IPCC Standard Output from Coupled Ocean-Atmosphere GCMs

WGCM Climate Simulation Panel¹ with the assistance of PCMDI²

For further information contact: taylor13@llnl.gov

¹ Panel members include: Gerald Meehl (chair), Curt Covey, Mojib Latif, Bryant McAvaney, John Mitchell, and Ron Stouffer.

² Contributing PCMDI members include: Karl E. Taylor, Curt Covey, Krishna AchutaRao, Michael Fiorino, Peter J. Gleckler, Thomas J. Phillips, and Kenneth R. Sperber.

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30 March 2007

· Introduction and format requirements

· Highest priority output fields

Table A1a: Monthly-mean 2-d atmosphere or land surface data (longitude, latitude, time:month)

Table A1b: Time-independent 2-d land surface data (longitude, latitude)

Table A1c: Monthly-mean 3-d atmosphere data (longitude, latitude, pressure, time:month)

Table O1a: Monthly-mean 1-d ocean data (latitude, region, time:month

Table O1b: Monthly-mean 2-d ocean data (latitude, depth, region, time:month)

Table O1c: Monthly-mean 0-d or 2-d ocean or sea ice data (longitude, latitude, time:month)

Table O1d: Time-independent 2-d ocean data (longitude, latitude)

Table O1e: Monthly-mean 3-d ocean data (longitude, latitude, depth, time:month)

Table A2a: Daily-mean 2-d atmosphere data (longitude, latitude, time:day)

Table A2b: Daily-mean 3-d atmosphere data (longitude, latitude, pressure, time:day)

Table A3: 3-bourly 2-d atmosphere data (longitude, latitude, time:3hour at 0, 3, 6, 9, 12, 15, 18, 21 Z)

Table A4: Extremes indices (longitude, latitude, time:year) from Frich et al. (their Table 1)

· Lower priority output fields

Table A1d: Monthly-mean ISCCP simulator data (longitude, latitude, pressure2, tau, time)

Table A5: Monthly-mean 2-d radiative forcing data (longitude, latitude, time)

Table A1e: Monthly-mean 2-d and 3-d sulfate aerosol fields

Table O1f: Monthly-mean 1-d and 2-d ocean fields

Table O1g: Monthly-mean 2-d sea ice fields (longitude, latitude, time:month)

Table A1f: Monthly-mean surface fields and prescribed land surface characteristics

· Experiments and time periods for which data should be submitted

Introduction and format requirements

The following list of standard output for coupled GCMs is intended both to serve the immediate needs of the IPCC and to become a recommended "core set" of variables for CMIP.

Modeling groups contributing output to the IPCC and CMIP database must ensure that it meets rather strict format and metadata requirements. These requirements yield files produced in network Common Data Form (netCDF; see http://www.unidata.ucar.edu/packages/netcdf), which has become the most popular form for exchanging ocean-atmosphere model output. The files will be "self-describing" and the metadata contained in the files will conform with the NetCDF Climate and Forecast (CF) Metadata Conventions (see http://www.cgd.ucar.edu/cms/eaton/cf-metadata). The new CF conventions for netCDF data generalize and extend the Cooperative Ocean / Atmosphere Research Data Service (COARDS) conventions developed in the 1990s. Note that the CF convention establishes standard names for climate and weather variables, which identify the physical quantity. These standard names are given in the tables below. Note that more than one field can be associated with the same standard name because different fields sampled in different ways (e.g., surface air temperature vs. upper air temperature) refer to the same physical quantity. Nevertheless, one can uniquely identify each stored field by considering additional metadata stored in the file (e.g., dimension information). Extended definitions of CF standard names, which basically answer the question "What do you mean precisely by this quantity?", may be found on the Web at http://www.cgd.ucar.edu/cms/eaton/cf-metadata/standard_name.html.

