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Major modification in progress for this article - please DO NOT UPDATE THIS PAGE or your edits will get overwritten when the new modified article will be published. Please edit the working copy of this page in the C3S Modified CKB articles or CAMS Modified CKB articles section. |
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For the time being and until further notice,ERA-Interim (1st January 1979 to 31st August 2019) shall continue to be accessibleaccess through the ECMWF Web API stopped on 01 June 2023 Its successor . ERA5 is now available from the Climate Data Store (CDS) (What are the changes from ERA-Interim to ERA5?) and users are strongly advised to migrate to ERA5 (How to download ERA5). For those users who still need access to ERA-Interim after 01 June 2023 (subject to further notice), they can do so via the Climate Data Store (CDS) API. |
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The data are archived in the ECMWF data archive MARS and datasets are available until 31 May 2023 through both the ECMWF Web interface and the ECMWF WebAPI, which is the programmatic way of retrieving data from the archive.
Until further notice, ERA-Interim is also available via the API of the C3S Climate Data Store.
Documentation is available on How to download ERA-Interim data from the ECMWF data archive (Member State users can access the data directly from MARS, in the usual manner).
...
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Download from ERA-Interim Wave data can be downloaded using the same mechanisms as atmospheric data. Please see How to download data via the ECMWF WebAPIERA-Interim For wave spectra you need to specify the additional parameters 'direction' and 'frequency'. Expand | | |
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Code Block | ||
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| ||
#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
"class": "ei",
"dataset": "interim",
"expver": "1",
"stream": "wave",
"type": "an",
"date": "2016-01-01/to/2016-01-31",
"time": "00:00:00/06:00:00/12:00:00/18:00:00",
"param": "251.140",
"direction": "1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24",
"frequency": "1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24/25/26/27/28/29/30",
"target": "2d_spectra_201601",
}) |
If you want to download the data in NetCDF format, please add the 'format' and 'grid' parameters:
Code Block | ||
---|---|---|
| ||
#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
......
"grid": "0.75/0.75", # Spatial resolution in degrees latitude/longitude
"format": "netcdf"
"target": "2d_spectra_201601.nc"
}) |
Decoding 2D wave spectra in GRIB
To decode wave spectra in GRIB format we recommend ecCodes. Wave spectra are encoded in a specific way that other tools might not decode correctly.
In GRIB, the parameter is called 2d wave spectra (single) because in GRIB, the data are stored as a single global field per each spectral bin (a given frequency and direction), but in NetCDF, the fields are nicely recombined to produce a 2d matrix representing the discretized spectra at each grid point.
The wave spectra are encoded in GRIB using a local table specific to ECMWF. Because of this, the conversion of the meta data containing the information about the frequencies and the directions are not properly converted from GRIB to NetCDF format. So rather than having the actual values of the frequencies and directions, values show index numbers (1,1) : first frequency, first direction, (1,2) first frequency, second direction, etc ....For ERA, because there are a total of 24 directions, the direction increment is 15 degrees with the first direction given by half the increment, namely 7.5 degree, where direction 0. means going towards the north and 90 towards the east (Oceanographic convention), or more precisely, this should be expressed in gradient since the spectra are in m^2 /(Hz radian)
The first frequency is 0.03453 Hz and the following ones are : f(n) = f(n-1)*1.1, n=2,30
Also note that it is NOT the spectral density that is encoded but rather log10 of it, so to recover the spectral density, expressed in m^2 /(radian Hz), one has to take the power 10 (10^) of the NON missing decoded values. Missing data are for all land points, but also, as part of the GRIB compression, all small values below a certain threshold have been discarded and so those missing spectral values are essentially 0. m^2 /(gradient Hz).
Decoding 2D wave spectra in NetCDF
The NetCDF wave spectra file will have the dimensions longitude, latitude, direction, frequency and time.
However, the direction and frequency bins are simply given as 1 to 24 and 1 to 30, respectively.
The direction bins start at 7.5 degree and increase by 15 degrees until 352.5, with 90 degree being towards the east (Oceanographic convention).
The frequency bins are non-linearly spaced. The first bin is 0.03453 Hz and the following bins are: f(n) = f(n-1)*1.1; n=2,30. The data provided is the log10 of spectra density. To obtain the spectral density one has to take to the power 10 (10 ** data). This will give the units 2D wave spectra as m**2 s radian**-1 . Very small values are discarded and set as missing values. These are essentially 0 m**2 s radian**-1.
