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 the others 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 driver (driver2oasis.f90) which mimics the atmosphere, is a single processor code which just reads some forcing files and provides that data to OASIS. It is a prototype for any atmospheric model which would send 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 land surface model is an independent executable which has its own restart mechanism and history system and can thus be developed independently from the atmospheric component.

The driver for OASIS : driver2oasis.f90

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

As the original ORCHIDEE driver (dim2_driver.f90) is outdated and inflexible it has been redeveloped here.

For the moment this code only reads compressed by gathering forcing files which contain a sufficient amount of meta-data (Description_Forcing_Files.pdf).

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 at lowest atmsophric level	K	TAIRDRIV	TEMPLEV1
Atmospheric moisture at lowest atmsophric level	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
Surface drag	s/m

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

The main, in the FORTRAN sense is very similar to the intersurf.f90 which as been used up to now. The main difference is that it obtains the atmospheric data by issuing the needed OASIS_get calls. It will perform the following actions :

• Set-up the domain decomposition of the grid it has been given.

• Set-up the restart and history mechanisms.

• Start the OASIS exchanges and obtain the atmospheric data.

• Call sechiba_main

• Send to the atmosphere the flux computed

• Write to the history files

• return to the next call to OASIS in order to obtain the atmospheric data for the next time step.

Configuration of this ORCHIDEE version

As OASIS does not provide a mechanism to exchange time and grid descriptions between two coupled models some extra information needed to be added to the run.def. Obviously the coupling mechanism itself also needs to be documented in the namecouple of OASIS.

Added information in the run.def

• The information on the length of the simulation which has always been used in the run.def is insufficient to ensure that the driver and ORCHIDEE run over the same period. In order to solve this the run.def now needs to include a start and an end date for the simulation. The two keywords are (The dates provided need to be in the ISO format and be exact (include hour, minutes and seconds).) :
  • START_DATE = 1990-01-01 0:0:0
  • END_DATE = 1990-03-01 0:0:0

• The time step is not any more provided as a decomposition of forcing time step but rather in actual seconds to be used (driver2oasis.f90 takes care of doing the right temporal interpolation) :
  • TIME_STEP = 900

• In the same way, the distribution of the average rainfall provided by the forcing over the finer time stepping of the model is given in seconds :
  • SPRED_PREC_SEC = 3600

• As the time interval to be simulated the more than one forcing file might be needed. Thus the option which names the forcing file accepts a list of files :
  • FORCING_FILE = WFDEI_CRU_2008.nc WFDEI_CRU_2009.nc

• The description of the region over which to run has been simplified as well (this is only read by the driver2oasis.f90 in the initialisation pahse (when run with the -init option) as after this step all this information is in the grid file) :
  • WEST_EAST = -19.3, 62.3
  • SOUTH_NORTH = -6.3, 62.3

• A file containing the grid description is needed to ensure that the driver and the model use the same geographical domain :
  • GRID_FILE = EuroMed_grid.nc

The grid description file

The grid file is generated by the driver2oasis or by the atmospheric model. In the case of an off-line simulation this file is generated in an initialisation call (driver2oasis -init) based on the forcing file. The program will read the forcing file, compute the land/ocean mask and other ancillary information. We will ensure that orchideeoasis.f90 (the main program for ORCHIDEE) will be able to read also grid description files of all atmospheric models it is coupled to. WRF will be the first case implemented.

This file needs to provide the following information to ORCHIDEE :

• Longitude of all points
• Latitude of all points
• land/sea mask (1 over land O over oceans)
• area of each grid box
• corners in lon and lat of the corners of the grid box
• Index of land point in the global grid
• neighbours
• ...

As more models are integrated in the coupling model the information to be provided by the grid file will be refined.

The namecouple

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