The detailed requirements for CMIP / IPCC contributions are contained in the document http://www-pcmdi.llnl.gov/ipcc/IPCC_output_requirements.htm. This document should be read carefully before preparing contributions. Perhaps the easiest way to meet these requirements is to rewrite your model output through CMOR, a software library available from PCMDI and further described in the next paragraph. Put briefly, the requirements involve metadata, coordinate systems, and file organization. Units and sign conventions of the data must conform to the tables below. Latitude-longitude grids must be rectilinear, i.e., have a unique set of longitudes that applies to all latitudes. Data on non-rectilinear grids (typically occurring for ocean output) must be interpolated to rectilinear grids before transmission to the PCMDI. (In this case, the original "native grid" data, if deemed of sufficient value, may also be provided to the PCMDI.) Vertical coordinates must be depth for ocean variables and pressure for atmosphere variables (with the exception of cloudiness, which as noted below should be provided on model levels). Three-dimensional atmosphere variables must be interpolated to standard pressure levels given below. We also recommend -- but do not require -- that the ocean depth levels match Levitus observations. Finally, we require that submitted files contain only one output variable per file, though they may have many time points per file. This file organization contrasts with the typical model output history files, which contain all variables for a single time step.

To facilitate adherence to these standards, the PCMDI has written (in FORTRAN 90) a standard output code called CMOR (pronounced "see more"; see http://www-pcmdi.llnl.gov/software-portal/cmor). This code structures the data uniformly and writes netCDF files in full compliance with IPCC requirements. Use of CMOR is being encouraged (and in some cases required) by various ongoing model intercomparison projects. The CMOR documentation in pdf format is available at http://www-pcmdi.llnl.gov/software/cmor/cmor_users_guide.pdf, and the source code is available at http://www-pcmdi.llnl.gov/software-portal/cmor/download. For further information, contact taylor13@llnl.gov.

The notes that appear in the following tables are meant to provide precise definitions of the requested fields. Sometimes it may be impossible to satisfy the requests; in these cases, any deviations from the specifications below should be described in the "history" and/or "comment" attributes associated with the variable

Model output overview

The model output fields listed below are identified as either being "highest priority" (tables A1a-A1c, A2-A4, and O1a-O1e) or "lower priority" (tables A1d-A1f, A5, and O1f-O1g). The fields appearing in the "lower priority" tables will in some cases be essential for carrying out analyses of high interest (e.g., the radiative forcing fields are needed to help determine why models have different responses to anthropogenic influences); placement in the "lower priority" table may reflect one or more of the following factors: 1) perceived to be difficult to calculate (or lack of agreement as to calculation method), 2) nominated late, after the "highest priority" tables had been officially released, or 3) generally perceived to be of somewhat less interest than other fields.

Some of the variables in the tables below were required for the original 1997 version of CMIP2 or have been requested for contributions to the CMIP pilot project "20th Century Climate in Coupled Models" (20C3M). Many of the additional fields were requested by AMIP, the atmosphere-only counterpart of CMIP in which ocean surface and sea ice boundary conditions are prescribed to match observations over the late 20th century (see AMIP2 standard output). Suggestions for additions or changes to these tables are welcome and will be considered for future model intercomparison and IPCC exercises.

In most cases, variables that appear in the same table will all be associated with a single climate component (i.e., atmosphere, ocean, land, or sea ice) and will all be a function of the same spatial dimensions. Also the temporal sampling (3-hourly, daily, monthly, or time-independent) and any spatial or temporal averaging will in most cases be the same within each table.