This recoding can be done with the Python xarray package, for example:
Code Block | ||
---|---|---|
| ||
import xarray as xr
import numpy as np
da = xr.open_dataarray('2d_spectra_201601.nc')
da = da.assign_coords(direction=np.arange(7.5, 352.5 + 15, 15))
da = da.assign_coords(frequency=np.full(30, 0.03453) * (1.1 ** np.arange(0, 30)))
da = 10 ** da
da = da.fillna(0)
da.to_netcdf(path='2d_spectra_201601_recoded.nc') |
Units of 2D wave spectra
Once decoded, the units of 2D wave spectra are m2 s radian-1
Instantaneous and Accumulated parameters
Instantaneous parameters represent an average over the model time step (30min). Accumulated parameters are accumulated from the start of the forecast, ie. from 00 UTC or 12 UTC to the time step selected. All the analysed fields are instantaneous instead forecast data could be either instantaneous or accumulated, depending on the parameter. More detailed information on parameters are shown in Parameter listing.
Minimum/maximum since the previous post processing
In ERA-Interim there are some parameters named '...since previous post-processing', for example 'Maximum temperature at 2 metres since previous post-processing'. This represents the maximum temperature between the previous archived forecast 'Step' and the forecast 'Step'. For example, 'Maximum temperature at 2 metres since previous post-processing' with start time 00 UTC and Step=9, is the maximum 2m temperature in the 3-hour period between 06 UTC and 09 UTC.
Monthly means
ERA-interim sub-daily data are monthly averaged on data with valid times or accumulation periods that fall within the calendar month in question. The different monthly means are:
- Synoptic Monthly Means (stream=mnth) are the monthly averages:
- for each analysis time (at the four main synoptic hours - 00, 06, 12, and 18 UTC)
- for each forecast start time (00 and 12 UTC) and step (3, 6, 9, 12 etc).
- Monthly Means of Daily Means (stream=moda) are only available for analysis and instantaneous forecast data.
- Monthly Means of Daily Forecast Accumulations (stream=mdfa) are similar to Monthly Means of Daily Means but are for accumulated fields (eg precipitation, radiation) and three step ranges are available.
See also Section 3 of the ERA-Interim archive documentation.
Data format
Model level fields are in GRIB2 format. All other fields are in GRIB1, unless otherwise indicated.?
Level listings
Pressure levels: 1000/975/950/925/900/875/850/825/800/775/750/700/650/600/550/500/450/400/350/300/250/225/200/175/150/125/100/70/50/30/20/10/7/5/3/2/1
Potential temperature levels: 265/275/285/300/315/320/330/350/370/395/430/475/530/600/700/850
Model levels: 1/to/60, which are described at L60 model level definitions
Potential vorticity level:
Mathinline |
---|
PV=\pm 2PVU |
Parameter listings
Tables 1-6 below describe the surface and single level parameters (levtype=sfc), Table 7 describes wave parameters, Table 8 describes the monthly mean exceptions for surface and single level and wave parameters and Tables 9-13 describe upper air parameters on various levtypes. Information on all ECMWF parameters is available from the ECMWF parameter database.
Parameters described as "instantaneous" below, are valid at the specified time.
Instantaneous, invariant, surface and single level parameters table
...
title | Table 1 |
---|
...
count
...
name
...
an
...
fc
...
paramId
...
units
...
1
...
Low vegetation cover
...
x
...
27
...
2
...
High vegetation cover
...
x
...
28
...
(0-1)
...
3
...
Low vegetation type (table index - see Table 10.1 in IFS CY31R1 Documentation)
...
x
...
29
...
...
4
...
High vegetation type (table index - see Table 10.1 in IFS CY31R1 Documentation)
...
x
...
30
...
...
5
...
Standard deviation of filtered subgrid orography
...
x
...
74
...
m
...
6
...
Surface geopotential
...
x
...
x
...
129
...
m2s-2
...
7
...
x
...
160
...
m
...
8
...
Anisotropy of orography X
...
x
...
161
...
~
...
9
...
Angle of sub-grid scale orography
...
x
...
162
...
~
...
10
...
