Introduction

Today ORCHIDEE is designed to be called by a subroutine independently on each processor. This is implemented for instance in the traditional driver (dim2_driver.f90) :

This assumes that the domain decomposition of the atmospheric model or the driver are the same. It also means that the processors which have no land point to deal with are waiting for their pairs to finish.

In order to avoid these issues and generalise the coupling of ORCHIDEE with other atmospheric components the OASIS-MCT coupler can be used. In this case the atmosphere can work on any number of processors and ORCHIDEE on a distinct set of processors. This is exemplified in the driver2oasis presented below. Here the atmosphere is a single processor code which just reads some forcing file and provides that data to OASIS. But it is a model for any atmospheric model which would provide the same fields from its n processors to OASIS.

In the OASIS approach to coupling a number of advantages are gained :

• The atmosphere can run on a different set of processors than the land surface and have its own domain decomposition.

• ORCHIDEE will only execute on the processors attributed to it and only treat land surface points.

• After the atmosphere has sent its data to OASIS it can continue to work on processes which do not require the input of the land surface.

• In the same way, once the land surface has returned the variables needed by the atmosphere (i.e. after solving the surface energy balance) it can continue to work and prepare the next time step.

The driver for OASIS : driver2oasis.f90

This code simulates an atmospheric model which would provide provide data for ORCHIDEE through OASIS.

As the original ORCHIDEE driver (dim2_driver.f90) is outdated and inflexible it has been redeveloped here.

For the moment this code only read compressed by gathering forcing files which contain a sufficient amount of meta-data (link to document).

This code is aimed at running only on one processor. It should not be a limiting factor in the execution time as it only reads once the forcing and then only does some simple time interpolations. Furthermore these interpolations are done while ORCHIDEE runs, i.e. it is not blocked by an oasis_get call.

The fields which OASIS exchanges between the atmosphere and the surface

From the atmosphere to ORCHIDEE

Name	units	ID on driver side	ID on ORCHIDEE side
Height of T and Q	m	ZLTQDRIV	HEIGHTTQ
Height of wind	m	ZLUVDRIV	HEIGHTUV
Atmospheric temperature	K	TAIRDRIV	TEMPLEV1
Atmospheric moisture	kg/kg	QAIRDRIV	HUMILEV1
Rainfall	kg/m2/dt	RAINDRIV	RAINFALL
Snowfall	kg/m2/dt	SNOWDRIV	SNOWFALL
Downward solar radiation	W/m2	SWDODRIV	SHOWDOWN
Downward longwave radiation	W/m2	LWDODRIV	LONWDOWN
Solar angle	??	SOLADRIV	SOLARANG
Eastward atmsopheric wind	m/s	UWINDRIV	EASTWIND
Northward wind	m/s	VWINDRIV	NORTWIND
Surface pressure	Pa	PRESDRIV	SURFPRES

From the surface to the atmosphere

Name	units	ID on driver side	ID on ORCHIDEE side
Surface evaporation	kg/m2/dt	EVAPDRIV	TOTEVAPS
Sensible heat flux	W/m2	SENSDRIV	FLUXSENS
Latent heat flux	W/m2	LATEDRIV	FLUXLATE
Diffuse freshwater flux to the oceans	m3/dt	COASDRIV	COASTFLO
River flux into the ocean	m3/dt	RIVEDRIV	RIVERFLO
Net CO2 flux	??	NECODRIV	FLUNECO2
??	??	LUCODRIV	FLULUCO2
Surface radiative temperature	K	TRADDRIV	TSURFRAD
Surface temperature	K	TNEWDRIV	TSURFNEW
Surface humidity	kg/kg	QSURDRIV	QSURFNEW
Near Infra-red albedo	-	ANIRDRIV	ALBEDNIR
Visible albedo	-	AVISDRIV	ALBEDVIS
Surface emissivity	-	EMISDRIV	EMISLONW
Surface roughness	m	ROUGDRIV	ROUGNESS

dt : is the time step used by the models. We do not expect ORCHIDEE and the atmospheric model to use different time steps.

The "main" which controls the ORCHIDEE execution : orchideeoasis.f90

Execution of the model coupled to the driver

As OASIS is linked into both models (driver2oasis and orchideeoasis) the execution of the coupled model is simply achieved by one mpirun command. It can have the following structure :

mpirun -n $PEDRIV driver2oasis : -n $PEORCH orchideeoasis

where : PEDRIV=1 and PEORCH=$(($NSLOTS-$PEDRIV)), i.e. only one processor is for the driver and all others work on ORCHIDEE.

Execution of the model coupled to WRF