WARNING

Some modifications have been made in this page since the meeting at C3S General Assembly at Toulouse
More details about those changes, and the final netCDF examples following the guide are expected to be made available around the 21st April 2017

Introduction

During these first stages of the proof-of-concept phase of C3S seasonal forecast activity, we have been working in standardise the data provision in netCDF.

The proposal is constrained by the CF convention, and we have tried not to diverge from specifications coming from other well established communities: SPECS and CMIP5/6

Additionally ACDD has been also taken into account when defining the data discovery related metadata.

In that sense, the following links are valuable sources of information that have informed the definition of this proposal:

CF convention

CF convention standard names tables

SPECS file content and format, data structure and metadata

CMIP5 list of variables

CMIP6 Data Request: MIP variables search

ACDD convention

NOTES about tables color coding

Along this document some issues are still at some level under discussion. To mark that it has been followed the following color coding scheme

Items to be discussed with EQC / Waiting for a proposal from EQC

Items under discussion within C3S seasonal forecast team

Encoding Guide

Global attributes

The following properties are intended to provide information about where the data came from and what has been done to it. This information is mainly for the benefit of human readers and data discovery mechanisms. The attribute values are all character strings. When an attribute appears both globally and as a variable attribute, the variable’s version has precedence.

Attribute Name	Value	Examples	Comment
Conventions	CF_convention_string C3S-0.1 [Other convention] :...	"CF-1.6 C3S-0.1"	Multiple conventions may be included (separated by blank spaces)
title	"<short institution name> seasonal forecast model output prepared for C3S" CF: Free text ACDD (highly recommended)	"ECMWF seasonal forecast model output prepared for C3S"	A short phrase or sentence describing the dataset. In many discovery systems, the title will be displayed in the results list from a search, and therefore should be human readable and reasonable to display in a list of such names <short institution name> is the first element of the comma-separated list of values of the corresponding "institution" attribute
references	URIs (such as a URL or DOI) for papers or other references. A valid doi is recommended CF: Free text	"doi:10.5194/gmd-8-1509-2015"	Published or web-based references that describe the data or methods used to produce it.
source	A methodology to build this attribute will be provided	"world model v.0.1" "model-generated, GloSea5-GC2" "IPSL-CM5A-LR (2010) : atmos : LMDZ4 (LMDZ4_v5, 96x95x39); ocean : ORCA2 (NEMOV2_3, 2x2L31); seaIce : LIM2 (NEMOV2_3); ocnBgchem : PISCES (NEMOV2_3); land : ORCHIDEE (orchidee_1_9_4_AR5)"	The method of production of the original data. If it was model-generated, source should name the model and its version, as specifically as it could be useful
institution	Controlled Vocabulary: "ECMWF, European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom" "Met Office, Exeter, United Kingdom" "Météo-France, Toulouse, France" "DWD, Deutscher Wetterdienst, Offenbach, Germany" "CMCC, Centro Euro-Mediterraneo sui Cambiamenti Climatici, Bologna, Italy" CF: Free text	"Météo-France, Toulouse, France"	Specifies where the original data was produced. The name of the institution principally responsible for originating this data. NOTE: The first element of the comma-separated list of values will be used as a shortened version of this attribute in some of the other global attributes ('summary', 'title')
contact	Copernicus User Support URI should be used http://copernicus-support.ecmwf.int CF: Free text	"http://copernicus-support.ecmwf.int"
project	"C3S Seasonal Forecast" should be used CF: Free text	"C3S Seasonal Forecast"
creation_date	SPECS: YYYY-MM-DDThh:mm:ss<zone> ISO 8601:2004 extended format	"2011-06-24T02:53:46Z"	The date on which this version of the data was created. Modification of values implies a new version, hence this would be assigned the date of the most recent values modification. Metadata changes are not considered when assigning the creation_date NOTE: The ACDD 1.3 names this attribute as "date_create". The name "creation_date" has been used following SPECS convention.
comment	Free text	"Produced by University of Hamburg for DWD at ECMWF HPC facilities" "Run by CMCC at CINECA"	Miscellaneous information about the data, not captured elsewhere.
forecast_type	"forecast" or "hindcast"	"forecast"	To identify the type of data
history	Each line should begin with a timestamp indicating the date and time of day when the program was executed CF: Free Text	"Produced using CDS Toolbox on 1/6/2016" "Model raw output postprocessing with modelling environment (IMDI) at DKRZ: URL: http://svn-mad.zmaw.de/svn/mad/Model/IMDI/trunk, REV: 3436 2011-07-17T15:14:45Z CMOR rewrote data to comply with CF standards and CMIP5 requirements."	To record relevant information, such as the command history which led to this file being produced. Provides an audit trail for modifications to the original data. Well-behaved generic netCDF filters will automatically append their name and the parameters with which they were invoked to the global history attribute of an input netCDF file
commit, iso_lineage or lineage	Free text (ISO Lineage model 19115-2)	"Produced using CDS Toolbox v1.0"	Trace of the tools/scripts used. Either this attribute or the "history" attribute needs to include information about the versioning of the software used to create the data. This is related with some of the EQC activities (WP4 of QA4SEAS). ISO 19115-2 defines a linage model where this is been considered.
summary	Controlled Vocabulary: "Seasonal Forecast data produced by <short institution name> as its contribution to the seasonal forecast activity of the Copernicus Climate Change Service (C3S). The data has global coverage with a 1-degree horizontal resolution and spans for around 6 months since the start date" ACDD (highly recommended)	"Seasonal Forecast data produced by DWD as its contribution to the seasonal forecast activity of the Copernicus Climate Change Service (C3S). The data has global coverage with a 1-degree horizontal resolution and spans for around 6 months since the start date"	A short paragraph describing the dataset <short institution name> is the first element of the comma-separated list of values of the corresponding "institution" attribute
keywords	Fixed string "Seasonal Forecasts, C3S, ECMWF, Copernicus, Climate Change, Climate Services, Earth Science Services, Environmental Advisories, Climate Advisories" ACDD (highly recommended)		A comma separated list of key words and phrases. NOTE: This attribute is likely to be modified in the future, once the contents of the Thesaurus for CDS faceting will be defined
forecast_reference_time	SPECS: YYYY-MM-DDThh:mm:ssZ NOTE: This is ISO 8601:2004 extended format, but time zone is required to be UTC	"2011-06-01T00:00:00Z"	time of the analysis from which the forecast was made Introduced as global attribute to keep compatibility with SPECS (note that works fine for SPECS data structure, i.e. one variable per start time per file)