Slope of sub-grid scale orography
...
x
...
163
...
~
...
11
...
Land-sea mask
...
x
...
x
...
172
...
(0 - 1)
Instantaneous, varying, surface and single level parameters table
...
title | Table 2 |
---|
If you want to download the data in NetCDF format, please add the 'format' and 'grid' parameters. Decoding 2D wave spectra in GRIB To decode wave spectra in GRIB format we recommend ecCodes. Wave spectra are encoded in a specific way that other tools might not decode correctly. In GRIB, the parameter is called 2d wave spectra (single) because in GRIB, the data are stored as a single global field per each spectral bin (a given frequency and direction), but in NetCDF, the fields are nicely recombined to produce a 2d matrix representing the discretized spectra at each grid point. The wave spectra are encoded in GRIB using a local table specific to ECMWF. Because of this, the conversion of the meta data containing the information about the frequencies and the directions are not properly converted from GRIB to NetCDF format. So rather than having the actual values of the frequencies and directions, values show index numbers (1,1) : first frequency, first direction, (1,2) first frequency, second direction, etc .... Also note that it is NOT the spectral density that is encoded but rather log10 of it, so to recover the spectral density, expressed in m^2 /(radian Hz), one has to take the power 10 (10^) of the NON missing decoded values. Missing data are for all land points, but also, as part of the GRIB compression, all small values below a certain threshold have been discarded and so those missing spectral values are essentially 0. m^2 /(gradient Hz). Decoding 2D wave spectra in NetCDF The NetCDF wave spectra file will have the dimensions longitude, latitude, direction, frequency and time. However, the direction and frequency bins are simply given as 1 to 24 and 1 to 30, respectively. The direction bins start at 7.5 degree and increase by 15 degrees until 352.5, with 90 degree being towards the east (Oceanographic convention). The frequency bins are non-linearly spaced. The first bin is 0.03453 Hz and the following bins are: f(n) = f(n-1)*1.1; n=2,30. The data provided is the log10 of spectra density. To obtain the spectral density one has to take to the power 10 (10 ** data). This will give the units 2D wave spectra as m**2 s radian**-1 . Very small values are discarded and set as missing values. These are essentially 0 m**2 s radian**-1. This recoding can be done with the Python xarray package, for example:
Units of 2D wave spectra Once decoded, the units of 2D wave spectra are m2 s radian-1 |
Instantaneous and Accumulated parameters
Instantaneous parameters represent an average over the model time step (30min). Accumulated parameters are accumulated from the start of the forecast, ie. from 00 UTC or 12 UTC to the time step selected. All the analysed fields are instantaneous instead forecast data could be either instantaneous or accumulated, depending on the parameter. More detailed information on parameters are shown in Parameter listing.
Minimum/maximum since the previous post processing
In ERA-Interim there are some parameters named '...since previous post-processing', for example 'Maximum temperature at 2 metres since previous post-processing'. This represents the maximum temperature between the previous archived forecast 'Step' and the forecast 'Step'. For example, 'Maximum temperature at 2 metres since previous post-processing' with start time 00 UTC and Step=9, is the maximum 2m temperature in the 3-hour period between 06 UTC and 09 UTC.
Monthly means
ERA-interim sub-daily data are monthly averaged on data with valid times or accumulation periods that fall within the calendar month in question. The different monthly means are:
- Synoptic Monthly Means (stream=mnth) are the monthly averages:
- for each analysis time (at the four main synoptic hours - 00, 06, 12, and 18 UTC)
- for each forecast start time (00 and 12 UTC) and step (3, 6, 9, 12 etc).
- Monthly Means of Daily Means (stream=moda) are only available for analysis and instantaneous forecast data.
- Monthly Means of Daily Forecast Accumulations (stream=mdfa) are similar to Monthly Means of Daily Means but are for accumulated fields (eg precipitation, radiation) and three step ranges are available.
See also Section 3 of the ERA-Interim archive documentation.
Data format
Model level fields are in GRIB2 format. All other fields are in GRIB1, unless otherwise indicated.
