L.Tuomi1, L.Axell2, V.Huess3, and J. Murawski4
1
Finnish Institute of Marine Research (FIMR), Finland
Swedish Meteorological and Hydrological Institute (SMHI), Sweden
3
Danish Meteorological Institute (DMI), Denmark
4
Federal Maritime and Hydrographic Agency (BSH), Germany
2
EVR1-CT-2002-20012
www. boos.org/papa/papa.html
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Abstract
This report describes different techniques to couple atmospheric and ocean models. Also
the effect of the coupling to the atmospheric and ocean model results is discussed. During
the past ten years there have been several experiments to couple atmospheric and ocean
models. In general the coupling has been reported to have a positive, although quite modest,
impact on both atmospheric and ocean model results. Several different techniques have
been used to couple the models. Most of the techniques are specific for the models and
computer environments used. Although at the moment there is no general coupling method
that is suitable for all the models and all computer environments, there has been attempts to
develop such a coupling library. Also special surface layer modules are being developed to
properly describe the air-sea interaction between the atmosphere and the ocean.
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Contents
1 Introduction.................................................................................................................3
2 Background.................................................................................................................3
3 Coupled models tested and used by PAPA partners...................................................4
3.1 Coupled atmosphere-wave models..............................................................4
3.2 Coupled atmosphere-ocean model...............................................................5
3.3 Coupled wave-ocean model.........................................................................5
4 Models used by PAPA partners..................................................................................6
5 Technical aspects of coupling.....................................................................................6
6 The need for development in models and coupling techniques..................................8
7 Summary.....................................................................................................................8
References......................................................................................................................8
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1 Introduction
Two-way coupling between the atmosphere and the ocean is important for the computation
of momentum, heat and moisture exchange across the air-sea interface. While the ocean
models are necessarily forced by the fields calculated by the atmospheric models, the
atmospheric models can carry out the computation of the atmospheric circulation without
accounting for the fields computed by the ocean model. Usually atmospheric models use
climatological values or real-time observations at the beginning of the forecast run for
parameters like SST and certain parameters that are dependent on the sea state like
Charnock parameter are kept constant. The ocean models usually get the atmospheric
forcement with large time interval (6 hours is common). This means that rapid changes for
example in the wind directions cannot be taken into account. One of the purpose of the
PAPA Model group is to prepare plans for a general coupling interface between
atmospheric and oceanographic models.
2 Background
The first effort in Europe to couple atmospheric and ocean models was during the MAST 2
project ECAWOM (1994-1996). The ECAWOM model (European Coupled AtmosphereWave-Ocean Model) consists of three independent models: the atmospheric model
HIRLAM (HIgh Resolution Limited Area Model), the wave model WAM (WAve Model)
and the ocean circulation model HAMSOM (HAMburg Self Ocean Model) in the version
of Programa de Clima Maritimo (Weisse and Alvarez, 1997). The model is divided into
four different modules: atmosphere, ocean, waves and coupling, which are controlled by a
main program. The parameters transferred between the models (Fig.1) are supplied to the
coupling module where they are stored and if necessary physically transferred and rescaled.
Figure 1. Parameters exchanged between the atmospheric model, the ocean model and the
wave model.
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The results from the fully coupled model showed that in a strong atmospheric cyclone,
compared with one-way coupling, two-way coupling attenuates the depth of the pressure
minimum, significantly reduces the wave height and surface wind speed, while the
momentum flux increases. The heat and moisture fluxes are increased if they are computed
using the same wave dependent roughness that is used for the momentum flux, while they
are decreased if they are computed using the Charnock relation. The effects are
proportionally larger for extreme storms (Lionello et al. 1997).
The first coupled model for operational forecasting in Europe was developed and set
up by ECMWF. The model consists of ECMWF's global atmospheric model (TL511L60)
and the wave model WAM which are tightly coupled e.g. the wave model is a subroutine of
the atmospheric model. The atmospheric model and the wave model interact with each
other every coupling time step, i.e. the atmospheric model is run for a coupling time step,
then the wave model is called using the lowest level winds produced by the atmospheric
model after which the atmospheric model is run for the next coupling time step, now using
the Charnock parameter as determined by the WAM model during the previous interaction
period (Janssen et al., 2001). The coupled atmosphere-wave model has been in operational
use at ECMWF since 1998. The results from the coupled model showed that the two-way
interaction has only a modest positive impact on the scores of the atmospheric parameters.
