4.6. L2P#
The altimeter L2P products are along-track files, usually corresponding to a satellite pass, processed from each mission data provider’s L1B product (usually referred to as SGDR) containing the waveforms. A retracking algorithm is applied to retrieve significant wave height from the instrument waveforms.
Additional post-processing is performed, like quality control, adjustment of significant wave height to a common reference, uncertainty estimation, etc… and complementary variables are also computed or added from other model or satellite sources. The content is fully consistent and standardised for each mission included in this dataset.
For altimeters, only the Ku band measurements are considered, when available (the only current exception being SARAL for which only Ka band is provided).
This section describes in detail the specific content of the version 4 L2P for Sea State CCI, configured as shown in the table Table 4.12, which can be used to locate appropriate information in this document.
netCDF File Contents |
Description |
|---|---|
Coordinate variables |
Information to permit locating data on non-orthogonal grids, as defined in Section 3.2 |
Geophysical data record variables |
environmental variables for 1st band altimeter (usually Ku) as defined in Section 4.6.1 |
Instrumental data record variables |
instrumental variables for 1st band altimeter (usually Ku) as defined in Section 4.6.2 |
Auxiliary data record variables |
auxiliary variables as defined in Section 4.6.3 |
Global Attributes |
A collection of required global attributes describing general characteristics of the file, as defined in section Section 3.3 |
4.6.1. L2P geophysical data record format specification#
The Table 4.13 provides an overview of the CCI Sea State L2P environment (geophysical) data record within a L2P file. In the following sections, each variable within the L2P data file is described in detail.
Variable Name |
Description |
Units |
|---|---|---|
Significant wave height, as retrieved from the altimeter retracker, averaged over 1 Hz cells, without bias correction. |
m |
|
RMS of the full resolution Hs measurements in |
m |
|
number of valid full resolution Hs measurements used to compute the Hs in |
1 |
|
Significant wave height, averaged over 1 Hz cells, with cross-mission bias correction. |
m |
|
Significant wave height, averaged over 1 Hz cells, with cross-mission bias correction and denoising. |
m |
|
Uncertainty of the significant wave height averaged over 1 Hz cells |
m |
|
Quality level (from 0 - worst to 3 - best) of the significant wave height averaged over 1 Hz cells |
code |
|
flag specifying the editing criteria on which a 1 Hz significant wave height measurement was rejected (meaning its quality level is not set to “good”). |
bit mask |
|
high-frequency noise attached to swh_adjusted, from EMD filter |
m |
|
first IMF attached to the denoising of swh_adjusted, from EMD filter |
m |
|
uncertainty estimated by EMD filter |
m |
4.6.1.1. swh#
The significant wave height (SWH), within 1 Hz cells, averaged from groups of full resolution 20 Hz (40 Hz for Saral, 18 Hz for Envisat) measurements calculated from the altimeter retracking, without any cross-mission bias correction.
For ERS-1, ERS-2, TOPEX, Sentinel-3 A & B and Sentinel-6, the 1 Hz measurements were estimated from the full resolution SWH measurements provided in the source Agency’s GDR & SGDR products. Refer to the processing details Section 4.5.2 for the specific source used for these missions.
For Jason-1, Jason-2, Jason-3, Envisat, SARAL and CryoSat-2, a specific retracking was performed, using the WHALES nadir altimetry retracker selected by the CCI Sea State experts. For more details on the WHALES retracker, refer to the processing details Section 4.5.2.
For all missions, the groups of full resolution measurements used to calculate the 1 Hz values are exactly the same as in the source Agency’s GDR & SGDR products. Both CCI and Agency files can be compared one to one, have the same number of measurements, and the same latitude, longitude, time for each 1 Hz measurement. A minimal number of 6 (12 for SARAL) valid points is required to estimate a valid 1 Hz measurement. For more information on how the full resolution measurements are compressed into 1 Hz values, refer to the processing details Section 4.5.3.
The swh variable in a L2P product follows the format shown in table Table 4.14.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh(time) ;
swh:_FillValue = 1.e+20 ;
swh:band = "Ku" ;
swh:long_name = "significant wave height, as estimated by the altimeter retracker, without any cross-mission bias correction" ;
swh:standard_name = "sea_surface_wave_significant_height" ;
swh:coverage_content_type = "physicalMeasurement" ;
swh:units = "m" ;
swh:ancillary_variables = "swh_quality swh_rejection_flags" ;
swh:coordinates = "lon lat" ;
4.6.1.2. swh_rms#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_rms(time) ;
swh_rms:_FillValue = 1.e+20 ;
swh_rms:band = "Ku" ;
swh_rms:long_name = "RMS of the full resolution significant wave height with a 1 Hz compressed measurement" ;
swh_rms:standard_name\' = "sea_surface_wave_significant_height standard_error" ;
swh_rms:coverage_content_type = "auxiliaryMeasurement" ;
swh_rms:units = "m" ;
swh_rms:coordinates = "lon lat" ;
4.6.1.3. swh_num_valid#
The number of valid points used to compute the significant wave height (SWH), within 1 Hz cells, from the full resolution measurements calculated from each altimeter waveform by the retracker.
The groups of full resolution SWH measurements used to calculate the 1 Hz values are exactly the same as in the source Agency’s GDR & SGDR products. Both CCI Sea State and Agency files can be compared one to one, have the same number of measurements, and the same latitude, longitude, time for each 1 Hz measurement.