Spatial Coordinates

Type (CMIP5)	Coordinate Name (CMIP5)	Dimension Names (CMIP5)	Axis	standard_name	long_name (CMIP5)	units (CF canonical units)	positive	valid_min (CMIP5)	valid_max (CMIP5)	bounds	Notes
double	lat	lat	Y	latitude	latitude	degrees_north	N/A	-90.	90.	lat_bnds	Values (1x1deg grid) prescribed: center of 1-degree cells dimension lat=180 [-89.5, -88.5 , ..., -0.5, 0.5 ... 89.5]
double	lon	lon	X	longitude	longitude	degrees_east	N/A	0.	360.	lon_bnds	Values (1x1deg grid) prescribed: center of 1-degree cells dimension lon=360 [0.5 , 1.5 , ..., 358.5, 359.5]
double	plev	plev	Z	air_pressure	pressure	Pa	down	N/A	N/A		This is also referred to as isobaric level by some tools Values prescribed: dimension plev=11 [925., 850., 700., 500., 400., 300., 200., 100., 50., 30., 10.] (NOTE: values here written in hPa, they should be Pa)
double	plev	plev	Z	air_pressure	pressure	Pa	down	N/A	N/A
double	depth	depth	Z	depth	depth	m	down	N/A	N/A	depth_bnds	Only used for soil model levels NOTE: Number and depth of levels is not prescribed by C3S
double	height	(scalar auxiliary coordinate)	Z	height	height	m	up or down	CMIP5: 2mtemp: 1. 10mu/v: 1.	CMIP5: 2mtemp: 10. 10mu/v: 30.		Used for single level fields (height, soil,SST) e.g. 2 m (for Temperature)
C3S: string	realization	str31=31	E	realization	realization	1	N/A	N/A	N/A		members are not a physical quantity. Realization is a discrete coordinate and the members its categorical values (ordered or non-ordered ones) SPECS approach: rXXiYYpZZ