Extreme events, where one would expect a substantial impact of two-way interaction do not
occur very frequently. The impact on the wave height scores was found to be more
substantial. However, the wave forecast depends to a large extent on the quality of the
small-scale features in the driving wind field.
Studies with coupled models have also been done for example by Doyle (1995,
2002), Lionello et al. (1998 and 2003) and Lalbeharry et al. (2000). Common to most of the
experiments with coupled models is that coupled models give slightly better results than
uncoupled models, but during extreme storms greater differences can be seen. The
significance of air-sea coupling to the model results has been questioned by Weisse et al.
(2000). However, they focused on the consequences for regional atmospheric modelling.
Consequences for wave and storm surge modelling were not considered.
3 Coupled models tested and used by PAPA partners
Different types of coupled models have been developed and tested by BSH, DMI, FIMR
and SMHI. FIMR has also used their coupled model operationally for a one-year period.
3.1 Coupled atmosphere-wave models
DMI has studied coupling between the atmospheric model HIRLAM and the wave model
WAM. The coupling was implemented into the models via an MPI interface. The wind
fields at 10 meters height were transferred from the atmospheric model to the wave model
and sea state dependent Charnock parameter from the wave model to the atmospheric
model every coupling time step.
This study was encouraged by the results of the ECMWF's coupled model (e.g.
Janssen et al. 2001). The purpose of this study was to evaluate the results obtained with a
more realistic description of the momentum exchange between the sea surface and the
overlying atmosphere. When comparing the coupled model results with the uncoupled
HIRLAM results Sattler et al. (2002) found that the coupling reduced wind speeds and had
an impact on the bias in wind speed regardless of the sign of the bias. The standard
deviation was reduced through the coupling. The results indicate that the synoptic scale
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forecast improvements are best achieved if the coupling is implemented for a large model
domain. The coupling affected the wave fields by reducing the wave height. This implies
that for the low wave height range, the coupling reduces the models tendency to
overestimate the wave height, whereas for the high waves the coupling unfortunately has a
tendency to reduce the already underestimated wave heights.
FIMR has developed a coupled model together with the Finnish Meteorological
Institute (FMI). The development of the model was encouraged by the coupled model
results in the ECAWOM project in which FIMR participated. The coupling technique is
based on the coupling in the ECAWOM model. The FIMR-FMI coupled model consists of
the atmospheric model HIRLAM and the wave model WAM. The models are tightly
coupled, e.g. the wave model is run as a subroutine of the atmospheric model. The
parameters exchanged are the lowest level wind speed and sea surface roughness/sea state
dependent Charnock parameter. The coupled model was operational for a one-year period.
Due to changes in the super computer the coupled model cannot be run at the moment.
To estimate the effect of the coupling on the atmospheric and wave parameters a
test period was selected when both the coupled model and pure atmospheric model were
run simultaneously. During the selected period a small positive impact (due to coupling) on
the atmospheric winds was seen. For the wave parameters the impact was greater mainly
due to the small wind input time step (two minutes). Significant differences between the
coupled and uncoupled models could only be seen during extreme events (Järvenoja and
Tuomi, 2002).
3.2 Coupled atmosphere-ocean model
SMHI has tested a loose coupling between the atmospheric model HIRLAM and the 2.5dimensional ice-ocean model BOBA-PROBE. Loose coupling in this case means that SST
and the initial sea ice distribution from the ice-ocean model were used as lower boundary
conditions in the atmospheric model and were kept constant during the whole forecast. The
ice-ocean model was theb run using forcing from the HIRLAM fields, which resullted in
new fields of ice and SST to be used by HIRLAM in the next forecasts. Gustafsson et al.
(1998) found that certain forecast situations require two-way coupling, such as sudden ice
breakup due to strong winds. However, for short-range forecasts it was considered
satisfactory to initialize the weather forecasting model with SST and sea ice conditions.
3.3 Coupled wave-ocean model
BSH has studied coupling of a three dimensional hydrodynamical model with a 2-D
spectral wave model. The wave model implemented is the k-model and the circulation
model used is that of the Federal Maritime and Hydrographic Agency (BSHcmod). The
model is a tightly coupled model where the wave model is run as a subroutine of the
circulation model. Water level variations and surface current fields from the circulation
model are transferred to the wave model. The radiation stress from the wave model is then
transferred to the circulation model.