Storage type |
Name |
Unit |
|---|---|---|
int |
|
1 (dimensionless) |
['byte swh_numval(time) ;',
'\tswh_numval:band = "Ku" ;',
'\tswh_numval:long_name = "number of full resolution valid points used to compute the 1 Hz significant wave height value" ;',
'\tswh_numval:standard_name = "sea_surface_wave_significant_height number_of_observations" ;',
'\tswh_numval:coverage_content_type = "auxiliaryMeasurement" ;',
'\tswh_numval:units = "1" ;',
'\tswh_numval:coordinates = "lon lat" ;']
4.6.1.4. swh_adjusted#
The bias-corrected significant wave height, in meters. The correction is based on cross-mission intercalibration as described in Section 4.5.6.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_adjusted(time) ;
swh_adjusted:_FillValue = 1.e+20 ;
swh_adjusted:band = "Ku" ;
swh_adjusted:long_name = "significant wave height, bias corrected" ;
swh_adjusted:standard_name = "sea_surface_wave_significant_height" ;
swh_adjusted:coverage_content_type = "physicalMeasurement" ;
swh_adjusted:units = "m" ;
swh_adjusted:ancillary_variables = "swh_quality swh_rejection_flags" ;
swh_adjusted:coordinates = "lat lon" ;
4.6.1.5. swh_denoised#
The bias-corrected and denoised significant wave height, in meters.
Important
This is the recommended significant wave height variable to be used for most applications.
A non-parametric denoising method based on Empirical Mode Decomposition (EMD, Huang
et al., 1998) and inspired by wavelet thresholding is applied to the
variable swh_adjusted to estimate the denoised significant wave height
(see Kopsinis and McLaughlin, 2009, Quilfen et al., 2018 and Quilfen and
Chapron, 2019ab), as detailed in Section 4.5.7.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_denoised(time) ;
swh_denoised:_FillValue = 1.e+20 ;
swh_denoised:units = "m" ;
swh_denoised:long_name = "significant wave height, bias corrected and denoised" ;
swh_denoised:standard_name = "sea_surface_wave_significant_height" ;
swh_denoised:comment = "EMD denoising by Quilfen et al." ;
swh_denoised:coordinates = "lat lon" ;
4.6.1.6. swh_uncertainty#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_uncertainty(time) ;
swh_uncertainty:_FillValue = 1.e+20 ;
swh_uncertainty:units = "m" ;
swh_uncertainty:long_name = "theoretical estimate of the uncertainty caused by speckle noise and sampling in 1-Hz averaged SWH values" ;
swh_uncertainty:standard_name = "sea_surface_wave_significant_height" ;
swh_uncertainty:comment = "EMD denoising by Quilfen et al." ;
swh_uncertainty:coordinates = "lat lon" ;
4.6.1.7. swh_quality_level#
Quality control of individual altimeter measurements is performed with checks
over the calculated 1 Hz values and ancillary variables. As a result, the
estimated significant wave height comes with a quality level provided in the
swh_quality_level variable. Its meaning is defined as described in
Table 4.20.
value |
meaning |
description |
|---|---|---|
0 |
undefined |
the measurement value is not defined or relevant (missing value, etc…), no quality check was applied. |
1 |
bad |
the measurement was qualified as not usable after quality check. |
2 |
acceptable |
the measurement may still be usable for some application or the quality check could not fully assess if it is a bad or good value (suspect). |
3 |
good |
the measurement is usable. |
For more details on how the quality level is set for a significant wave height measurement, refer to the processing details Section 4.5.4.
The swh_quality_level variable in a L2P product follows the format shown in
table Table 4.21.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
bit code |
byte swh_quality_level(time) ;
swh_quality_level:band = "Ku" ;
swh_quality_level:flag_values = 0b, 1b, 2b, 3b ;
swh_quality_level:flag_meanings = "undefined bad acceptable good" ;
swh_quality_level:long_name = "quality of significant wave height measurement" ;
swh_quality_level:standard_name = "sea_surface_wave_significant_height status_flag" ;
swh_quality_level:coverage_content_type = "qualityInformation" ;
swh_quality_level:coordinates = "lat lon" ;
4.6.1.8. swh_rejection_flags#
When SWH measurements were rejected as bad, the reason (quality test) for
which they were rejected is reported in the related swh_rejection_flags
variable. Refer to editing for details on the tests performed for
the quality check of the measurements.
Section 4.5.4 provides the meaning of each flag possibly raised, stored as a specific bit of an integer.
