The aqua-planet

OpenIFS includes a number of idealised configurations. In this article we explain how to configure the model for an 'aqua-planet' simulation. An aqua-planet is one that includes no topography and no land. It's a highly simplified representation of Earth but useful for a number of studies.

The aquaplanet configuration (enabled by namelist variable LAQUA) is based on the Neale & Hoskins publications:

Neale, R. B. and Hoskins B. J., 2000a, A standard test for AGCMs and their physical parameterizations. I: The proposal, Atmos. Sci. Letters, 1, pp. 101-107, DOI: 10.1006/asle.2000.0022
Neale, R. B. and Hoskins, B. J., 2000b, A standard test for AGCMs and their physical parameterizations. II: Results for The Met. Office Model, Atmos. Sci. Letters, 1, pp. 108-114, DOI: 10.1006/asle.2000.0024

More recent general publications on aqua-planet simulations:

Medeiros, B., et al,2008, Aquaplanets, climate sensitivity, and low clouds. Journal of Climate, doi: 10.1175/2008JCLI1995.1
Blackburn, M., et al., 2013, The Aqua-Planet Experiment (APE): CONTROL SST Simulation. Journal of the Meteorological Society of Japan, 91A. pp. 17-56. ISSN 2186-9057 doi: 10.2151/jmsj.2013-A02
Blackburn, M. and Hoskins, B. J., 2013, Context and aims of the Aqua-Planet Experiment. Journal of the Meteorological Society of Japan, 91A. pp. 1-15. ISSN 2186-9057 doi: 10.2151/jmsj.2013-A01

See also for example, the PCMDI Aqua-Planet website.

Please note that the aqua-planet and other idealized configurations are a research tool and therefore may change between model versions.

For further assistance with idealised configurations of OpenIFS, please contact: openifs-support@ecmwf.int.

Code changes

There is a small error in OpenIFS 38r1 in the code that sets the SST for the 3KW1 case of Neale & Hoskins. This code incorrectly sets the latitudinal width to 30^o instead of 60^o.
This error is corrected in later versions of OpenIFS.

The required code changes are small.

In the file: src/ifs/phys_ec/gp_sstaqua.F90, find the lines 233:238:

        ! 3KW1
        RCHI=3.0_JPRB
        IF( ABS(PGEMU(IOFF+JROF-1)) <= SIN(RPHID) ) THEN
          PSD_VF(JROF,YSD_VF%YSST%MP,IBL)=PSD_VF(JROF,YSD_VF%YSST%MP,IBL)+&
           &RCHI*COS(PGELAM(IOFF+JROF-1)-RLAMBDA0)*&
           &COS(0.5_JPRB*ZPI*(ASIN(PGEMU(IOFF+JROF-1))/RPHID))**2

The use of RPHID needs to be corrected to 60^o:

        ! 3KW1
        RCHI=3.0_JPRB
        IF( ABS(PGEMU(IOFF+JROF-1)) <= SIN(2._JPRB*RPHID) ) THEN                   !  correct use of RPHID on this line
          PSD_VF(JROF,YSD_VF%YSST%MP,IBL)=PSD_VF(JROF,YSD_VF%YSST%MP,IBL)+&
           &RCHI*COS(PGELAM(IOFF+JROF-1)-RLAMBDA0)*&
           &COS(0.5_JPRB*ZPI*(ASIN(PGEMU(IOFF+JROF-1))/(2._JPRB*RPHID)))**2        !  correct use of RPHID on this line

Configuration

Configuring OpenIFS for an aqua-planet simulation requires appropriate changes to:

Namelists: to enable the idealized configuration and initial fields
Initial files: to set orography & land-sea-mask.

In the sections below, we walk through the namelist options explaining how to configure the aqua-planet.

Dynamical core and SST

The model namelist NAMDYNCORE controls the configuration for idealized cases, including the aqua-planet.

To enable the aqua-planet edit the fort.4 file (containing the input model namelist) and make sure the settings are as below. You may find this namelist is blank in your file as it does not need altering for normal forecasts, in which case add the following:

&NAMDYNCORE
LDYNCORE=true,       ! switches on idealized planet configurations.
LAQUA=true,          ! switches on idealized aqua planet.
MSSTSCHEME=1,        ! controls SST choice for aquaplanet following Neale & Hoskins (2000a) (see ifs/module/yomdyncore.F90 for more details).
                     !  1 = Control; 2 = Peaked; 3 = Flat;  4 = Qobs;  5 = Control5N;  6 = 1KEQ, chi=1.0;  
                     !  7 = 3KEQ, chi=3.0, width=15 deg;  8 = 3KW1, chi=3.0, width=30 deg; 9 = const. SST
/

The SST modifications can be further changed using the variables defined in ifs/phys_ec/gp_sstaqua.F90:

RLAMBDA0       : Longitude of maximum SST anomaly
RLAMBDAD, RPHID: Half width in long/lat of SST anomaly
RCHI           : Maximum magnitude of SST anomaly

For more details of the SST configuration, see: ifs/phys_ec/gp_sstaqua.F90 and refer to the Neale & Hoskins (2001) publication.