The coupled model was tested only with an artificial wind field of magnitude 20 m/s
to study coupled wave-current interaction for near coastal regions. During a storm surge
with high water level an accurate wave setup prediction is fundamental. The additional
wave induced flow is also important for the transport of sediment and suspended matter and
therefore for the time evolution of the coastal environment.
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The modulation of the sea state by changing water depth especially in tidal regions is
significant.
4 Models used by PAPA partners
The variety of models used by PAPA partners can be seen in table 1. There are only few
commonly used models like the wave model WAM and the ocean circulation model
HIROMB. A few models are shared between the institutes, like BSHcmod. For such a
variety of models, a general coupling method is not easily found. Although the physics of
the boundary layer and therefore also the variables transferred can be agreed on to some
extent, the technical transfer of parameters can be done in several different ways.
5 Technical aspects of coupling
Several methods to couple models are known. Coupling can be done via an MPI interface
(like DMI) or models can be tightly coupled like models of BSH, FIMR/FMI and ECMWF
model, or loosely coupled as at SMHI. Also some general coupling libraries have been
developed. An example of this kind of libraries is OASIS (Ocean Atmosphere Sea Ice Soil).
It was developed in CERFACS to answer the requirements of climate modelling (Valcke et
al. 2000).
In general the coupling through a special library is more flexible to changes in
model versions or to implementations of new models. However, the use of general coupling
libraries requires special knowledge and usually sets requirements also for the computer
resources. Other types of coupling, like running the ocean model as a subroutine of the
atmospheric model, are more commonly used at the moment.
A Coupling library should be able to couple different types of models and it should
work on different computer platforms. The coupling library should also be able to do the
following tasks
1.
2.
3.
4.
5.
Exchange of coupling fields
Horizontal interpolation of coupling fields
Check the values transferred
Mask land points
Time interpolation of the coupling fields
Running coupled models sets requirements also for the computer resources. As
described previously better results with coupled models can be achieved if models are run
with high resolution in large areas. Also according to Janssen et al. (2001) to obtain an
impact of the ocean waves on the atmospheric circulation a high spatial resolution
atmosphere (and wave model) is required. In operational short-range
forecasting the time consumed in the model run has to be relatively small. To be able to run
large model areas with high-resolution super computers or linux clusters etc.
have to be used and usually parallel model codes are needed. It is not possible to obtain
parallel versions of all model codes and some of the parallelized model codes work only on
certain super computer types.
Table 1. Models used by PAPA partners
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Wave models
HYPAS/HYPNE
k-Model
OSW
SPM
SWAN
WAM
WAVEWATCH
Ocean circulation BSHcmod
models
MSI circulation model
HIROMB
Mike21
MIT GCM
ocean model by Andrejev
and Sokolov
POM
Drift models
Ice models
Ecological models
SMHI
BSH
RDANH
NWAHEM (SPB SOI)
NWAHEM (SPB SOI)
DMI, FIMR, MIG, MSI,
NWAHEM (SPB SOI)
NWAHEM (SPB SOI)
BSH, DMI
MSI
RDANH,SMHI,SYKE
DMI
FIMR
FIMR,
NWAHEM
(SPB SOI)
MIG, NWAHEM (SPB
SOI)
BSHdmod.L/BSHdmod.E BSH, DMI
MikeSA/PA
DMI
FIMR-PATS
FIMR
SEATRACKWEB
RDANH, SMHI
Gulf of Finland drift SYKE
model
part of BSHcmod
BSH, DMI
FIMR ice model
FIMR
BOBA-PROBE
SMHI
HIROMB
SMHI
3-D Baltic Sea ecosystem SYKE
model
MSI ecological model
MSI
Norwecom, Branded and FIMR
non-branded
complex
ecological models
SCOBI
SMHI
SPM transport
BSH
in BSHdmod.E
6 The need for development in models and coupling techniques
As discussed previously, ocean models are always forced by the fields calculated by an
atmospheric model. However the atmospheric models are usually run without calculated
SST or sea state. Instead climatological values or the latest observations are used. The
models have been built in a way that they to some extent take into account the exchanges of
momentum, heat and moisture in the air-sea interface. Ocean models, however, usually
have simplified boundary layers. For example in the WAM model the surface stress is
calculated from the surface winds using equations for a neutral boundary layer in all
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conditions. Atmospheric models usually have more accurate description of the boundary
layer but the feedback from the ocean is ignored.