Highest priority output fields

Monthly-mean and time-independent data

Table A1a: Monthly-mean 2-d atmosphere or land surface data (longitude, latitude, time:month).
	CF standard_name	output variable name	units	notes
1	air_pressure_at_sea_level	psl	Pa
2	precipitation_flux	pr	kg m^-2 s^-1	includes both liquid and solid phases.
3	air_temperature	tas	K	near-surface (usually, 2 meter) air temperature; the CMOR singleton dimension default value of 2 m can be overridden, if absolutely necessary, by redefining axis "height1".
4	moisture_content_of_soil_layer	mrsos	kg m^-2	water in all phases in the upper 0.1 meters of soil, and averaged over the land portion of the grid cell (i.e., compute by dividing the total mass of water contained in the soil layer of the grid cell by the land area in the grid cell); report as "missing" or 0.0 where the land fraction is 0; the CMOR singleton dimension default value of 0.1 m can be overridden, if absolutely necessary, by redefining axis "depth1".
5	soil_moisture_content	mrso	kg m^-2	water in all phases summed over all soil layers, and averaged over the land portion of the grid cell (i.e., compute by dividing the total mass of water contained in the soil layer of the grid cell by the land area in the grid cell); report as "missing" or 0.0 where the land fraction is 0.
6	surface_downward_eastward_stress	tauu	Pa
7	surface_downward_northward_stress	tauv	Pa
8	surface_snow_thickness	snd	m	this thickness when multiplied by the average area of the grid cell covered by snow yields the time-mean snow volume. Thus, for time means, compute as the weighted sum of thickness (averaged over the snow-covered portion of the grid cell) divided by the sum of the weights, with the weights equal to the area covered by snow. report as 0.0 in snow-free regions.
9	surface_upward_latent_heat_flux	hfls	W m^-2
10	surface_upward_sensible_heat_flux	hfss	W m^-2
11	surface_downwelling_longwave_flux_in_air	rlds	W m^-2
12	surface_upwelling_longwave_flux_in_air	rlus	W m^-2
13	surface_downwelling_shortwave_flux_in_air	rsds	W m^-2
14	surface_upwelling_shortwave_flux_in_air	rsus	W m^-2
15	surface_temperature	ts	K	"skin" temperature (i.e., SST for open ocean)
16	surface_air_pressure	ps	Pa	not mean sea-level pressure
17	snowfall_flux	prsn	kg m^-2 s^-1
18	convective_precipitation_flux	prc	kg m^-2 s^-1
19	atmosphere_water_vapor_content	prw	kg m^-2	vertically integrated through the atmospheric column
20	soil_frozen_water_content	mrfso	kg m^-2	summed over all soil layers, and averaged over the land portion of the grid cell (i.e., compute by dividing the total mass of frozen water contained in the soil layer of the grid cell by the land area in the grid cell); report as "missing" or 0.0 where the land fraction is 0.
21	surface_runoff_flux	mrros	kg m^-2 s^-1	compute as the total surface runoff leaving the land portion of the grid cell divided by the land area in the grid cell; report as "missing" or 0.0 where the land fraction is 0.
22	runoff_flux	mrro	kg m^-2 s^-1	compute as the total runoff (including "drainage" through the base of the soil model) leaving the land portion of the grid cell divided by the land area in the grid cell; report as "missing" or 0.0 where the land fraction is 0.
23	surface_snow_amount_where_land	snw	kg m^-2	compute as the mass of surface snow on the land portion of the grid cell divided by the land area in the grid cell; report as "missing" or 0.0 where the land fraction is 0; exclude snow on vegetation canopy or on sea ice.
24	surface_snow_area_fraction_where_land	snc	%	fraction of grid cell covered by snow that lies on land; exclude snow that lies on sea ice.
25	surface_snow_melt_flux_where_land	snm	kg m^-2 s^-1	compute as the total surface melt water on the land portion of the grid cell divided by the land area in the grid cell; report as 0.0 for snow-free land regions; report as 0.0 or "missing" where the land fraction is 0.
26	eastward_wind	uas	m s^-1	near-surface (usually, 10 meters) eastward component of wind; the CMOR singleton dimension default value of 10 m can be overridden, if absolutely necessary, by redefining axis "height2".
27	northward_wind	vas	m s^-1	near-surface (usually, 10 meters) northward component of wind; the CMOR singleton dimension default value of 10 m can be overridden, if absolutely necessary, by redefining axis "height2".
28	specific_humidity	huss	1 (i.e., dimensionless fraction)	near-surface (usually, 2meters) specific humidity; the CMOR singleton dimension default value of 2 m can be overridden, if absolutely necessary, by redefining axis "height1".
29	toa_incoming_shortwave_flux	rsdt	W m^-2	incident shortwave at the top of the atmosphere
30	toa_outgoing_shortwave_flux	rsut	W m^-2	at the top of the atmosphere
31	toa_outgoing_longwave_flux	rlut	W m^-2	at the top of the atmosphere (to be compared with satellite measurements)
32	net_downward_radiative_flux_at_top_of_ atmosphere_model	rtmt	W m^-2	i.e., at the top of that portion of the atmosphere where dynamics are explicitly treated by the model.
33	net_downward_shortwave_flux_in_air	rsntp	W m^-2	at 200 hPa only; the CMOR singleton dimension default value of 200 hPa can be overridden, if absolutely necessary, by redefining axis "pressure1".
34	net_upward_longwave_flux_in_air	rlntp	W m^-2	at 200 hPa only; the CMOR singleton dimension default value of 200 hPa can be overridden, if absolutely necessary, by redefining axis "pressure1".
35	net_downward_shortwave_flux_in_air_ assuming_clear_sky	rsntpcs	W m^-2	at 200 hPa only; method "2" is recommended for calculating clear-sky fluxes; the CMOR singleton dimension default value of 200 hPa can be overridden, if absolutely necessary, by redefining axis "pressure1".
36	net_upward_longwave_flux_in_air_ assuming_clear_sky	rlntpcs	W m^-2	at 200 hPa only; method "2" is recommended for calculating clear-sky fluxes; the CMOR singleton dimension default value of 200 hPa can be overridden, if absolutely necessary, by redefining axis "pressure1".
37	surface_downwelling_shortwave_flux_in_ air_assuming_clear_sky	rsdscs	W m^-2	method "2" is recommended for calculating clear-sky fluxes
38	surface_upwelling_shortwave_flux_in_ air_assuming_clear_sky	rsuscs	W m^-2	method "2" is recommended for calculating clear-sky fluxes
39	surface_downwelling_longwave_flux_in_ air_assuming_clear_sky	rldscs	W m^-2	method "2" is recommended for calculating clear-sky fluxes
40	toa_outgoing_longwave_flux_assuming_ clear_sky	rlutcs	W m^-2	method "2" is recommended for calculating clear-sky fluxes
41	toa_outgoing_shortwave_flux_assuming_ clear_sky	rsutcs	W m^-2	method "2" is recommended for calculating clear-sky fluxes
42	cloud_area_fraction	clt	%	for the whole atmospheric column, as seen from the surface or the top of the atmosphere. Include both large-scale and convective cloud.
43	atmosphere_cloud_condensed_water_content	clwvi	kg m^-2	include both liquid and ice phases, consider all the mass of condensed water in the column and divide by its area (in the longitude-latitude plane)
44	atmosphere_cloud_ice_content	clivi	kg m^-2	consider all the mass of ice-phase water in the column and divide by its area (in the longitude-latitude plane)