If idealized initial states are used (see below) rather than real initial fields, it can be useful to introduce some initial noise into the vorticity field. For starting from analysed fields this is not needed:

&NAMDYNCORE
NOISEVOR=1,      ! if 1 add initial noise in vorticity

Mass conservation

Enable mass conservation correction to prevent mass drifting in long integrations.
Edit namelist NAMDYN in the fort.4 file and add (or change) this variable:

&NAMDYN
LMASCOR=true,        ! if true, apply mass correction.

Edit namelist NAMCT0 in fort.4 to set the mass correction frequency:

&NAMCT0
NFRMASSCON=24,      ! frequency of mass conversation fixups (time-steps)

Model physics

Aqua-planet simulations normally use the full model physics. By setting LEPHYS=false, the model would run an aqua-planet with all the physics switched off.

Ozone

For aqua-planet simulations the prognostic ozone scheme and transport of ozone should be disabled using these logicals in their namelists:

&NAMGFL
   YO3_NL%LGP=false,       ! turns off transport of prognostic ozone

&NAEPHY
   LEO3CH=false,           ! turns off prognostic ozone & uses climatological ozone instead

Physics namelist

The physics namelist NAEPHY is included here for completeness. If necessary edit the fort.4 file (containing the input model namelist) and make sure the settings are as below.

Note in this example, the model physics is turned on (enabled).

For more information on these switches see the relevant modules in the model code: yoephy.F90 (and yoewcou.F90 for the coupling to the wave model).

&NAEPHY
LEPHYS=true,     ! switch the full ECMWF physics package on/off.
LEVDIF=true,     ! turn on/off the vertical diffusion scheme.
LESURF=true,     ! turn on/off the interface surface processes.
LECOND=true,     ! turn on/off the large-scale condensation processes.
LECUMF=true,     ! turn on/off the mass-flux cumulus convection.
LEPCLD=true,     ! turn on/off the prognostic cloud scheme.
LEEVAP=true,     ! turn on/off the evaporation of precipitation
LEVGEN=true,     ! turn on/off Van Genuchten hydrology (with soil type field)
LESSRO=true,     ! turn on/off orographic (VIC-type) runoff
LECURR=false,    ! if true, ocean current boundary condition is used.
LEGWDG=true,     ! turn on/off gravity wave drag.
LEGWWMS=true,    ! turn on/off the Warner-McIntyre-Scinocca non-orographic gravity wave drag scheme.
LEOZOC=true,     ! turn on/off the climatological ozone.
LEQNGT=true,     ! turn on/off the negative humidity fixer.
LERADI=true,     ! turn on/off the radiation scheme.
LERADS=true,     ! turn on/off the interactive surface radiative properties.
LESICE=true,     ! turn on/off the interactive sea-ice processes.
LEO3CH=false,    ! turn on/off the ozone chemistry (for prognostic ozone).
LEDCLD=true,     ! turn on/off the diagnostic cloud scheme.
LDUCTDIA=false,  ! turn on/off computation and archiving of ducting diagnostics.
LWCOU=false,     ! turn on/off coupled wave model (n.b. always off for OpenIFS model version 38r1).
LWCOU2W=false,   ! turn on/off two-way interaction with the wave model (n.b. always off for OpenIFS model version 38r1).
/

Initialisation

OpenIFS (and IFS) can be initialized either with real fields or idealized starting conditions.This is controlled both by the namelist switch N3DINI and by the code in the model.

It is crucial that the orography, surface pressure and land-sea-mask are set correctly (see below).

&NAMCT0
N3DINI=0,     ! normally 0 to initialise the model from a real atmosphere, but see ifs/module/yomdyncore.F90 for other possibilities.

Orography, surface pressure and land-sea mask

In order to correctly run the aqua-planet in OpenIFS (and IFS), it is important that the orography field in the model is set to zero to ensure a flat surface. The surface pressure must also be adjusted to be consistent with the flat orography and the land-sea-mask in the model set to zero to indicate all sea-points.

There are two ways in which this can be achieved; either take an existing set of model initial files (the ICM*INI* files) and interpolate pressure to the surface, or run the model and adjust the orography and pressure whilst the model is running to prevent a sudden 'shock'.

Also see

ECMWF took part in the 2012 Dynamical Core Model Intercomparison Project (DCMIP). More information can be found here: https://www.earthsystemcog.org/projects/dcmip-2012/

Acknowledgements

OpenIFS would like to thank Sylvie Malardel (ECMWF) and Peter Bechtold (ECMWF) for their assistance in preparing this material