the
aboveground biomass is distributed in 10 different leaf angle classes

About 10 years ago the University of Bayreuth started to work with
ACASA. The first issue was to use a sensitivity analysis to check
whether the model could be applied for a Central European spruce
forest

The eddy covariance data on sensible heat, latent heat, and carbon dioxide flux have been energy balance corrected for comparison with the simulations.

For the correction of the energy fluxes, the residual (Res)
arises from the following equation:

The discrepancy between measured and simulated NEE can be an effect of
the unclosed energy balance on the

Footprint models are currently a commonly used tool for the
identification of the source area of flux measurements

In a study by

The footprint climatology of the mainly forested
Waldstein–Weidenbrunnen site is well described in many publications
like

As an overview, the footprint climatology for the period from 26 June
to 17 July 2011 has been calculated and is
a superposition of all individual
footprints. Figure

Footprint climatology of the Köhlerloh clearing (26 June
to 17 July 2011) at the turbulence mast (TM) for the three stability
classes and the combination of all classes;

In the following, we compare for the clearing the measured and
the energy-balance-corrected fluxes with the tile approach of the
model according to (i) the relevant footprint near the measurement
point and (ii) the land cover distribution of the whole clearing. It
was found that for the footprint of a 2.25 m height there are no
significant differences between the modeling approaches due to the
small size of the footprint (not shown). The results for TM with the
footprint for a 5.5 m measuring height are shown in
Fig.

From the visual comparison of Figs.

Comparison of ACASA model simulations with measured turbulent
fluxes at the TM (5.5 m measurement height) for the first GDP;

Same as Fig.

Neither the modeled nor the measured fluxes are free of errors and
could be used as an independent parameter. Therefore, the errors are
assumed to be similar and an orthogonal regression analysis for evenly
distributed errors has been applied

The analysis of the eddy covariance data shows that in all cases,
except for the latent heat in the second golden day period, the fluxes
are underestimated and an energy balance correction is necessary to
obtain an agreement with the modeled data. All results are very
similar for both golden day periods. For a discussion of the results
included in Fig.

The effect of the different energy balance corrections is not equal
for both sites. For the forest site, both corrections of the sensible
heat flux agree quite well with the model, with slightly better values
for the buoyancy correction. For the latent heat flux of the forest
site, the measurements corrected according to the Bowen ratio are in
better agreement with the simulations. For

Example of the regression analysis of the measured and the
simulated (ACASA) data for GDP 1 for

Results of the comparison of the measured and the modeled
flux data for different correction methods with an orthogonal
regression for binned data points for the clearing with and
without footprint weighting (for details, see
Fig.

Results of the comparison of the measured and the modeled flux data for different correction methods with an orthogonal regression for binned data points for the forest (absolute values in

However, for the clearing with

The correction of the NEE data seems to be necessary, but the
underestimation by the model is still given. This could be an
overestimation by the measured fluxes due to the increased mechanical
turbulence and consequently also turbulent fluxes caused by the
heterogeneous forest structure, as discussed by

In this section we used the modeled data, which close the energy
balance by definition as an etalon, for the validation of the
correction methods. Therefore, the modeled Bowen ratio was assumed to
be true and the measured fluxes have been corrected in such a way that
the energy balance was closed and the Bowen ratio agreed with the
modeled Bowen ratio. Finally, the fraction of the residual attributed
to the sensible heat flux has been determined and is shown in
Fig.

Fraction of the residual attributed to the sensible heat flux
for the forest site (GDP 1, 2, and 3), under the assumption that the
model calculated the true Bowen ratio, and according to the
correction methods with the Bowen ratio

Main daily cycle of the

The comparison of the modeled and the measured fluxes has shown that
the ACASA model determines the fluxes for high and low vegetation with
high accuracy and within the typical measurement uncertainty

The ACASA model, which was originally developed for tall vegetation, can also be used with high accuracy for low vegetation, as was demonstrated for the clearing, if the plant-specific parameters are appropriately implemented in the model. Therefore, it is applicable to consider ACASA in tile approaches for heterogeneous areas for tall and short vegetation or clearings.

The footprint-averaged tile approach did not produce significantly
better results than the tile approach for the whole clearing, as was
done by

The small underestimation by the model of fluxes above the forest is
probably a local overestimation of the fluxes at the turbulence tower
(TT), which is adjacent to an area of the forest where higher fluxes are
possible

Assuming that the ACASA model is well parameterized and the available
energy is accurately distributed to the sensible and the latent heat
flux, a good agreement has been found with the energy-balance-corrected measurements. The correction with the buoyancy flux leads to
better results, but this depends on the Bowen ratio; i.e., for

An overview of the instrumentation and important
measurements at the Waldstein–Weidenbrunnen site is provided by

The paper is based on the master's degree thesis of KG, who was advised by AR and TF and supported by WB (application of the footprint modeling and turbulence data calculation), EF (plant parameters), and RDP and KTPU (application of ACASA). The additional analysis in this paper and the writing was done by KG, who was advised by TF.

The authors declare that they have no conflict of interest.

The first author acknowledges Ina Tegen for providing the opportunity to finalize this paper at the Leibniz Institute for Tropospheric Research. We thank the editor and the three anonymous reviewers for the helpful comments.

This research was funded within the DFG projects FO 226/16-1 and ME 2100/4-1 as well the DFG PAK 446 project, mainly through the subprojects ME 2100/5-1 and FO226/21-1, the German Federal Ministry of Education, Science, Research and Technology (PT BEO 51-0339476 D), and the BaCaTeC (Bayerisch-Kalifornische Hochschulzentrum) project Modellierung des Energieaustausches zwischen der Atmosphäre und Waldökosystemen. Partial support came from a grant from the US National Science Foundation EF1137306 to the Massachusetts Institute of Technology, sub-award 5710003122 to the University of California, Davis. This publication was funded by the German Research Foundation (DFG) and the University of Bayreuth within the funding program Open Access Publishing. Edited by: Andreas Ibrom Reviewed by: three anonymous referees