Level listings
Pressure levels: 1000/975/950/925/900/875/850/825/800/775/750/700/650/600/550/500/450/400/350/300/250/225/200/175/150/125/100/70/50/30/20/10/7/5/3/2/1
Potential temperature levels: 265/275/285/300/315/320/330/350/370/395/430/475/530/600/700/850
Model levels: 1/to/60, which are described at L60 model level definitions
Potential vorticity level:
Mathinline |
---|
PV=\pm 2PVU |
Parameter listings
Tables 1-6 below describe the surface and single level parameters (levtype=sfc), Table 7 describes wave parameters, Table 8 describes the monthly mean exceptions for surface and single level and wave parameters and Tables 9-13 describe upper air parameters on various levtypes. Information on all ECMWF parameters is available from the ECMWF parameter database.
Parameters described as "instantaneous" below, are valid at the specified time.
Instantaneous, invariant, surface and single level parameters table
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Instantaneous, varying, surface and single level parameters table
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1
2
3Forecasts are only available up to a range of 12-hours |
Forecast accumulated surface and single level parameters
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Forecast minimum/maximum
...
count
...
name
...
an
...
fc
...
paramId
...
units
...
1
...
Sea ice fraction
...
x
...
x
...
31
...
(0 - 1)
...
2
...
Snow albedo
...
x
...
x
...
32
...
(0 - 1)
...
3
...
Snow density
...
x
...
x
...
33
...
kg m-3
...
4
...
Sea surface temperature
...
x
...
x
...
34
...
K
...
5
...
Sea ice temperature layer 11
...
x
...
x
...
35
...
K
...
6
...
Sea ice temperature layer 21
...
x
...
x
...
36
...
K
...
7
...
Sea ice temperature layer 31
...
x
...
x
...
37
...
K
...
8
...
Sea ice temperature layer 41
...
x
...
x
...
38
...
K
...
9
...
Volumetric soil water level 12
...
x
...
x
...
39
...
m3 m-3
...
10
...
Volumetric soil water level 22
...
x
...
x
...
40
...
m3 m-3
...
11
...
Volumetric soil water level 32
...
x
...
x
...
41
...
m3 m-3
...
12
...
Volumetric soil water level 42
...
x
...
x
...
42
...
m3 m-3
...
13
...
Convective available potential energy (CAPE)
...
x
...
59
...
J kg-1
...
14
...
Total column liquid water
...
x
...
78
...
kg m-2
...
15
...
Total column ice water
...
x
...
79
...
kg m-2
...
16
...
Surface pressure3
...
x
...
x
...
134
...
Pa
...
19
...
Soil temperature level 1
...
x
...
x
...
139
...
K
...
20
...
Snow depth
...
x
...
x
...
141
...
m of water equivalent
...
21
...
Charnock parameter
...
x
...
x
...
148
...
~
...
22
...
Mean sea level pressure
...
x
...
x
...
151
...
Pa
...
23
...
Boundary layer height
...
x
...
159
...
m
...
24
...
Total cloud cover
...
x
...
x
...
164
...
(0 - 1)
...
25
...
10 metre U wind component
...
x
...
x
...
165
...
m s-1
...
26
...
10 metre V wind component
...
x
...
x
...
166
...
m s-1
...
27
...
2 metre temperature
...
x
...
x
...
167
...
K
...
28
...
2 metre dewpoint temperature
...
x
...
x
...
168
...
K
...
29
...
Soil temperature level 2
...
x
...
x
...
170
...
K
...
32
...
Soil temperature level 3
...
x
...
x
...
183
...
K
...
33
...
Low cloud cover
...
x
...
x
...
186
...
(0 - 1)
...
34
...
Medium cloud cover
...
x
...
x
...
187
...
(0 - 1)
...
35
...
High cloud cover
...
x
...
x
...
188
...
(0 - 1)
...
36
...
Skin reservoir content
...
x
...
x
...
198
...
m of water equivalent
...
37
...
Total column ozone
...
x
...
206
...
kg m-2
...
38
...
Instantaneous eastward turbulent surface stress
...
x
...
229
...
N m-2
...
39
...
Instantaneous northward turbulent surface stress
...
x
...
230
...
N m-2
...
40
...
Instantaneous surface heat flux
...
x
...
231
...
W m-2
...
41
...
Instantaneous moisture flux (evaporation)
...
x
...
232
...
kg m-2 s
...
43
...
Skin temperature
...
x
...
x
...
235
...
K
...
44
...
Soil temperature level 4
...
x
...
x
...