bit |
meaning |
|---|---|
nb_of_valid_swh_too_low |
the measurement was considered as invalid as there are indications of unsuitable waveforms for a proper SWH calculation. In particular there was no remaining 20 Hz values after all checks (such as distance to the closest shoreline) and outlier tests. |
sea_ice |
the measurement has possible ice contamination. The sea ice fraction is taken from an external source (such as the CCI Sea Ice microwave based daily maps). Sea ice contamination is defined as areas where the sea ice fraction is greater than a minimal threshold (corresponding to 10% of ice in the current configuration). |
swh_validity |
the SWH measurement was considered as invalid (out of the ]0, 30] meter range for instance). |
swh_rms_outlier |
the measurement was considered as invalid when the RMS of the SWH measurements used to estimate each 1 Hz SWH measurement was beyond the acceptable threshold for a given range of SWH. |
outlier |
the measurement was considered as invalid when performing the SWH outlier test, based on the neighbouring measurements within a 100 km window. |
The swh_rejection_flags variable in a L2P product follows the format shown in
table Table 4.23.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
bit mask |
byte swh_rejection_flags(time) ;
swh_rejection_flags:band = "Ku" ;
swh_rejection_flags:flag_masks = 1b, 2b, 4b, 8b, 16b ;
swh_rejection_flags:flag_meanings = "nb_of_valid_swh_too_low swh_validity sea_ice swh_rms_outlier outlier_test" ;
swh_rejection_flags:long_name = "consolidated instrument and sanity check flags raised when downgrading the swh quality level" ;
swh_rejection_flags:standard_name = "sea_surface_wave_significant_height status_flag" ;
swh_rejection_flags:coverage_content_type = "qualityInformation" ;
swh_rejection_flags:coordinates = "lat lon" ;
4.6.1.9. swh_emd_noise#
The high-frequency noise of SWH returned by the application of the
Empirical Mode Decomposition (EMD, Huang et al., 1998) to the bias-corrected
significant wave height (swh_adjusted), in meters. See Section 4.5.7
for more details on the SWH denoising.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_emd_noise(time) ;
swh_emd_noise:_FillValue = 1.e+20 ;
swh_emd_noise:units = "m" ;
swh_emd_noise:long_name = "standard deviation of the ensemble of noisy signals used to estimate swh_denoised" ;
swh_emd_noise:comment = "EMD denoising by Quilfen et al." ;
swh_emd_noise:coordinates = "lat lon" ;
4.6.1.10. swh_emd_imf1#
The first Intrinsic Mode Function of SWH returned by the application of the
Empirical Mode Decomposition (EMD, Huang et al., 1998) to the bias-corrected
significant wave height (swh_adjusted), in meters. See Section 4.5.7
for more details on the SWH denoising.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_emd_imf1(time) ;
swh_emd_imf1:_FillValue = 1.e+20 ;
swh_emd_imf1:units = "m" ;
swh_emd_imf1:long_name = "first IMF attached to swh_adjusted" ;
swh_emd_imf1:comment = "EMD denoising by Quilfen et al." ;
swh_emd_imf1:coordinates = "lat lon" ;
4.6.1.11. swh_emd_uncertainty#
The SWH uncertainty returned by the application of the
Empirical Mode Decomposition (EMD, Huang et al., 1998) to the bias-corrected
significant wave height (swh_adjusted), in meters. See Section 4.5.7
for more details on the SWH denoising.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m (meter) |
double swh_emd_uncertainty(time) ;
swh_emd_uncertainty:_FillValue = 1.e+20 ;
swh_emd_uncertainty:units = "m" ;
swh_emd_uncertainty:long_name = "uncertainty attached to swh_adjusted" ;
swh_emd_uncertainty:standard_name = "sea_surface_wave_significant_height standard_error" ;
swh_emd_uncertainty:comment = "EMD denoising by Quilfen et al." ;
swh_emd_uncertainty:coordinates = "lat lon" ;
4.6.2. L2P instrumental data record format specification#
The Table 4.27 provides an overview of the CCI Sea State L2P instrumental data record within a L2P file, mainly the altimeter backscatter in available sensing bands (Ku, C, and Ka for SARAL). In the following sections, each variable within the L2P data file is described in detail.
Note
Altimeters do not have all the same sensing bands and some of these variables may therefore be missing for some missions.
Variable Name |
Description |
Units |
|---|---|---|
Ku band backscatter coefficient, as calculated from the retracking |
dB |
|
RMS of the Ku band backscatter coefficient, within 1Hz cells, of the 20 Hz measurements calculated from the retracking |
dB |
|
number of valid points used to compute Ku band backscatter coefficient, within 1Hz cells, of the 20 Hz measurements calculated from the retracking |
1 |
|
Quality level (from 0 - worst to 3 - best) of the Ku band sigma0 averaged over 1 Hz cells |
||
flag specifying the editing criteria on which a 1 Hz Ku band sigma0 measurement was rejected (meaning its quality level is not set to “good”). |
||
C band backscatter coefficient, as calculated from the retracking |
dB |
|
RMS of the C band backscatter coefficient, within 1Hz cells, of the 20 Hz measurements calculated from the retracking |
dB |
|
number of valid points used to compute C band backscatter coefficient, within 1Hz cells, of the 20 Hz measurements calculated from the retracking |
1 |
|
Quality level (from 0 - worst to 3 - best) of the C band sigma0 averaged over 1 Hz cells |
||
flag specifying the editing criteria on which a 1 Hz C band sigma0 measurement was rejected (meaning its quality level is not set to “good”). |
4.6.2.1. sigma0_ku#
The Ku-band backscatter coefficients (sigma0), within 1 Hz cells, averaged from groups of full resolution 20 Hz (18 Hz for Topex) measurements calculated from the altimeter retracking, without any cross-mission bias correction. The Ku-band sigma0 is only provided for Ku-band altimeters (excluding SARAL/AltiKa for instance).
The 1 Hz measurements were estimated from the full resolution sigma0 measurements provided in the source Agency’s GDR & SGDR products, including when SWH was estimated with the CCI Sea State selected retracker (WHALES). Refer to the processing details Section 4.5.2 for the specific source used for each mission.
For all missions, the groups of full resolution measurements used to calculate the 1 Hz values are exactly the same as in the source Agency’s GDR & SGDR products. Both CCI and Agency files can be compared one to one, have the same number of measurements, and the same latitude, longitude, time for each 1 Hz measurement. A minimal number of 6 valid points is required to estimate a valid 1 Hz measurement. For more information on how the full resolution measurements are compressed into 1 Hz values, refer to the processing details Section 4.5.3.
The sigma0_ku variable in a L2P product follows the format shown in table
Table 4.28.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
dB |
double sigma0_ku(time) ;
sigma0_ku:_FillValue = 1.e+20 ;
sigma0_ku:band = "Ku" ;
sigma0_ku:long_name = "backscatter coefficient" ;
sigma0_ku:standard_name = "surface_backwards_scattering_coefficient_of_radar_wave" ;
sigma0_ku:coverage_content_type = "physicalMeasurement" ;
sigma0_ku:units = "dB" ;
sigma0_ku:coordinates = "lon lat" ;
4.6.2.2. sigma0_ku_rms#
4.6.2.3. sigma0_ku_num_valid#
The number of valid points used to compute the Ku band backscatter coefficient (sigma0), within 1 Hz cells, from the full resolution measurements calculated from each altimeter waveform by the source Agency’s retracker.