NOTE about the horizontal coordinates: The regridding procedure to provide the data in the 1-degree grid must take into account that the full definition of the gird cells is done by the cell boundaries (lat_bnds, lon_bnds)

Time Coordinates

Type

Coordinate Name

Dimension Names

Axis

standard_name

long_name

calendar

units

bounds

Notes

double

reftime

N/A

forecast_reference_time

"Start date of the forecast"

gregorian

UDUNITS time units
e.g.
"hours since YYYY-MM-DD hh:mm:ss TZhh:TZmm"

N/A

In SPECS it is a "global_attribute"
It has been additionally introduced here as a coordinate variable to ease future netCDF management (e.g. file merging)

double

leadtime

N/A

forecast_period

"Time elapsed since the start of the forecast"

N/A

SPECS: days
C3S: requested units can be relaxed to equivalent time units

leadtime_bnds

The interval of time between the forecast reference time and the valid time

Boundaries not needed when this time coordinate is used for instantaneous values (note that "time:point" is used as cell_method in those cases)

When boundaries are required, the value of the coordinate must be in the centre of the correspondent time cell boundaries

double

time

leadtime

N/A

time

"Verification time of the forecast"

gregorian

SPECS: "days since 1850-01-01"

C3S: requested units can be relaxed to equivalent time units

time_bnds

Time for which the forecast is valid

Boundaries not needed when this time coordinate is used for instantaneous values (note that "time:point" is used as cell_method in those cases)

When boundaries are required, the value of the coordinate must be in the centre of the correspondent time cell boundaries

NOTE: Definitions for "leadtime" and "time" have been taken from SPECS. The introduction of "reftime" as a variable has been adapted from SPECS global attribute description for the forecast reference time.

NOTE: Even though there are different requested time steps among the variables (6h, 12h, 24h), just one set of time axes has been defined, as that would be enough when applying the requirement of "one variable per file"

"leadtime" has been selected as dimension (instead of "time") for both time and leadtime. That means "leadtime" is the coordinate and "time" is an auxiliary coordinate

This diverges from SPECS (where "time" was the name of the dimension and the coordinate, and "leadtime" was an auxiliary coordinate)
Here it has been done like that because
1. both reftime and leadtime are the relevant (let's say "orthogonal") coordinates coming from time = reftime + leadtime
2. doing like that has some advantages when merging netcdf files ("leadtime" can be easily shared by different variables in a merged file, while "time" cannot)

Cell boundaries

As described in section 7.1 Cell Boundaries of CF convention.

To represent cells we add the attribute bounds to the appropriate coordinate variable(s). The value of bounds is the name of the variable that contains the vertices of the cell boundaries. We refer to this type of variable as a "boundary variable." A boundary variable will have one more dimension than its associated coordinate or auxiliary coordinate variable. The additional dimension should be the most rapidly varying one, and its size is the maximum number of cell vertices. Since a boundary variable is considered to be part of a coordinate variable’s metadata, it is not necessary to provide it with attributes such as long_name and units

Bounds Name	Dimensions	Comments
time_bnds	leadtime, bnds
leadtime_bnds	leadtime, bnds
lat_bnds	lat, bnds	Values (1x1deg grid) prescribed: [-90., 89.], [-89., -88.], ... [89., 90.]
lon_bnds	lon, bnds	Values (1x1deg grid) prescribed: [0., 1.], [1., 2.], ... [359., 360.]
depth_bnds	depth,bnds	Should define the full vertical extent of the soil model layers

Grid mapping

As described in section 5.6 Grid Mappings and Projections of CF convention.

