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 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

ACDD convention

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 [Other convention] :...	"CF-1.6" "CF-1.6 C3S-0.1"	Multiple conventions may be included (separated by blank spaces)
title	A controlled vocabulary will be provided CF: Free text ACDD (highly recommended)	"IPSL-CM5A-LR model output prepared for CMIP5 RCP4.5"	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
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	A controlled vocabulary will be provided CF: Free text	"Met Office"	Specifies where the original data was produced. The name of the institution principally responsible for originating this data.
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 SPECS conventions will be followed
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 A way of labelling hindcasts and forecasts to match them needs to be devised.
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	The content will be provided ACDD (highly recommended): Text, defined phrase		A short paragraph describing the dataset
keywords	The content will be provided ACDD (highly recommended) : text, controlled vocabulary		A comma separated list of key words and phrases.
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: This 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_bounds	Values (1x1deg grid) prescribed: dimension lat=181 [-90. , -89. , ..., 0., ... 90.]
double	lon	lon	X	longitude	longitude	degrees_east	N/A	0.	360.	lon_bounds	Values (1x1deg grid) prescribed: dimension lon=360 [0. , 1. , ..., 358., 359.]
double	plev	plev	Z	air_pressure	pressure	Pa	down	N/A	N/A	bounds?	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	depth	depth	Z	depth	depth	m	down	N/A	N/A	depth_bounds	Only used for soil model levels NOTE: Number and depth of levels is not prescribed by C3S
double	height	height	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	C3S: realization_dim CF: a different name is needed for dim/variable (see comment)	E	realization	realization	1	N/A	N/A	N/A		members are not a physical quantity. Realization is a discrete coordinate and the mebers it categorical values (ordered or non-ordered ones)

Time Coordinates

Type

Coordinate Name

Dimension Names

Axis

standard_name

long_name

calendar

units

bounds

Notes

double

reftime

N/A

T

forecast_reference_time

"Start date of the forecast"

gregorian

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

bounds?

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

double

leadtime

time

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

bounds?

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

double

time

T

time

"Verification time of the forecast"

gregorian

SPECS: "days since 1850-01-01"

C3S: requested units can be relaxed to equivalent time units

time_bounds

Time for which the forecast is valid

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"

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_bounds	time,bounds	where to put the time coordinate (beginning, middle, end of the bounds) ? e.g. time=20160922 06 timebounds = [20160922 00, 20160922 06] for 24h time steps: [0,24] is that convention always valid?
lat_bounds	lat, bounds	Values (1x1deg grid) prescribed: [-90., 89.], [-89., -88.], ... [89., 90.]
lon_bounds	lon, bounds	Values (1x1deg grid) prescribed: [0., 1.], [1., 2.], ... [359., 360.]
depth_bounds	depth,bounds	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 latitude_longitude ;
    latitude_longitude:grid_mapping_name = "latitude_longitude" ;

NOTE: Wouldn't a different name be more appropriate? (e.g. CRS as in CF examples)

Variables

NOTE: coordinates should list first of all the auxiliary coordinate(s) and then all the other coordinates.... SHOULD reftime, leadtime and realization be included as well????

NOTE: type double/real????

Static Fields

attributes

name
(CMIP5)

dimensions

standard_name

long_name
(CMIP5)

units

coordinates

grid_mapping

NOTES

sftlf

lat,lon

land_area_fraction

"Land Area Fraction"

1

"lat lon"

latitude_longitude

orog

lat,lon

surface_altitude

"Surface Altitude"

m

"lat lon"

latitude_longitude

Surface Fields (defined at a given height level)