The groups of full resolution sigma0 measurements used to calculate the 1 Hz values are exactly the same as in the source Agency’s GDR & SGDR products. Both CCI Sea State and Agency files can be compared one to one, have the same number of measurements, and the same latitude, longitude, time for each 1 Hz measurement.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
1 |
4.6.2.4. sigma0_ku_quality_level#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
enumerate |
byte sigma0_ku_quality_level(time) ;
sigma0_ku_quality_level:band = "Ku" ;
sigma0_ku_quality_level:flag_values = 0b, 1b, 2b, 3b ;
sigma0_ku_quality_level:flag_meanings = "undefined bad acceptable good" ;
sigma0_ku_quality_level:long_name\' = "quality of compressed backscatter coefficient" ;
sigma0_ku_quality_level:standard_name = "surface_backwards_scattering_coefficient_of_radar_wave status_flag" ;
sigma0_ku_quality_level:coverage_content_type = "qualityInformation" ;
sigma0_ku_quality_level:coordinates = "lat lon" ;
4.6.2.5. sigma0_ku_rejection_flags#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
bit code |
byte sigma0_ku_rejection_flags(time) ;
sigma0_ku_rejection_flags:band = "Ku" ;
sigma0_ku_rejection_flags:flag_masks = 1b, 2b, 4b ;
sigma0_ku_rejection_flags:flag_meanings = "nb_of_valid_sigma0_too_low sigma0_validity sea_ice" ;
sigma0_ku_rejection_flags:long_name = "consolidated instrument and sanity check flags raised when downgrading backscatter coefficient quality level" ;
sigma0_ku_rejection_flags:standard_name = "surface_backwards_scattering_coefficient_of_radar_wave status_flag\n" ;
sigma0_ku_rejection_flags:coverage_content_type = "qualityInformation" ;
sigma0_ku_rejection_flags:coordinates = "lon lat" ;
4.6.3. L2P auxiliary data record format specification#
The Table 4.32 provides an overview of the CCI Sea State L2P auxiliary data records within a L2P file. In the following sections, each variable within this table is described in detail.
Variable Name |
Description |
Units |
|---|---|---|
Distance to the nearest shoreline |
m |
|
Water depth to sea floor |
m |
|
Water depth to sea floor |
1 |
|
Total column cloud liquid water |
kg m-2 |
|
2 metre temperature |
K |
|
Sea surface temperature |
K |
|
10 metre U wind component |
m s-1 |
|
10 metre V wind component |
m s-1 |
|
Surface pressure |
Pa |
|
Significant height of combined wind waves and swell |
||
Peak wave period |
s |
|
Mean wave period based on first moment of swell |
s |
|
Significant wave height of first swell partition |
m |
|
Mean wave direction of first swell partition |
degree |
|
Mean wave period |
s |
|
Mean wave direction |
degree |
|
Significant height of wind waves |
m |
|
Mean direction of wind waves |
degree |
|
Mean period of wind waves |
s |
|
Significant height of wind and swell waves |
m |
|
Mean period T02 |
s |
|
Mean period T0m1 |
s |
|
Electromagnetic bias coefficient |
1 |
|
Wave peak frequency |
s-1 |
|
Wave mean direction (from) |
degree |
|
skewness of P(z,sx,sy=0) |
1 |
|
2D wavenumber peakedness |
m rad-1 |
4.6.3.1. distance_to_coast#
The distance to the nearest coastline for each ocean measurement was extracted from the Distance to Nearest Coastline grid at 0.01 degree resolution, provided by the NASA Goddard Space Flight Center (GSFC) Ocean Color Group and available at: http://www.pacioos.hawaii.edu/metadata/dist2coast_1deg.html
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m |
double distance_to_coast(time) ;
distance_to_coast:_FillValue = 1.e+20 ;
distance_to_coast:units = "m" ;
distance_to_coast:source = "GSFC" ;
distance_to_coast:coordinates = "lat lon" ;
4.6.3.2. bathymetry#
The same bathymetry source was used for all mission to get the ocean sea floor depth. We selected the 15 arc second General Bathymetric Chart of the Oceans (GEBCO), 2024 (https://doi.org/10.5285/1c44ce99-0a0d-5f4f-e063-7086abc0ea0f), available at: https://www.gebco.net.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m |
double bathymetry(time) ;
bathymetry:_FillValue = 1.e+20 ;
bathymetry:long_name = "ocean depth" ;
bathymetry:standard_name = "sea_floor_depth_below_mean_sea_level" ;
bathymetry:units = "m" ;
bathymetry:coverage_content_type = "auxiliaryInformation" ;
bathymetry:source = "The GEBCO_2024 Grid, www.gebco.net, doi: 10.5285/1c44ce99-0a0d-5f4f-e063-7086abc0ea0f" ;
bathymetry:institution = "BODC" ;
bathymetry:positive = "up" ;
bathymetry:coordinates = "lon lat" ;
4.6.3.3. sea_ice_fraction#
Sea ice concentration is provided as an ancillary variable but also used in the SWH editing procedure. We use an external sea ice concentration product to discard possibly ice contaminated measurements. Because no products provides a complete temporal coverage (missed acquisitions, non continuities between different microwave radiometer missions, infrequent updates of some datasets), we had to use different sources, as reported in Section 4.5.5.3.