When the coordinate variables for a horizontal grid are longitude and latitude, a grid mapping variable with grid_mapping_name of latitude_longitude may be used to specify the ellipsoid and prime meridian.

char hcrs ;
    hcrs:grid_mapping_name = "latitude_longitude" ;

Variables

Common attributes

The following attributes must be included in all the variables

attribute name	value	comment
grid_mapping	hcrs
_FillValue	1.0e20	Those attributes must be included JUST when the variable has missing values
missing_value	1.0e20

Static Fields

attributes

name
(CMIP5)

dimensions

standard_name

long_name
(CMIP5)

units

coordinates

NOTES

sftlf

lat,lon

land_area_fraction

"Land Area Fraction"

1

"lat lon"

orog

lat,lon

surface_altitude

"Surface Altitude"

m

"lat lon"

Surface Fields (defined at a given height level)

name (CMIP5/6&C3S)	dimensions	standard_name	long_name (CMIP5/6&C3S)	units	coordinates	cell_methods	NOTES
		attributes
tas	time,lat,lon	air_temperature	"Near-Surface Air Temperature"	K	"height time lat lon"	"time: point" C3S: required CF: recommended	height is usually 2m
tasmax	time,lat,lon	air_temperature	"Daily Maximum Near-Surface Air Temperature"	K	"height time lat lon"	"time: maximum (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional	height is usually 2m C3S: The interval is required to have a value<=3 hours)
tasmin	time,lat,lon	air_temperature	"Daily Minimum Near-Surface Air Temperature"	K	"height time lat lon"	"time: minimum (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional	height is usually 2m C3S: The interval is required to have a value<=3 hours)
tdps	time,lat,lon	dew_point_temperature	"2m dewpoint temperature"	K	"height time lat lon"	"time: point" C3S: required CF: recommended	height is usually 2m
uas	time,lat,lon	x_wind	Eastward Near-Surface Wind	m s-1	"height time lat lon"	"time: point" C3S: required CF: recommended	height is usually 10m
vas	time,lat,lon	y_wind	Northward Near-Surface Wind	m s-1	"height time lat lon"	"time: point" C3S: required CF: recommended	height is usually 10m
wsgmax N/A in CMIP	time,lat,lon	wind_speed_of_gust	"Maximum Wind Speed of Gust" N/A in CMIP	m s-1	"height time lat lon"	"time: maximum (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional	height is usually 10m C3S: The interval is required to have a value<=3 hours)

Surface Fields (not defined at a height level)

