Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 102 -
Comparison of Methods for Area-Averaging Surface Energy Fluxes over Heterogeneous
Land Surfaces Using High-Resolution Non-Hydrostatic Simulations
Günther Heinemann 1 and Michael Kerschgens 2
1
Meteorologisches Institut der Universität Bonn, Auf dem Hügel 20, D-53121 Bonn, Germany;
2
Inst. für Geophysik und Meteorologie, Universität zu Köln, Kerpener Str. 13, 50937 Köln, Germany;
e-mail: gheinemann@uni-bonn.de
1. Introduction
The quantification of land surface heterogeneity effects for
the exchange processes between land surfaces and the atmosphere
is of vital interest for the energy budget of the
atmospheric boundary layer and for the atmospheric branch
of the hydrological cycle. The results presented in this paper
are based on high-resolution non-hydrostatic model simulations
for the LITFASS area near Berlin. This area represents
a highly heterogeneous landscape of 20x20 km² around the
Meteorological Observatory Lindenberg (MOL) of the
German Weather Service (DWD).
Figure 1: Idealized simulation of the 250m model for the
potential temperature and wind vector (only every second
vector) at 30 m valid at 11 UTC 17 June 1998. Stations of
the LITFASS-98 experiment are indicated.
2. Methods and model
Model simulations were carried out using the nonhydrostatic
model FOOT3DK (Shao et al., 2001) of the
University of Köln with resolutions of 1 km (F1) and 250 m
(F250). The performance of different area-averaging methods
for the turbulent surface fluxes were tested for the LIT-
FASS area, namely the aggregation method, the mosaic
method and the tile method. For the tile method, the experimental
setup of the surface energy balance stations of the
LITFASS-98 experiment was investigated (Beyrich et al.,
2002). Two different simulation types are considered: (1)
realistic topography and idealized synoptic forcing, and (2)
realistic topography and realistic synoptic forcing for LIT-
FASS-98 cases. A one-way nesting procedure is used for
nesting FOOT3DK in ‘Lokalmodell‘ (LM) of the DWD.
3. Results
The effect of the land surface heterogeneity on the lowlevel
wind field is most pronounced for weak wind situations.
Fig.1 shows the fields of the potential temperature
and the wind vector at 30 m valid for a simulation without
geostrophic forcing at local noon. The variations of the
landuse lead to mesoscale circulation patterns. These
patterns are most pronounced over the large forest area in
the western part of the F250 domain, where large roughness
lengths are associated with large sensible heat fluxes.
Another type of mesoscale circulation patterns can be seen
associated with the (relatively cold) lakes, where land-sea
breezes develop.
H 0 ,E 0 in W/m²
300
200
100
0
3 6 9 12 15 18 21
UTC
E 0
E 0,LS agg
E 0,LS mos
E 0,tile
Figure 2: Daily course of E 0 computed by different
averaging methods for the whole LITFASS area for an
idealized simulation of the F250 model (nesting in F1 with
a geostrophic forcing of 8 m s -1 ). Indices: agg = aggregation,
mos = mosaic, tile = tile method.
Fig. 2 shows the daily course of the latent heat flux E0
computed by different methods for the LITFASS area for
a simulation with a geostrophic forcing of 8 m s -1 . The
mosaic method shows good results, if the wind speed is
sufficiently high. During weak wind convective conditions,
errors are particularly large for the latent heat flux
on the 20x20 km² scale. The aggregation method yields
generally higher errors than the mosaic method, which
even increase for higher wind speeds. The main reason is
the strong surface heterogeneity associated with the lakes
and forests in the LITFASS area. The main uncertainty of
the tile method is the knowledge of the area coverage in
combination with the representative positions of the stations.
References
Beyrich et al., Experimental determination of turbulent
fluxes over the heterogeneous LITFASS area: Selected
results from the LITFASS-98 experiment, Theor.
Appl. Climatol. 73, 3–18, 2002.
Shao, Y. Sogalla, M. Kerschgens, M.J., Brücher, W.,
Treatment of land surface heterogeneity in a
mesoscale atmospheric model. Meteorol. Atm Phys.
78, 157-181, 2001.