The sea ice concentration is expressed as a fraction.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
1 |
double sea_ice_fraction(time) ;
sea_ice_fraction:_FillValue = 1.e+20 ;
sea_ice_fraction:units = "1" ;
sea_ice_fraction:long_name = "fraction of sea ice in water" ;
sea_ice_fraction:standard_name = "sea_ice_fraction" ;
sea_ice_fraction:source = "" ;
sea_ice_fraction:coverage_content_type = "auxiliaryInformation" ;
sea_ice_fraction:institution = "" ;
string sea_ice_fraction:source_files = "ice_conc_nh_ease2-250_icdr-amsr2_202210271200.nc", "ice_conc_sh_ease2-250_icdr-amsr2_202210271200.nc" ;
sea_ice_fraction:coordinates = "lat lon" ;
4.6.3.4. era5_total_column_cloud_liquid_water#
The total column cloud liquid water in the atmosphere, from ERA5 model
reanalysis (tclw variable), in kg per m2.
This parameter is the amount of liquid water contained within cloud droplets in a column extending from the surface of the Earth to the top of the atmosphere. Rain water droplets, which are much larger in size (and mass), are not included in this parameter. This parameter represents the area averaged value for a model grid box. Clouds contain a continuum of different sized water droplets and ice particles. The ECMWF Integrated Forecasting System (IFS) cloud scheme simplifies this to represent a number of discrete cloud droplets/particles including: cloud water droplets, raindrops, ice crystals and snow (aggregated ice crystals). The processes of droplet formation, phase transition and aggregation are also highly simplified in the IFS.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
kg m-2 |
float era5_total_column_cloud_liquid_water(time) ;
era5_total_column_cloud_liquid_water:_FillValue = 1.e+20f ;
era5_total_column_cloud_liquid_water:units = "kg m-2" ;
era5_total_column_cloud_liquid_water:long_name = "Total column cloud liquid water" ;
era5_total_column_cloud_liquid_water:source = "ECWMF ERA5 Reanalysis" ;
era5_total_column_cloud_liquid_water:source_variable = "tclw" ;
era5_total_column_cloud_liquid_water:coordinates = "lat lon" ;
4.6.3.5. era5_2m_air_temperature#
The air temperature at 2 meter height, from ERA5 model reanalysis (t2m
variable), in Kelvin.
This parameter is the temperature of air at 2m above the surface of land, sea or inland waters. 2m temperature is calculated by interpolating between the lowest model level and the Earth’s surface, taking account of the atmospheric conditions. This parameter has units of kelvin (K). Temperature measured in kelvin can be converted to degrees Celsius (°C) by subtracting 273.15.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
K |
float era5_2m_air_temperature(time) ;
era5_2m_air_temperature:_FillValue = 1.e+20f ;
era5_2m_air_temperature:units = "K" ;
era5_2m_air_temperature:long_name = "2 metre temperature" ;
era5_2m_air_temperature:source = "ECWMF ERA5 Reanalysis" ;
era5_2m_air_temperature:source_variable = "t2m" ;
era5_2m_air_temperature:coordinates = "lat lon" ;
4.6.3.6. era5_sea_surface_temperature#
The sea surface temperature, from ERA5 model reanalysis (sst variable), in
Kelvin.
This parameter (SST) is the temperature of sea water near the surface. In ERA5, this parameter is a foundation SST, which means there are no variations due to the daily cycle of the sun (diurnal variations). SST, in ERA5, is given by two external providers. Before September 2007, SST from the HadISST2 dataset is used and from September 2007 onwards, the OSTIA dataset is used. This parameter has units of kelvin (K). Temperature measured in kelvin can be converted to degrees Celsius (°C) by subtracting 273.15.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
K |
float era5_sea_surface_temperature(time) ;
era5_sea_surface_temperature:_FillValue = 1.e+20f ;
era5_sea_surface_temperature:units = "K" ;
era5_sea_surface_temperature:long_name = "Sea surface temperature" ;
era5_sea_surface_temperature:source = "ECWMF ERA5 Reanalysis" ;
era5_sea_surface_temperature:source_variable = "sst" ;
era5_sea_surface_temperature:coordinates = "lat lon" ;
4.6.3.7. era5_eastward_wind#
The zonal wind speed at 10 meter height, from ERA5 model reanalysis (u10
variable), in meter per second.
This parameter is the eastward component of the 10m wind. It is the horizontal speed of air moving towards the east, at a height of ten metres above the surface of the Earth, in metres per second. Care should be taken when comparing this parameter with observations, because wind observations vary on small space and time scales and are affected by the local terrain, vegetation and buildings that are represented only on average in the ECMWF Integrated Forecasting System (IFS). This parameter can be combined with the V component of 10m wind to give the speed and direction of the horizontal 10m wind.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m s-1 |
4.6.3.8. era5_northward_wind#
The meridian wind speed at 10 meter height, from ERA5 model reanalysis (v10
variable), in meter per second.
This parameter is the northward component of the 10m wind. It is the horizontal speed of air moving towards the north, at a height of ten metres above the surface of the Earth, in metres per second. Care should be taken when comparing this parameter with observations, because wind observations vary on small space and time scales and are affected by the local terrain, vegetation and buildings that are represented only on average in the ECMWF Integrated Forecasting System (IFS). This parameter can be combined with the U component of 10m wind to give the speed and direction of the horizontal 10m wind.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m s-1 |
4.6.3.9. era5_surface_pressure#
The atmospheric pressure at sea level, from ERA5 model reanalysis (sp
variable), in Pascal.