name (CMIP5/6&C3S)	dimensions	standard_name	long_name (CMIP5/6&C3S)	units	cell_methods
		attributes
psl	time,lat,lon	air_pressure_at_sea_level	"Sea Level Pressure"	Pa	"time: point" C3S: required CF: recommended
clt	time,lat,lon	cloud_area_fraction	"Total Cloud Fraction"	1	"time: point" C3S: required CF: recommended
tsl	time,lat,lon	soil_temperature	"Temperature of Soil"	K	"time: point" C3S: required CF: recommended
tso	time,lat,lon	sea_surface_temperature	"Sea Surface Temperature"	K	"time: point" C3S: required CF: recommended
sitemptop N/A in CMIP	time,lat,lon	sea_ice_temperature	"Surface temperature of sea ice" N/A in CMIP	K	"time: point" C3S: required CF: recommended
sic	time,lat,lon	sea_ice_area_fraction	"Sea Ice Area Fraction"	1	"time: point" C3S: required CF: recommended
N/A in CMIP	time,depth,lat,lon	mass_content_of_water_in_soil_layer	N/A in CMIP	kg m^-2	"time: point" C3S: required CF: recommended
lwesnw N/A in CMIP	time,lat,lon	lwe_thickness_of_surface_snow_amount	"Liquid Water Equivalent of Surface Snow Amount" N/A in CMIP	m	"time: point" C3S: required CF: recommended
rhosn N/A in CMIP	time,lat,lon	snow_density	"Snow density" N/A in CMIP	kg m^-3	"time: point" C3S: required CF: recommended
lweprs N/A in CMIP	time,lat,lon	lwe_thickness_of_stratiform_precipitation_amount	"Liquid Water Equivalent of Stratiform Precipitation Amount" N/A in CMIP	m	"time: sum"
lweprc N/A in CMIP	time,lat,lon	lwe_thickness_of_convective_precipitation_amount	"Liquid Water Equivalent of Convective Precipitation Amount" N/A in CMIP	m	"time: sum"
lwepr N/A in CMIP	time,lat,lon	lwe_thickness_of_precipitation_amount	"Liquid Water Equivalent of Total Precipitation Amount" N/A in CMIP	m	"time: sum"
lweprsn N/A in CMIP	time,lat,lon	lwe_thickness_of_snowfall_amount	"Liquid Water Equivalent of Snowfall Amount" N/A in CMIP	m	"time: sum"
hfss	time,lat,lon	surface_upward_sensible_heat_flux	"Surface Upward Sensible Heat Flux"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
hfls	time,lat,lon	surface_upward_latent_heat_flux	"Surface Upward Latent Heat Flux"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
rsds	time,lat,lon	surface_downwelling_shortwave_flux_in_air	"Surface Downwelling Shortwave Radiation"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
rlds	time,lat,lon	surface_downwelling_longwave_flux_in_air	"Surface Downwelling Longwave Radiation"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
rss	time,lat,lon	surface_net_downward_shortwave_flux	"Net Shortwave Surface Radiation"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
rls	time,lat,lon	surface_net_downward_longwave_flux	"Net Longwave Surface Radiation"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
rst N/A in CMIP	time,lat,lon	toa_net_downward_shortwave_flux	"TOA Net Shortwave Radiation" N/A in CMIP	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
rlt N/A in CMIP	time,lat,lon	toa_net_downward_longwave_flux	"TOA Net Longwave Radiation" N/A in CMIP	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is r equired. CF: interval is optional
rsdt	time,lat,lon	toa_incoming_shortwave_flux	"TOA Incident Shortwave Radiation"	W m^-2	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
tauu	time,lat,lon	surface_downward_eastward_stress	"Surface Downward Eastward Wind Stress"	Pa	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
tauv	time,lat,lon	surface_downward_northward_stress	"Surface Downward Northward Wind Stress"	Pa	"time: mean (interval: <value> <unit>)" C3S: interval is required. CF: interval is optional
lwee N/A in CMIP	time,lat,lon	lwe_thickness_of_water_evaporation_amount	"Liquid Water Equivalent of Evaporation Amount" N/A in CMIP	m	"time: sum"
N/A in CMIP	time,lat,lon	runoff_amount	N/A in CMIP	kg m^-2	"time: sum"
N/A in CMIP	time,lat,lon	surface_runoff_amount	N/A in CMIP	kg m^-2	"time: sum"
N/A in CMIP	time,lat,lon	subsurface_runoff_amount	N/A in CMIP	kg m^-2	"time: sum"

Pressure Level Fields

name (CMIP5)	dimensions	standard_name	long_name (CMIP5)	units	cell_methods
		attributes
zg	time,plev,lat,lon	geopotential_height	"Geopotential Height"	m	"time: point" C3S: required CF: recommended
ta	time,plev,lat,lon	air_temperature	"Air Temperature"	K	"time: point" C3S: required CF: recommended
hus	time,plev,lat,lon	specific_humidity	"Specific Humidity"	1	"time: point" C3S: required CF: recommended
N/A in CMIP	time,plev,lat,lon	atmosphere_relative_vorticity	N/A in CMIP	s^-1	"time: point" C3S: required CF: recommended
N/A in CMIP	time,plev,lat,lon	divergence_of_wind	N/A in CMIP	s^-1	"time: point" C3S: required CF: recommended
ua	time,plev,lat,lon	x_wind	"Eastward Wind"	m s^-1	"time: point" C3S: required CF: recommended
va	time,plev,lat,lon	y_wind	"Northward Wind"	m s^-1	"time: point" C3S: required CF: recommended

When boundaries are required, the value of the coordinate must be in the centre of the correspondent time cell boundaries

Space shortcuts

Page tree

Introduction

Encoding Guide

Global attributes

Spatial Coordinates

Time Coordinates

Cell boundaries

Grid mapping

Variables

Common attributes

Static Fields

Surface Fields (defined at a given height level)

Surface Fields (not defined at a height level)

Pressure Level Fields

Space shortcuts

Page tree

Guide to NetCDF encoding for C3S providers

Introduction

Encoding Guide

Global attributes

Spatial Coordinates

Time Coordinates

Cell boundaries

Grid mapping

Variables

Common attributes

Static Fields

Surface Fields (defined at a given height level)

Surface Fields (not defined at a height level)

Pressure Level Fields