Table A1b: Time-independent 2-d land surface data (longitude, latitude).
	CF standard_name	output variable name	units	notes
1	surface_altitude	orog	m	height above the geoid; as defined here, "the geoid" is a surface of constant geopotential that, if the ocean were at rest, would coincide with mean sea level. Under this definition, the geoid changes as the mean volume of the ocean changes (e.g., due to glacial melt, or global warming of the ocean). Report here the height above the present-day geoid. Over ocean, report as 0.0
2	land_area_fraction	sftlf	%
3	land_ice_area_fraction	sftgif	%	fraction of grid cell occupied by "permanent" ice (i.e., glaciers).
4	soil_moisture_content_at_ field_capacity	mrsofc	kg m^-2	divide the total water holding capacity of all the soil in the grid cell by the land area in the grid cell; report as "missing" or 0.0 outside land areas.

Table A1c: Monthly-mean 3-d atmosphere data (longitude, latitude, pressure, time:month). Except for cloud area fraction, this data must be provided on pressure levels, including at least the following standard levels:1000, 925, 850, 700, 600, 500, 400, 300, 250, 200, 150, 100, 70, 50, 30, 20, 10 hPa.
	CF standard_name	output variable name	units	notes
1	cloud_area_fraction_in_atmosphere_layer	cl	%	unlike all other fields in this table, the cloud fraction should be reported for each model layer (not interpolated to standard pressures). Include both large-scale and convective cloud.
2	air_temperature	ta	K
3	eastward_wind	ua	m s^-1
4	northward_wind	va	m s^-1
5	specific_humidity	hus	1 (i.e., dimensionless fraction)
6	lagrangian_tendency_of_air_pressure	wap	Pa s^-1	commonly referred to as "omega", this represents the vertical component of velocity in pressure coordinates (positive down)
7	geopotential_height	zg	m
8	relative_humidity	hur	%
9	mole_fraction_of_o3_in_air	tro3	1e-9 (i.e., ppbv)	if climatologically specified, report only for 1 year.