This parameter is the pressure (force per unit area) of the atmosphere at the surface of land, sea and inland water. It is a measure of the weight of all the air in a column vertically above a point on the Earth’s surface. Surface pressure is often used in combination with temperature to calculate air density. The strong variation of pressure with altitude makes it difficult to see the low and high pressure weather systems over mountainous areas, so mean sea level pressure, rather than surface pressure, is normally used for this purpose. The units of this parameter are Pascals (Pa). Surface pressure is often measured in hPa and sometimes is presented in the old units of millibars, mb (1 hPa = 1 mb= 100 Pa).
Storage type |
Name |
Unit |
|---|---|---|
float |
|
Pa |
float era5_surface_pressure(time) ;
era5_surface_pressure:_FillValue = 1.e+20f ;
era5_surface_pressure:units = "Pa" ;
era5_surface_pressure:long_name = "Surface pressure" ;
era5_surface_pressure:standard_name = "surface_air_pressure" ;
era5_surface_pressure:source = "ECWMF ERA5 Reanalysis" ;
era5_surface_pressure:source_variable = "sp" ;
era5_surface_pressure:coordinates = "lat lon" ;
4.6.3.10. era5_swh#
The significant height of combined wind waves and swell, from ERA5/WAM model
reanalysis (swh variable), in meters.
This parameter represents the average height of the highest third of surface ocean/sea waves generated by wind and swell. It represents the vertical distance between the wave crest and the wave trough. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of both. More strictly, this parameter is four times the square root of the integral over all directions and all frequencies of the two-dimensional wave spectrum. This parameter can be used to assess sea state and swell. For example, engineers use significant wave height to calculate the load on structures in the open ocean, such as oil platforms, or in coastal applications.
Note
Note that ERA5 WAM model assimilates altimeter data. For a fully independent SWH estimate, use the WW3 SWH (see Section 4.6.3.20).
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m |
float era5_swh(time) ;
era5_swh:_FillValue = 1.e+20f ;
era5_swh:units = "m" ;
era5_swh:long_name = "Significant height of combined wind waves and swell" ;
era5_swh:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_swh:source_variable = "swh" ;
era5_swh:standard_name = "sea_surface_wave_significant_height" ;
era5_swh:coordinates = "lat lon" ;
4.6.3.11. era5_peak_wave_period#
The peak wave period of combined wind waves and swell, from ERA5/WAM model
reanalysis (pp1d variable), in seconds.
This parameter represents the period of the most energetic ocean waves generated by local winds and associated with swell. The wave period is the average time it takes for two consecutive wave crests, on the surface of the ocean/sea, to pass through a fixed point. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). This parameter is calculated from the reciprocal of the frequency corresponding to the largest value (peak) of the frequency wave spectrum. The frequency wave spectrum is obtained by integrating the two-dimensional wave spectrum over all directions. The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of both.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float era5_peak_wave_period(time) ;
era5_peak_wave_period:_FillValue = 1.e+20f ;
era5_peak_wave_period:units = "s" ;
era5_peak_wave_period:long_name = "Peak wave period" ;
era5_peak_wave_period:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_peak_wave_period:source_variable = "pp1d" ;
era5_peak_wave_period:standard_name = "sea_surface_wave_period_at_variance_spectral_density_maximum" ;
era5_peak_wave_period:coordinates = "lat lon" ;
4.6.3.12. era5_swell_mean_period#
The mean wave period of swell based on first moment for swell, from ERA5/WAM
model reanalysis (p1ps variable), in seconds.
This parameter is the reciprocal of the mean frequency of the wave components associated with swell. All wave components have been averaged proportionally to their respective amplitude. This parameter can be used to estimate the magnitude of Stokes drift transport in deep water associated with swell. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of all swell only. Moments are statistical quantities derived from the two-dimensional wave spectrum.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float era5_swell_mean_period(time) ;
era5_swell_mean_period:_FillValue = 1.e+20f ;
era5_swell_mean_period:units = "s" ;
era5_swell_mean_period:long_name = "Mean wave period based on first moment for swell" ;
era5_swell_mean_period:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_swell_mean_period:source_variable = "p1ps" ;
era5_swell_mean_period:standard_name = "sea_surface_primary_swell_mean_period_from_variance_spectral_density_second_frequency_moment" ;
era5_swell_mean_period:coordinates = "lat lon" ;
4.6.3.13. era5_swell_swh#
The significant wave height of first swell partition, from ERA5/WAM
model reanalysis (p140121 variable), in meters.
This parameter represents the average height of the highest third of surface ocean/sea waves associated with the first swell partition. Wave height represents the vertical distance between the wave crest and the wave trough. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. In many situations, swell can be made up of different swell systems, for example, from two distant and separate storms. To account for this, the swell spectrum is partitioned into up to three parts. The swell partitions are labelled first, second and third based on their respective wave height. Therefore, there is no guarantee of spatial coherence (the first might be from one system at one location and another system at the neighbouring location). More strictly, this parameter is four times the square root of the integral over all directions and all frequencies of the first swell partition of the two-dimensional swell spectrum. The swell spectrum is obtained by only considering the components of the two-dimensional wave spectrum that are not under the influence of the local wind. This parameter can be used to assess swell. For example, engineers use significant wave height to calculate the load on structures in the open ocean, such as oil platforms, or in coastal applications.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m |
float era5_swell_swh(time) ;
era5_swell_swh:_FillValue = 1.e+20f ;
era5_swell_swh:units = "m" ;
era5_swell_swh:long_name = "Significant wave height of first swell partition" ;
era5_swell_swh:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_swell_swh:source_variable = "p140121" ;
era5_swell_swh:standard_name = "sea_surface_primary_swell_wave_significant_height" ;
era5_swell_swh:coordinates = "lat lon" ;
4.6.3.14. era5_swell_direction#
The mean wave direction of first swell partition, from ERA5/WAM
model reanalysis (p140122 variable), in degrees.