236
...
K
...
45
...
Temperature of snow layer
...
x
...
x
...
238
...
K
...
46
...
Forecast albedo
...
x
...
243
...
~
...
47
...
Forecast surface roughness
...
x
...
244
...
m
...
48
...
Forecast logarithm of surface roughness for heat
...
x
...
245
...
~
1
...
title | Ice layers |
---|
...
2
...
title | Soil layers |
---|
...
3Forecasts are only available up to a range of 12-hours
Forecast accumulated surface and single level parameters
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Clear sky surface photosynthetically active radiation | 20 | W m-2s | 2 | Snow evaporation | 44 | m | 3 | Snow melt | 45 | m | 4 | Large-scale precipitation fraction 50 s 5 | Downward UV radiation at the surface 57 W m-2 s 6 | Surface photosynthetically active radiation | 58 | W m-2 s | 7 | Large-scale precipitation | 142 | m of water | 8 | Convective precipitation | 143 | m of water | 9 | Snowfall | 144 | m of water equivalent | 10 | Boundary layer dissipation 145 W m-2 s 11 | Surface sensible heat flux 146 W m-2 s 12 | Surface latent heat flux 147 W m-2 s 13 | Downward surface solar radiation 169 W m-2 s 14 Downward surface thermal radiation 175 W m-2 s 15 | Surface solar radiation 176 W m-2 s 16 | Surface thermal radiation 177 W m-2 s 17 | Top solar radiation 178 W m-2 s 18 Top thermal radiation 179 W m-2 s 19 East-West turbulent surface stress 180 N m-2 s 20 North-South turbulent surface stress 181 N m-2 s 21 East-West gravity wave surface stress 195 N m-2 s 22 North-South gravity wave surface stress 196 N m-2 s 23 Gravity wave dissipation 197 W m-2 s 24 | Runoff | 205 | m of water | 25 Top net solar radiation, clear sky 208 W m-2 s 26 Top net thermal radiation, clear sky 209 W m-2 s 27 Surface solar radiation, clear sky 210 W m-2 s 28 Surface thermal radiation, clear sky 211 W m-2 s 29 TOA incident solar radiation 212 W m-2 s 30 Total precipitation 228 m of water 31 Convective snowfall 239 m of water equivalent 32 Large-scale snowfall 240 m of water equivalent |
Forecast minimum/maximum surface and single level parameters
...
title | Table 4 |
---|
...
count
...
name
...
paramId
...
units
...
1
...
Wind gusts at 10 m
...
49
...
2
...
Max. temp. at 2 m since previous post-processing
...
201
...
K
...
3
...
Min. temp. at 2 m since previous post-processing
...
202
...
K
|
Vertical single level integrals for budgets
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Vertical single level integrals for budgets
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Wave and gridded ERS altimeter parameters
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Wave and gridded ERS altimeter parameters
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1Forecasts are also available at a range of 3-hours 2Available from late 1991 3Available for 30 frequencies and 24 directions |
Monthly mean surface and single level parameters: Exceptions from Tables 1-4
...
title | Table 7 |
---|
...
count
...
name
...
an
...
fc
...
paramId
...
units
...
1
...
Magnitude of surface stress (accumulated)
...
48
...
2
...
Wind gusts at 10 m
...
no mean
...
49
...
m s-1
...
3
...
Geopotential
...
x
...
no mean
...
129
...
m-2 s-2
...
4
...
Land/sea mask
...
x
...
no mean
...
172
...
(0,1)
...
5
...
Max. temp. at 2 m since previous post-processing
...
no mean
...
201
...
K
...
6
...
Min. temp. at 2 m since previous post-processing
...
no mean
...
202
...
K
...
7
...
x
...
x
...
207
...
m s-1
Accumulated model full levels parameters to validate clear sky radiation
...
title | Table 8 |
---|
...
count
...
name
...
an
...
fc
...
paramId
...
units
...
1
...
Short wave radiative tendency
...
x
...
100
...
2
...
Long wave radiative tendency
...
x
...
x
...
101
...
K
...
3
...
Clear sky short wave radiative tendency
...
x
...
K
...
4
...
Clear sky long wave radiative tendency
...
x
...
x
...
103
...
