Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Objective Calibration of the Land Surface Model SEWAB
S.Huneke, K.-P.Johnsen, J.Geyer, H.Lohse, H.-T.Mengelkamp
GKSS Research Centre, D-21502 Geesthacht, Germany, (huneke@gkss.de)
1. Introduction
A complex land surface model is generally characterized
by a multitude of parameters, which are not exactly
known a priori. Therefore a model calibration is needed.
The success of a manual calibration essentially depends
on the experience of the modeler and their knowledge of
the basic approaches and interactions in the model. A
manual calibration therefore is always subjective to some
extent. Moreover, it can be extremely time consuming.
Methods of automatic calibration can improve these
shortcomings.
Following, the land surface model SEWAB (Surface
Energy and Water Balance) [1] is calibrated by use of the
global optimization algorithm SCE-UA (Shuffled
Complex Evolution – University of Arizona).
For the calibration and validation period measurements of
turbulent heat fluxes during the LITFASS 2003 campaign
(LITFASS = ‘Lindenberg Inhomogeneous Terrain –
Fluxes between Atmosphere and Surface: a Long-term
<strong>Study</strong>’) [2] within the project EVA-GRIPS (Regional
Evaporation at Grid/Pixel Scale over heterogeneous
Land Surfaces) are used. The measurement site is around
the Meteorological Observatory Lindenberg (MOL) of
the Deutscher Wetterdienst (DWD).
2. Measurements
On different types of landuse micrometeorological
stations (energy budget stations) have been measured
latent and sensible heat fluxes during the vegetation
period in May and June 2003.
Fig.1: Latent heat flux measurement over different types
of landuse in the LITFASS area on June 17, 2003, 30-
min-average.
Fig.2: Daytime evolution of the sensible heat flux over
three different types of landuse on June 17, 2003, 30min-average.
The systems have used the eddy covariance method to
obtain the turbulent fluxes. At every side a CSAT3 sonic
anemometer and a krypton hygrometer was installed.
Figures 1 and 2 show considerable differences across the
LITFASS area in the turbulent heat fluxes on June 17.
3. Calibration
In order to get an optimal parameter set for simulating the
turbulent heat fluxes, the SEWAB model is calibrated
with the SCE-UA algorithm. It minimizes an objective
function, which compares the measured and simulated
data (Fig. 3). In general the Nash-Sutcliff criteria is used
as an independent objective function.
model input:
I = {i 1 ,...,i m }
model parameters
θ = {θ1,...,θk} SEWAB-model
SEWAB output:
Φ (θ,Ι) = {γ (θ,Ι) 1,...,γ(θ,Ι) n}
optimization
-algorithm
SCE-UA:
E(θ) = min
objective function:
E(θ) = Φ (θ,Ι) − O
observation:
O = {o 1 ,...,o n }
Fig. 3: Flow diagram of SEWAB calibration with the
SCE-UA-algorithm.