This parameter is the mean direction of waves in the first swell partition. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. In many situations, swell can be made up of different swell systems, for example, from two distant and separate storms. To account for this, the swell spectrum is partitioned into up to three parts. The swell partitions are labelled first, second and third based on their respective wave height. Therefore, there is no guarantee of spatial coherence (the first swell partition might be from one system at one location and a different system at the neighbouring location). The units are degrees true, which means the direction relative to the geographic location of the north pole. It is the direction that waves are coming from, so 0 degrees means “coming from the north” and 90 degrees means “coming from the east”.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
degree |
float era5_swell_direction(time) ;
era5_swell_direction:_FillValue = 1.e+20f ;
era5_swell_direction:units = "degrees" ;
era5_swell_direction:long_name = "Mean wave direction of first swell partition" ;
era5_swell_direction:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_swell_direction:source_variable = "p140122" ;
era5_swell_direction:standard_name = "sea_surface_primary_swell_wave_from_direction" ;
era5_swell_direction:coordinates = "lat lon" ;
4.6.3.15. era5_mean_wave_period#
The mean wave period, from ERA5/WAM model reanalysis (mwp variable), in
seconds.
This parameter is the average time it takes for two consecutive wave crests, on the surface of the ocean/sea, to pass through a fixed point. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). This parameter is a mean over all frequencies and directions of the two-dimensional wave spectrum. The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of both. This parameter can be used to assess sea state and swell. For example, engineers use such wave information when designing structures in the open ocean, such as oil platforms, or in coastal applications.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float era5_mean_wave_period(time) ;
era5_mean_wave_period:_FillValue = 1.e+20f ;
era5_mean_wave_period:units = "s" ;
era5_mean_wave_period:long_name = "Mean wave period" ;
era5_mean_wave_period:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_mean_wave_period:source_variable = "mwp" ;
era5_mean_wave_period:standard_name = "sea_surface_wave_mean_period_from_variance_spectral_density_second_frequency_moment" ;
era5_mean_wave_period:coordinates = "lat lon" ;
4.6.3.16. era5_mean_wave_direction#
The mean wave direction, from ERA5/WAM model reanalysis (mwd variable), in
degrees.
This parameter is the mean direction of ocean/sea surface waves. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). This parameter is a mean over all frequencies and directions of the two-dimensional wave spectrum. The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of both. This parameter can be used to assess sea state and swell. For example, engineers use this type of wave information when designing structures in the open ocean, such as oil platforms, or in coastal applications. The units are degrees true, which means the direction relative to the geographic location of the north pole. It is the direction that waves are coming from, so 0 degrees means “coming from the north” and 90 degrees means “coming from the east”.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
degree |
float era5_mean_wave_direction(time) ;
era5_mean_wave_direction:_FillValue = 1.e+20f ;
era5_mean_wave_direction:units = "Degree true" ;
era5_mean_wave_direction:long_name = "Mean wave direction" ;
era5_mean_wave_direction:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_mean_wave_direction:source_variable = "mwd" ;
era5_mean_wave_direction:standard_name = "sea_surface_wave_mean_period" ;
era5_mean_wave_direction:coordinates = "lat lon" ;
4.6.3.17. era5_windwave_swh#
The significant height of wind waves, from ERA5/WAM model reanalysis (shww
variable), in meters.
This parameter represents the average height of the highest third of surface ocean/sea waves generated by the local wind. It represents the vertical distance between the wave crest and the wave trough. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of wind-sea waves only. More strictly, this parameter is four times the square root of the integral over all directions and all frequencies of the two-dimensional wind-sea wave spectrum. The wind-sea wave spectrum is obtained by only considering the components of the two-dimensional wave spectrum that are still under the influence of the local wind. This parameter can be used to assess wind-sea waves. For example, engineers use significant wave height to calculate the load on structures in the open ocean, such as oil platforms, or in coastal applications.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m |
float era5_windwave_swh(time) ;
era5_windwave_swh:_FillValue = 1.e+20f ;
era5_windwave_swh:units = "m" ;
era5_windwave_swh:long_name = "Significant height of wind waves" ;
era5_windwave_swh:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_windwave_swh:source_variable = "shww" ;
era5_windwave_swh:standard_name = "sea_surface_wind_wave_significant_height" ;
era5_windwave_swh:coordinates = "lat lon" ;
4.6.3.18. era5_windwave_direction#
The mean direction of wind waves, from ERA5/WAM model reanalysis (mdww
variable), in degrees.
The mean direction of waves generated by local winds. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of wind-sea waves only. It is the mean over all frequencies and directions of the total wind-sea wave spectrum. The units are degrees true, which means the direction relative to the geographic location of the north pole. It is the direction that waves are coming from, so 0 degrees means “coming from the north” and 90 degrees means “coming from the east”.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
degree |
float era5_windwave_direction(time) ;
era5_windwave_direction:_FillValue = 1.e+20f ;
era5_windwave_direction:units = "degrees" ;
era5_windwave_direction:long_name = "Mean direction of wind waves" ;
era5_windwave_direction:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_windwave_direction:source_variable = "mdww" ;
era5_windwave_direction:standard_name = "sea_surface_wind_wave_from_direction" ;
era5_windwave_direction:coordinates = "lat lon" ;
4.6.3.19. era5_windwave_period#
The mean period of wind waves, from ERA5/WAM model reanalysis (mpww
variable), in seconds.
