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Chances of the global-regional two-way nesting approach
Philip Lorenz and Daniela Jacob
Max Planck Institute for Meteorology, Hamburg, Germany; philip.lorenz@zmaw.de
1. Introduction
Large scale atmospheric processes influence smaller ones,
which in turn affect the evolution of the regional climate:
this paradigm is the basis for one-way nested simulations
with regional climate models (RCMs) driven by data from
general circulation models (GCMs).
However, small scale processes are at least influencing large
scale processes too and could have in key regions significant
impacts on the evolution of the general circulation: this
paradigm was the main motivation for the development of a
two-way nested GCM-RCM climate model system, in which
feedback from the RCM to the GCM takes place in a
selected region (see figure 1). This feedback is accounting
for processes which are not resolved by the relative coarse
resolution of the GCM, but which are resolved by the finer
resolution of the RCM.
One-way nesting
Two-way nesting
GCM RCM GCM RCM
Figure 1. Illustration of one-way nesting versus twoway
nesting.
Another motivation for performing two-way in contrast to
one-way nested RCM simulations is a higher compatibility
between the RCM's internal dynamics and the lateral
boundary conditions. The GCM adapts to the large scale
state of the RCM by the two-way nesting technique
feedback, and is providing therefore more consistent
boundary data.
For investigating the former mentioned questions, a twoway
nested global – regional climate model system has been
developed and applied in several simulations. A description
of the model system and introductions to different analysis
will be given within the following sections.
2. The two-way nested model system
The developed two-way nested model system consists of
Max Planck Institute (MPI-M) climate models, the GCM
ECHAM4 and the RCM REMO. Both the spectral GCM
ECHAM4 and the grid-point RCM REMO use the same set
of physical parameterisations. The feedback from the RCM
to the GCM on all prognostic variables takes place at every
time of the GCM (24 min. in T42 resolution). More details
about the two-way nested model system are given in Lorenz
and Jacob (2005).
space), and the RCM in a 0.5° (~55 km) horizontal
resolution.
3. Influence on the general circulation
For the analysis of the effects of the two-way nesting
approach on the general circulation, a stand-alone
ECHAM4 run has been performed and compared against
two-way nested GCM-RCM runs. At least for the two-way
nested region covering the tropical Maritime Continent a
positive influence on the general circulation was analyzed
and published in Lorenz and Jacob (2005).
4. One-way versus two-way nested RCM
simulations
Additional one-way nested RCM simulations have been
carried out for some of the domains used for the two-way
nested GCM-RCM simulations. The comparison of oneway
versus two-way RCM results reveal a significant
reduction of typical RCM boundary artefacts (like
unrealistic precipitation close to the lateral boundaries) in
the two-way nested simulations, and furthermore an
influence on the interior of the regional model domains.
5. Influence of the lateral boundary data update
frequency in one-way nested simulations
Most state of the art RCM’s use 6-hourly output from
GCMs or (re-)analysis as lateral boundary data. Within
the framework of the two-way nested model system it is
possible to perform one-way nested RCM simulations
using GCM output down to a time interval of 24 minutes,
which is the internal time step of ECHAM4 in T42
resolution. For the domain covering the Maritime
Continent only very small differences were found between
RCM runs with the usual 6 hourly update frequency and
runs with an increased update frequency of 24 minutes.
6. Conclusions
An overview of the major results of the investigations
within the two-way nested model system framework will
be presented; and prospects and limits of the two-way
nesting approach will be discussed.
References
Lorenz, P. and Jacob, D., Influence of regional scale
information on the global circulation: a two-way
nesting climate simulation, Geophysical Research
Letters, Vol.32, L18706, doi:10.1029/2005GL023351,
2005
10-year integrations using observed sea surface temperature
data (AMIP; 1980-1989) have been carried out with the twoway
nested model system for different two-way nested
regions. In all simulations the GCM was applied in a
spectral T42 horizontal resolution (~250 km in grid-point