Table O1a: Monthly-mean 1-d ocean data (latitude, region, time:month). Zonal mean over all oceans and also zonal mean for individual ocean basins (Atlantic, Indian, and Pacific basins: divide roughly at 20E and 120E).
	CF standard_name	output variable name	units	notes
1	northward_ocean_heat_transport	hfogo	W	transport by all ocean-related processes, both explicitly simulated and parameterized (e.g., any contribution from the 'bolus velocity' in the Gent-McWilliams parameterization), including sea water and sea ice contributions.

Table O1b: Monthly-mean 2-d ocean data (latitude, depth, region, time:month). Zonal mean over all oceans and also zonal mean for individual ocean basins (Atlantic, Indian, and Pacific basins: divide roughly at 20E and 120E). Data must be provided on depth levels. We recommend that these match the 33 standard levels of Levitus observations: 0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 4500, 5000, and 5500 meters.
	CF standard_name	output variable name	units	notes
1	ocean_meridional_overturning_streamfunction	stfmmc	m³ s^-1	Note that the units do not include mass. This should include only the explicitly calculated, purely advective component and should exclude contributions of the 'bolus velocity' that arise, for example, in the Gent-McWilliams parameterization.

Table O1c: Monthly-mean 0-d or 2-d ocean or sea ice data (longitude, latitude, time:month).
	CF standard_name	output variable name	units	notes
1	sea_surface_height_above_geoid	zos	m	this height, when multiplied by the area fraction of the grid cell covered by ocean (or sea ice), yields the volume of sea water above the geoid. As defined here, "the geoid" is a surface of constant geopotential that, if the ocean were at rest, would coincide with mean sea level. Under this definition, the geoid changes as the mean volume of the ocean changes (e.g., due to glacial melt, or global warming of the ocean). Report zos as "missing" over grid cells that are entirely land. There are a couple of acceptable options for reporting this field: 1) if the geoid is defined to relate to the instantaneous volume of the ocean, the global mean of zos will always be zero, and 2) if the geoid is defined relative to a time-mean sea level over some period, then the global mean of zos will be time-dependent. In either case a global mean time-series of sea level should also be reported as described in the next two table entries immediately below. In general IPCC analysis of global mean sea level changes will not rely on zos. It is recommended that in reporting zos, the atmospheric "inverted barometer" effect be omitted, since it can be easily calculated from the reported mean sea level pressure field. The "comment" attribute associated with zos should indicate whether or not the atmospheric "inverted barometer" influence on zos has been included. Additionally, it should be noted in the "comment" attribute whether zos is obtained directly, as in a free-surface model, or has been derived, for example, from geostrophy using diagnosed velocities at some level or from geostrophy relative to an assumed level of quiescence.
2	global_average_thermosteric_ sea_level_change	zostoga	m	a function only of time, zostoga is the contribution to change in global mean sea level, relative to some fixed distance from the center of the earth, due only to thermal structure changes. The fixed reference height should be invariant across all IPCC simulations by a model. In a rigid-lid model this quantity can be calculated by using a reference 3D salinity field to compute density as the 3D temperature field evolves. If only the total sea level change (due to thermosteric changes, water flux input from land/glaciers/atmosphere, and salinity influences on density) is available, omit zostoga, and report only zosga (see next table entry below). Please note in the "comment" attribute any assumptions or methodological details related to calculation of this time-series.
3	global_average_sea_level_change	zosga	m	a function only of time, zosga is the total change in global mean sea level, relative to some fixed distance from the center of the earth, due to thermosteric changes, water flux input from land/glaciers/atmosphere, and salinity influences on density. If the model cannot be trusted to provide estimates of the water flux input from land/glaciers, there is no need to report zosga (since salinity influences are of secondary importance and the thermosteric contribution is reported by zostoga). Note that to good approximation the difference between zostoga and zosga yields the global mean change in sea level due to water budget imbalances (presumably, resulting largely from changes in glacial mass). Please note in the "comment" attribute any assumptions or methodological details related to calculation of this time-series.