K
Accumulated model half or model full levels parameters to support chemical transport modelling
...
title | Table 9 |
---|
...
count
...
name
...
an
...
fc
...
paramId
...
units
...
1
...
Updraught mass flux
...
x
...
104
...
2
...
Downdraught mass flux
...
x
...
x
...
105
...
kg m-2
...
3
...
Updraught detrainment rate
...
x
...
x
...
106
...
kg m-2
...
4
...
Downdraught detrainment rate
...
x
...
x
...
107
...
kg m-2
...
5
...
Total precipitation profile
...
x
...
x
...
108
...
kg m-2
...
6
...
Turbulent diffusion coefficient for heat
...
x
...
x
...
109
...
m2
...
Accumulated model full levels net tendencies
...
title | Table 10 |
---|
...
count
...
name
...
an
...
fc
...
paramId
...
units
...
1
...
T tendency
...
x
...
x
...
110
...
K
...
2
...
q tendency
...
x
...
x
...
111
...
kg kg-1
...
3
...
u tendency
...
x
...
112
...
4
...
v tendency
...
x
...
x
...
113
...
m s-1
Parameters on isentropic surfaces
...
title | Table 11 |
---|
...
count
...
name
...
gg1
...
sh2
...
paramId
...
units
...
1
...
Montgomery potential
...
53
...
2
...
Pressure
...
x
...
54
...
Pa
...
3
...
Potential vorticity3
...
x
...
60
...
m2 s-1 K kg-1
...
4
...
Eastward wind component
...
131
...
m s-2
...
5
...
Northward wind component
...
132
...
m s-2
...
6
...
Specific humidity
...
x
...
133
...
kg/kg
...
1Gaussian grid
2Spherical harmonics
3Only PV is archived at 320 K
Parameters on the PV = ± 2 PVU surface
...
title | Table 12 |
---|
...
count
...
name
...
paramId
...
units
...
1
...
Potential temperature
...
3
...
2
...
Pressure
...
54
...
Pa
...
3
...
Geopotential
...
129
...
m2 s-2
...
4
...
Eastward wind component
...
131
...
m s-2
...
5
...
Northward wind component
...
132
...
m s-2
...
6
...
Specific humidity
...
133
...
kg/kg
...
Observations
The observations (satellite and in-situ) used as input into ERA-Interim are listed in tables below.
Satellite Data
...
title | Table 13 |
---|
...
Measurement
(sensitivities exploited in ERA5 / variables analysed)
...
BT (column water vapour, cloud liquid water,
precipitation and ocean surface wind speed)
...
1Forecasts are also available at a range of 3-hours 2Available from late 1991 3Available for 30 frequencies and 24 directions |
Monthly mean surface and single level parameters: Exceptions from Tables 1-4
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Accumulated model full levels parameters to validate clear sky radiation
Expand | ||||||||||||||||||||||||||||||
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Accumulated model half or model full levels parameters to support chemical transport modelling
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Accumulated model full levels net tendencies
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Parameters on isentropic surfaces
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1Gaussian grid 2Spherical harmonics 3Only PV is archived at 320 K |
Parameters on the PV = ± 2 PVU surface
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Observations
The observations (satellite and in-situ) used as input into ERA-Interim are listed in tables below.
Satellite Data
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In-situ data, provided by WMO WIS
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Computation of near-surface humidity and snow cover
Near-surface humidity
Near-surface humidity is not archived directly in ERA datasets, but the archive contains near-surface (2m from the surface) temperature (T), dew point temperature (Td), and surface pressure (sp) from which you can calculate specific and relative humidity at 2m:
- Specific humidity can be calculated over water and ice using equations 6.4 and 6.5 from Part IV, Physical processes section (Chapter 6, section 6.6.1b) in the documentation of the IFS for CY31R1. Use the 2m dew point temperature and surface pressure (which is approximately equal to the pressure at 2m) in these equations.
- Relative humidity should be calculated: RH = 100 * es(Td)/es(T)
The relative humidity can be calculated with respect to saturation over water, ice or mixed phase by defining es(T) with respect to saturation over water, ice or mixed phase (water and ice). The usual practice is to define near-surface relative humidity with respect to saturation over water.
