We developed a coupled regional climate system model based on the CCLM
regional climate model. Within this model system, using OASIS3-MCT as a
coupler, CCLM can be coupled to two land surface models (the Community Land
Model (CLM) and VEG3D), the NEMO-MED12 regional ocean model for the
Mediterranean Sea, two ocean models for the North and Baltic seas (NEMO-NORDIC
and TRIMNP+CICE) and the MPI-ESM Earth system model. We first present the
different model components and the unified OASIS3-MCT interface which handles
all couplings in a consistent way, minimising the model source code
modifications and defining the physical and numerical aspects of the
couplings. We also address specific coupling issues like the handling of
different domains, multiple usage of the MCT library and exchange of 3-D
fields. We analyse and compare the computational performance of the different
couplings based on real-case simulations over Europe. The usage of the LUCIA
tool implemented in OASIS3-MCT enables the quantification of the contributions
of the coupled components to the overall coupling cost. These individual
contributions are (1) cost of the model(s) coupled, (2) direct cost of
coupling including horizontal interpolation and communication between the
components, (3) load imbalance, (4) cost of different usage of processors by
CCLM in coupled and stand-alone mode and (5) residual cost including i.a. CCLM
additional computations. Finally a procedure for finding an optimum processor
configuration for each of the couplings was developed considering the time to
solution, computing cost and parallel efficiency of the simulation. The
optimum configurations are presented for sequential, concurrent and mixed
(sequential+concurrent) coupling layouts. The procedure applied can be
regarded as independent of the specific coupling layout and coupling details.
We found that the direct cost of coupling, i.e. communications and horizontal
interpolation, in OASIS3-MCT remains below 7 % of the CCLM stand-alone cost
for all couplings investigated. This is in particular true for the exchange of
450 2-D fields between CCLM and MPI-ESM. We identified remaining limitations
in the coupling strategies and discuss possible future improvements of the
computational efficiency