4	sea_surface_temperature	tos	K	this may differ from "surface temperature" in regions of sea ice.
5	sea_ice_area_fraction	sic	%	fraction of grid cell covered by sea ice.
6	sea_ice_thickness	sit	m	this thickness, when multiplied by the average area of the grid cell covered by sea ice, yields the time-mean sea ice volume. Thus, for time means, compute as the weighted sum of thickness (averaged over the sea ice-covered portion of the grid cell) divided by the sum of the weights, with the weights equal to the area covered by sea-ice; Report as 0.0 in regions free of sea ice.
7	eastward_sea_ice_velocity	usi	m s^-1	report as "missing" in regions free of sea ice.
8	northward_sea_ice_velocity	vsi	m s^-1	report as "missing" in regions free of sea ice.
9	water_flux_into_ocean	wfo	kg m^-2 s^-1	precipitation minus evaporation, plus runoff, melting of sea ice and any water flux correction calculated considering only the ocean-portion of each grid cell
10	ocean_barotropic_streamfunction	stfbarot	m³ s^-1	units do not include mass.
11	heat_flux_correction_where_ocean	hfcorr	W m^-2	if applicable, should be positive down (i.e., added to ocean); the total flux correction entering the ocean portion of the grid cell should be divided by the ocean area in the grid cell (in this context, ocean includes sea ice); report only for a single year and a single run, assuming this field is the same from year to year and for all runs.
12	water_flux_correction_where_ocean	wfcorr	kg m^-2s^-1	if applicable, should be positive down (i.e., added to ocean); the total flux correction entering the ocean portion of the grid cell should be divided by the ocean area in the grid cell (in this context, ocean includes sea ice); report only for a single year and a single run, assuming this field is the same from year to year and for all runs.
13	eastward_momentum_flux_ correction_where_ocean	tauucorr	Pa	if applicable, should be positive down (i.e., added to ocean); the total flux correction entering the ocean portion of the grid cell should be divided by the ocean area in the grid cell (in this context, ocean includes sea ice); report only for a single year and a single run, assuming this field is the same from year to year and for all runs.
14	northward_momentum_flux_ correction_where_ocean	tauvcorr	Pa	if applicable, should be positive down (i.e., added to ocean); the total flux correction entering the ocean portion of the grid cell should be divided by the ocean area in the grid cell (in this context, ocean includes sea ice); report only for a single year and a single run, assuming this field is the same from year to year and for all runs.

Table O1d: Time-independent 2-d ocean data (longitude, latitude).
	CF standard_name	output variable name	units	notes
1	sea_floor_depth_below_geoid	zobt	m	this height, when multiplied by the area fraction of the grid cell covered by ocean (or sea ice), yields the volume of water below the geoid. As defined here, "the geoid" is a surface of constant geopotential that, if the ocean were at rest, would coincide with mean sea level. Under this definition, the geoid changes as the mean volume of the ocean changes (e.g., due to glacial melt, or global warming of the ocean). Report here the sea floor depth for present day.
2	prescribed_heat_flux_into_slab_ocean	qflux	W m^-2	the so-called q-flux added to slab ocean cell, which is meant to account for convergence (or divergence) of heat by the ocean circulation. It should be computed as the total qflux energy added to the ocean-portion of the grid cell divided by the ocean area in the grid cell; report as "missing" or 0.0 where the ocean fraction is 0. Report only for slab ocean experiments.

Table O1e: Monthly-mean 3-d ocean data (longitude, latitude, depth, time:month). Data must be provided on depth levels. We recommend that these match the 33 standard levels of Levitus observations: 0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 4500, 5000, and 5500 meters. For the 3-d ocean fields, it is likely that storage space constraints will limit relatively quick access to output from only a single member of each ensemble, so in prioritizing your processing, consider initially sending PCMDI the 3-d ocean output from only 1 member of the ensemble.
	CF standard_name