This parameter is the average time it takes for two consecutive wave crests, on the surface of the ocean/sea generated by local winds, to pass through a fixed point. The ocean/sea surface wave field consists of a combination of waves with different heights, lengths and directions (known as the two-dimensional wave spectrum). The wave spectrum can be decomposed into wind-sea waves, which are directly affected by local winds, and swell, the waves that were generated by the wind at a different location and time. This parameter takes account of wind-sea waves only. It is the mean over all frequencies and directions of the total wind-sea spectrum.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float era5_windwave_period(time) ;
era5_windwave_period:_FillValue = 1.e+20f ;
era5_windwave_period:units = "s" ;
era5_windwave_period:long_name = "Mean period of wind waves" ;
era5_windwave_period:source = "ECMWF ERA5 / WAM Reanalysis" ;
era5_windwave_period:source_variable = "mpww" ;
era5_windwave_period:standard_name = "sea_surface_wind_wave_mean_period" ;
era5_windwave_period:coordinates = "lat lon" ;
4.6.3.20. ww3_swh#
The significant height of combined wind waves and swell, from Ifremer WW3 model
hindcast (hs variable), in meters.
Note
Note that Ifremer WW3 hindcast does not assimilate altimeter data and is therefore a fully independent SWH estimate, contrary to ERA5 reanalysis.
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m |
float ww3_swh(time) ;
ww3_swh:_FillValue = 1.e+20f ;
ww3_swh:long_name = "significant height of wind and swell waves" ;
ww3_swh:standard_name = "sea_surface_wave_significant_height" ;
ww3_swh:units = "m" ;
ww3_swh:source = "Ifremer WW3 Hindcast" ;
ww3_swh:source_variable = "hs" ;
ww3_swh:coordinates = "lat lon" ;
4.6.3.21. ww3_mean_wave_period#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float ww3_mean_wave_period(time) ;
ww3_mean_wave_period:_FillValue = 1.e+20f ;
ww3_mean_wave_period:long_name = "mean period T02" ;
ww3_mean_wave_period:standard_name = "sea_surface_mean_wave_mean_period_from_variance_spectral_density_second_frequency_moment" ;
ww3_mean_wave_period:units = "s" ;
ww3_mean_wave_period:source = "Ifremer WW3 Hindcast" ;
ww3_mean_wave_period:source_variable = "t02" ;
ww3_mean_wave_period:coordinates = "lat lon" ;
4.6.3.22. ww3_mean_wave_period_t0m1#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float ww3_mean_wave_period_t0m1(time) ;
ww3_mean_wave_period_t0m1:_FillValue = 1.e+20f ;
ww3_mean_wave_period_t0m1:long_name = "mean period T01" ;
ww3_mean_wave_period_t0m1:standard_name = "sea_surface_mean_wave_mean_period_from_variance_spectral_density_inverse_frequency_moment" ;
ww3_mean_wave_period_t0m1:units = "s" ;
ww3_mean_wave_period_t0m1:source = "Ifremer WW3 Hindcast" ;
ww3_mean_wave_period_t0m1:source_variable = "t01" ;
ww3_mean_wave_period_t0m1:coordinates = "lat lon" ;
4.6.3.23. ww3_emb#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float ww3_emb(time) ;
ww3_emb:_FillValue = 1.e+20f ;
ww3_emb:long_name = "EM-bias" ;
ww3_emb:units = "1" ;
ww3_emb:comment = "EM bias coefficient" ;
ww3_emb:source = "Ifremer WW3 Hindcast" ;
ww3_emb:source_variable = "emb" ;
ww3_emb:coordinates = "lat lon" ;
4.6.3.24. ww3_peak_wave_period#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
s |
float ww3_peak_wave_period(time) ;
ww3_peak_wave_period:_FillValue = 1.e+20f ;
ww3_peak_wave_period:units = "s" ;
ww3_peak_wave_period:standard_name = "sea_surface_wave_period_at_variance_spectral_density_maximum" ;
ww3_peak_wave_period:coordinates = "lat lon" ;
4.6.3.25. ww3_mean_wave_direction#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
degree |
float ww3_mean_wave_direction(time) ;
ww3_mean_wave_direction:_FillValue = 1.e+20f ;
ww3_mean_wave_direction:long_name = "wave mean direction" ;
ww3_mean_wave_direction:standard_name = "sea_surface_wave_from_mean_direction" ;
ww3_mean_wave_direction:units = "degree" ;
ww3_mean_wave_direction:source = "Ifremer WW3 Hindcast" ;
ww3_mean_wave_direction:source_variable = "dir" ;
ww3_mean_wave_direction:coordinates = "lat lon" ;
4.6.3.26. ww3_wave_skewness#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
1 |
float ww3_wave_skewness(time) ;
ww3_wave_skewness:_FillValue = 1.e+20f ;
ww3_wave_skewness:long_name = "skewness" ;
ww3_wave_skewness:units = "1" ;
ww3_wave_skewness:comment = "skewness of P(z,sx,sy=0)" ;
ww3_wave_skewness:source = "Ifremer WW3 Hindcast" ;
ww3_wave_skewness:source_variable = "skw" ;
ww3_wave_skewness:coordinates = "lat lon" ;
4.6.3.27. ww3_wavenumber_peakdness#
Storage type |
Name |
Unit |
|---|---|---|
float |
|
m rad-1 |
float ww3_wavenumber_peakdness(time) ;
ww3_wavenumber_peakdness:_FillValue = 1.e+20f ;
ww3_wavenumber_peakdness:long_name = "k-peakedness" ;
ww3_wavenumber_peakdness:units = "m/rad" ;
ww3_wavenumber_peakdness:comment = "2D wavenumber peakedness" ;
ww3_wavenumber_peakdness:source = "Ifremer WW3 Hindcast" ;
ww3_wavenumber_peakdness:source_variable = "qkk" ;
ww3_wavenumber_peakdness:coordinates = "lat lon" ;