name (CMIP5)	dimensions	standard_name	long_name (CMIP5)	units	coordinates	cell_methods	grid_mapping	NOTES
		attributes
tas	time,lat,lon	air_temperature	"Near-Surface Air Temperature"	K	"height time lat lon"	"time: point" C3S: required CF: recommended	latitude_longitude	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: required. CF: interval is optional	latitude_longitude	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: required. CF: interval is optional	latitude_longitude	height is usually 2m C3S: The interval is required to have a value<=3 hours)
	time,lat,lon	dew_point_temperature		K	"height time lat lon"	"time: point" C3S: required CF: recommended	latitude_longitude	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	latitude_longitude	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	latitude_longitude	height is usually 10m
	time,lat,lon	wind_speed_of_gust		m s-1	"height time lat lon"	"time: maximum (interval: <value> <unit>)" C3S: required. CF: interval is optional	latitude_longitude	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)	dimensions	standard_name	long_name (CMIP5)	units	cell_methods	grid_mapping	NOTES
		attributes
psl	time,lat,lon	air_pressure_at_sea_level	"Sea Level Pressure"	Pa	"time: point" C3S: required CF: recommended	latitude_longitude
clt	time,lat,lon	cloud_area_fraction	"Total Cloud Fraction"	1	"time: point" C3S: required CF: recommended	latitude_longitude
tsl	time,lat,lon	soil_temperature	"Temperature of Soil"	K	"time: point" C3S: required CF: recommended	latitude_longitude
tso	time,lat,lon	sea_surface_temperature	"Sea Surface Temperature"	K	"time: point" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	sea_ice_temperature		K	"time: point" C3S: required CF: recommended	latitude_longitude
sic	time,lat,lon	sea_ice_area_fraction	"Sea Ice Area Fraction"	1	"time: point" C3S: required CF: recommended	latitude_longitude
	time,depth,lat,lon	mass_content_of_water_in_soil_layer		kg m^-2	"time: point" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	lwe_thickness_of_surface_snow_amount		m	"time: point" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	snow_density		kg m^-3	"time: point" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	lwe_thickness_of_stratiform_precipitation_amount		m	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	lwe_thickness_of_convective_precipitation_amount		m	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	lwe_thickness_of_precipitation_amount		m	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	lwe_thickness_of_snowfall_amount		m	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_surface_downward_sensible_heat_of_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude	instantaneous, instead of integral? does it works with SW rad??? would it work with "instantaneous" 24h mean?
	time,lat,lon	integral_of_surface_downward_latent_heat_of_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_surface_net_downward_shortwave_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_surface_net_downward_longwave_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_toa_net_downward_shortwave_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_toa_net_downward_longwave_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_toa_incoming_shortwave_flux_wrt_time		W s m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_surface_downward_eastward_stress_wrt_time		Pa s	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	integral_of_surface_downward_northward_stress_wrt_time		Pa s	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	lwe_thickness_of_water_evaporation_amount		m	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	runoff_amount		kg m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	surface_runoff_amount		kg m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude
	time,lat,lon	subsurface_runoff_amount		kg m^-2	"time: sum" C3S: required CF: recommended	latitude_longitude

Pressure Level Fields

name (CMIP5)	dimensions	standard_name	long_name (CMIP5)	units	cell_methods	grid_mapping
		attributes
	time,plev,lat,lon	geopotential		m² s^-2	"time: point" C3S: required CF: recommended	latitude_longitude
ta	time,plev,lat,lon	air_temperature	"Air Temperature"	K	"time: point" C3S: required CF: recommended	latitude_longitude
hus	time,plev,lat,lon	specific_humidity	"Specific Humidity"	1	"time: point" C3S: required CF: recommended	latitude_longitude
	time,plev,lat,lon	atmosphere_relative_vorticity		s^-1	"time: point" C3S: required CF: recommended	latitude_longitude
	time,plev,lat,lon	divergence_of_wind		s^-1	"time: point" C3S: required CF: recommended	latitude_longitude
ua	time,plev,lat,lon	x_wind	"Eastward Wind"	m s^-1	"time: point" C3S: required CF: recommended	latitude_longitude
va	time,plev,lat,lon	y_wind	"Northward Wind"	m s^-1	"time: point" C3S: required CF: recommended	latitude_longitude

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Introduction

Encoding Guide

Global attributes

Spatial Coordinates

Time Coordinates

Cell boundaries

Grid mapping

Variables

Static Fields

Surface Fields (defined at a given height level)

Surface Fields (not defined at a height level)

Pressure Level Fields

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Guide to NetCDF encoding for C3S providers

Introduction

Encoding Guide

Global attributes

Spatial Coordinates

Time Coordinates

Cell boundaries

Grid mapping

Variables

Static Fields

Surface Fields (defined at a given height level)

Surface Fields (not defined at a height level)

Pressure Level Fields