To properly describe the air-sea interaction between the atmospheric models and
ocean models a realistic parameterisation of the physical processes in the boundary layer is
required. This is attempted for example in the PRISM project (Program for Integrated Earth
System Modelling), where a special surface layer module is being developed (Guilyardi et
al., 2003). The project aims at designing a flexible environment to easily assemble and run
earth system models with physically based conservation of fluxes across coupling
interfaces. However the feedback from the waves to the atmosphere and the ocean are
presently not included in the PRISM surface layer model.
7 Summary
Coupling of atmospheric and ocean models has been found useful, although the significant
improvements are during extreme storms. However, these storms are just the ones that need
to be predicted properly. Coupling can be done with several techniques even though the
parameters exchanged based on the physics of air sea interaction are well defined. Different
technical methods to couple the models can be selected depending on the models used and
technical resources. Considering all the models used by the PAPA partners and the
different computer resources, a general coupling method is difficult to recommend. Each
institute should choose the method best suitable for their models and computer
environments.
When planning the development and implementation of coupled models for
operational use one should also consider that for operational use, short execution times are
essential. This usually means running the models in super computers. The ability to use
coupled models in operational forecasting depends on the national computer resources and
available parallel model codes.
References
Doyle, J.D., 1995: Coupled ocean wave/atmosphere mesoscale model simulations of
cyclogeneses. Tellus, 47A, 766-778.
Doyle, J D., 2002: Coupled Atmosphere-Ocean Wave Simulations under High Wind
Conditions. Mon. Wea. Rev., 130, 3087-3099.
Guilyardi, E., R. Budich, G. Brasseur and G. Komen (ed). 2003: PRISM System
Specification Handbook, version1.0.
Gustafsson, N., L.Nyberg and A.Omstedt, 1998: Coupling of a High-Resolution
Atmospheric Model and Ocean Model for the Baltic Sea., Mon. Wea. Rev., 126, 28222846.
Janssen, P.A.E.M., J.D. Doyle, J. Bidlot, B.Hansen, L.Isaksen and P.Viterbo, 2001: Impact
and feedback of ocean waves on the atmosphere. ECMWF Technical Memorandum
No.341, 32p.
Järvenoja, S. and L. Tuomi, 2002: Coupled atmosphere-wave model for FMI and FIMR,
Hirlam Newsletter.
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Lalbeharry, R., J.Mailhot, S.Desjardins, and L.Wilson, 2000: Examination of the impact of
a coupled atmospheric and ocean wave system. Part II: Ocean wave aspects. J. Phys.
Oceanogr., 30, 402-415.
Lionello,P., H.Bergstöm, A.Buzzi, H.Hersbach, J.Højstrup, U.Högström, P.A.E.M.Janssen,
H.K.Johnson, K.K.Kahma, G.Komen, S.E.Larsen, V.K.Makin, P.Malguzzi,
K.Mastenbroek, W.Oost, H.Pettersson, A.S.Smedman, H.J.Vested, L.Zampato, 1997:
ECAWOM: MA group Final Report.
Lionello, P., P.Malguzzi and A.Buzzi, 1998: Coupling between the atmospheric circulation
and the ocean wave field: An idealized case. J. Phys. Oceanogr., 28, 161-177.
Lionello,P., G.Martucci and M.Zampieri,2003: Implementation of a coupled atmospherewave-ocean model in the Mediterranean Sea: Sensitivity of the short time scale evolution to
the air-sea coupling mecanisms. The Global Atmosphere and Ocean System, 9, 65-95.
Sattler K., J.She, B.H. Sass, L.Laursen, L. Landberg, M.Nielsen, H.S.Christensen, 2002:
Enhanced description of the wind climate in Denmark for determination of wind resources.
Scientific Report, 02-09, Danish Meteorological Institute.
Valcke,S., L. Terray and A. Piacentini, 2000: Oasis 2.4, Ocean atmosphere sea ice soil:
user's guide. Technical Report, CERFACS, Toulouse, France.
Weisse, R. and E.F.Alvarez, 1997: The European Coupled Atmosphere Wave Ocean Model
ECAWOM. Max-Planck-Institut für Meteorologie, Rep. No. 238, Hamburg, Germany,
34pp.
Weisse,R., H.Heyen and H. von Stroch, 2000: Sensitivity of a regional atmosphere model
to a sea state-dependent roughness and the need for ensemble calculations. Mon. Wea.
Rev.,128,3631-3642.
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