In the ECMWF model (IFS), snow is represented by an additional layer on top of the uppermost soil level. The whole grid box may not be covered in snow. The snow cover gives the fraction of the grid box that is covered in snow. The method for calculating snow cover depends on the particular version of the IFS and for ERA-Interim is computed directly using snow water equivalent (ie parameter SD (141.128)) as:
Panel | ||
---|---|---|
| ||
snow_cover (SC) = min(1, RW*SD/15 ) where RW is density of water equal to 1000 and RSN is density of snow (parameter 33.128). |
The Physical depth of snow where there is snow cover is equal to RW*SD/(RSN*SC) where RSN = density of snow (parameter 33.128). For more in depth information see:
- https://www.the-cryosphere.net/11/923/2017/tc-11-923-2017.pdf
- https://www.ecmwf.int/sites/default/files/elibrary/2013/13946-using-reanalyses-studying-eurasian-snow-cover-and-its-relationship-circulation-variability.pdf
- https://journals.ametsoc.org/doi/pdf/10.1175/JHM-D-12-012.1
Surface elevation and orography
In ERA-Interim, and often in meteorology, altitudes (the altitude of the land and sea surface, or specific altitudes in the atmosphere) are not represented as geometric altitude (in metres above the spheroid), but as geopotential height (in metres above the geoid). However, ECMWF archive the geopotential (in m2/s2), not the geopotential height.
In order to calculate the geopotential height of the land and sea surface (the so called surface geopotential height, or orography):
- First, download the surface geopotential (i.e. the geopotential of the land and sea surface): surface geopotential is available on model levels (at level=1), where it is archived in spectral form.
- Then divide the surface geopotential by g=9.80665 to obtain the surface geopotential height in metres.
In order to define the surface geopotential in ERA-Interim, the ECMWF model uses surface elevation data interpolated from GTOPO30, with some fixes for Antarctica and Greenland. See Chapter 10 Climatological data, of Part IV. Physical processes, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
Notes
- Over oceans the surface geopotential field shows 'spectral ripples' (Know issue KI6, see ERA-Interim known issues page for details), which are a reflection of the fact that the ERA-Interim model is a spectral model and the grid point surface geopotential has been spectrally fitted.
- The model levels are hybrid pressure/sigma, eg for ERA-Interim. See Chapter 2 Basic equations and discretization of Part III. Dynamics and numerical procedures, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
- The definition of the 60 model levels, for ERA-Interim, and the corresponding half-level, ph, and full-level, pf, values of pressure (for a standard atmosphere with a surface pressure of 1013.250 hPa), geopotential and geopotential heights can be found at http://www.ecmwf.int/en/forecasts/documentation-and-support/60-model-levels
Known issues
Please see the ERA-Interim known issues page for guidance and workarounds.
How to acknowledge and cite ERA-Interim
Guidelines
In addition to the terms and conditions of the license(s), users must:
- cite the CDS catalogue entry:
Copernicus Climate Change Service (2023): ERA-Interim atmospheric reanalysis. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), DOI: 10.24381/cds.f2f5241d (Accessed on DD-MMM-YYYY)
- provide clear and visible attribution to the Copernicus programme and to each data product used:
Copernicus programme:
[Generated using/Contains modified] Copernicus Climate Change Service information [year]. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.
Products:
Dee, D.P., Uppala, S.M., Simmons, A.J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M.A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A.C.M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A.J., Haimberger, L., Healy, S.B., Hersbach, H., Hólm, E.V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally, A.P., Monge-Sanz, B.M., Morcrette, J.J., Park, B.K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.N. and Vitart, F. (2011): ERA-Interim global atmospheric reanalysis. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), DOI: 10.24381/cds.f2f5241d (Accessed on DD-MMM-YYYY)
...
BT (column water vapour, cloud liquid water,
precipitation and ocean surface wind speed)
...
BT (T, humidity, column water vapour,
cloud liquid water, precipitation and ocean surface wind speed)
...
In-situ data, provided by WMO WIS
...
title | Table 14 |
---|
...
Computation of near-surface humidity and snow cover
Near-surface humidity
Near-surface humidity is not archived directly in ERA datasets, but the archive contains near-surface (2m from the surface) temperature (T), dew point temperature (Td), and surface pressure[1] (sp) from which you can calculate specific and relative humidity at 2m:
- Specific humidity can be calculated over water and ice using equations 6.4 and 6.5 from Part IV, Physical processes section (Chapter 6, section 6.6.1b) in the documentation of the IFS for CY31R1. Use the 2m dew point temperature and surface pressure (which is approximately equal to the pressure at 2m) in these equations.
- Relative humidity should be calculated: RH = 100 * es(Td)/es(T)
The relative humidity can be calculated with respect to saturation over water, ice or mixed phase by defining es(T) with respect to saturation over water, ice or mixed phase (water and ice). The usual practice is to define near-surface relative humidity with respect to saturation over water.
[1] Access to surface pressure varies by dataset. For example, for ERA-Interim surface pressure is available from the Web Interface and from the WebAPI, while for ERA-40 surface pressure is not available from the Web Interface, but only via the WebAPI.
In the ECMWF model (IFS), snow is represented by an additional layer on top of the uppermost soil level. The whole grid box may not be covered in snow. The snow cover gives the fraction of the grid box that is covered in snow. The method for calculating snow cover depends on the particular version of the IFS and for ERA-Interim is computed directly using snow water equivalent (ie parameter SD (141.128)) as:
Panel | ||
---|---|---|
| ||
snow_cover (SC) = min(1, RW*SD/15 ) where RW is density of water equal to 1000 and RSN is density of snow (parameter 33.128). |
The Physical depth of snow where there is snow cover is equal to RW*SD/(RSN*SC) where RSN = density of snow (parameter 33.128). For more in depth information see:
- https://www.the-cryosphere.net/11/923/2017/tc-11-923-2017.pdf
- https://www.ecmwf.int/sites/default/files/elibrary/2013/13946-using-reanalyses-studying-eurasian-snow-cover-and-its-relationship-circulation-variability.pdf
- https://journals.ametsoc.org/doi/pdf/10.1175/JHM-D-12-012.1
Surface elevation and orography
In ERA-Interim, and often in meteorology, altitudes (the altitude of the land and sea surface, or specific altitudes in the atmosphere) are not represented as geometric altitude (in metres above the spheroid), but as geopotential height (in metres above the geoid). However, ECMWF archive the geopotential (in m2/s2), not the geopotential height.
In order to calculate the geopotential height of the land and sea surface (the so called surface geopotential height, or orography):
- First, download the surface geopotential (i.e. the geopotential of the land and sea surface), http://apps.ecmwf.int/datasets/data/interim-full-invariant/?param=129.128
- Then divide the surface geopotential by g=9.80665 to obtain the surface geopotential height in metres.
In order to define the surface geopotential in ERA-Interim, the ECMWF model uses surface elevation data interpolated from GTOPO30, with some fixes for Antarctica and Greenland. See Chapter 10 Climatological data, of Part IV. Physical processes, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
Notes
- The surface geopotential is also available on model levels (at level=1), where it is archived in spectral form.
- Over oceans the surface geopotential field shows 'spectral ripples' (Know issue KI6, see ERA-Interim known issues page for details), which are a reflection of the fact that the ERA-Interim model is a spectral model and the grid point surface geopotential has been spectrally fitted.
- The model levels are hybrid pressure/sigma, eg for ERA-Interim. See Chapter 2 Basic equations and discretization of Part III. Dynamics and numerical procedures, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
- The definition of the 60 model levels, for ERA-Interim, and the corresponding half-level, ph, and full-level, pf, values of pressure (for a standard atmosphere with a surface pressure of 1013.250 hPa), geopotential and geopotential heights can be found at http://www.ecmwf.int/en/forecasts/documentation-and-support/60-model-levels
Known issues
Please see the ERA-Interim known issues page for guidance and workarounds.
How to acknowledge and cite ERA-Interim
To acknowledge the ERA-Interim data, please refer to the ERA-Interim licence for details on the wording to use.
To cite the source of the data, you may use the following data citation (as part of the bibliography):
Tip | ||
---|---|---|
| ||
European Centre for Medium-range Weather Forecast (ECMWF) (2011): The ERA-Interim reanalysis dataset, Copernicus Climate Change Service (C3S) (accessed <insert date of access here>), available from https://www.ecmwf.int/en/forecasts/datasets/archive-datasets/reanalysis-datasets/era-interim |
...
Reports
2004-2007
- IFS Documentation CY31R1 (model used to produced ERA-Interim)
...