L4

4.8. L4#

The altimeter L4 products are monthly gridded (1°x1°) significant wave height(SWH) statistics files, processed from each mission L2P series.

It provides statistics computed from swh_denoised variable in L2P (SWH bias corrected - adjusted - and denoised), retaining only the good quality measurements (quality_level equal to 3) within these L2P files. All missions processed as L2P and adjusted to each other are included and merged together within these monthly statistics.

For altimeters, only the Ku band measurements are considered, when available (the only current exception being SARAL for which only Ka band is provided). 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.

When processing the statistics described below, each satellite pass is first resampled onto the 1°x1° product grid. When several consecutive 1 Hz SWH measurements from the same satellite pass are contained within a given grid cell, the median of all these SWH measurements is retained as the representative value for the cell and satellite pass. This is why we refer in the following to statistics of median sea surface height. Each median value is from the same satellite pass and therefore corresponds to measurements within seconds of each other.

This section describes in detail the specific content of the version 4 L4 for Sea State CCI, configured as shown in the table Table 4.67, which can be used to locate appropriate information in this document.

Table 4.67 Summary description of the contents of a CCI Sea State L4 data product#

netCDF File Contents

Description

Coordinate variables

Information to permit locating data on orthogonal grids, as defined in Section 3.2

Statistical data record variables

significant wave height statistics for 1st band altimeter (usually Ku) as defined in Section 4.8.1

Global Attributes

A collection of required global attributes describing general characteristics of the file, as defined in section Section 3.3

4.8.1. L4 statistical data record format specification#

The Table 4.68 provides an overview of the CCI Sea State 4 significant wave height statistics data record within a L4 file. In the following sections, each variable within the L4 data file is described in detail.

Table 4.68 Summary description of CCI Sea State L4 SWH statistics records#

Variable Name

Description

Units

swh_mean

Mean of median significant wave height values

m

swh_max

Maximum median significant wave height value

m

swh_rms

RMS of median significant wave height values

m

swh_count

Number of median significant wave height values

1

swh_sum

Total of median significant wave height values

m

swh_squared_sum

Total of median significant wave height squared values

m2

swh_log_sum

Total of median significant wave height log values

m

swh_log_squared_sum

Total of median significant wave height log squared values

m2

swh_count_greater_than_0.50

Number of median significant wave height values greater than 0.5 m

1

swh_count_greater_than_1.00

Number of median significant wave height values greater than 1.0 m

1

swh_count_greater_than_1.50

Number of median significant wave height values greater than 1.5 m

1

swh_count_greater_than_2.00

Number of median significant wave height values greater than 2 m

1

swh_count_greater_than_2.50

Number of median significant wave height values greater than 2.5 m

1

swh_count_greater_than_3.00

Number of median significant wave height values greater than 3.0 m

1

swh_count_greater_than_3.50

Number of median significant wave height values greater than 3.5 m

1

swh_count_greater_than_4.00

Number of median significant wave height values greater than 4.0 m

1

swh_count_greater_than_5.00

Number of median significant wave height values greater than 5.0 m

1

swh_count_greater_than_6.00

Number of median significant wave height values greater than 6.0 m

1

swh_count_greater_than_8.00

Number of median significant wave height values greater than 8.0 m

1

swh_count_greater_than_10.00

Number of median significant wave height values greater than 10.0 m

1

4.8.1.1. swh_mean#

The mean, within 1°x1° grid cells, of all the (per cell) median significant wave height from all the satellite passes within a given month, in meters.

The swh_mean variable in a L4 product follows the format shown in table Table 4.69.

Table 4.69 CDL example description of swh_mean variable#

Storage type

Name

Unit

float

swh_mean

m (meter)

 
double swh_mean(time, lat, lon) ;
	swh_mean:_FillValue = NaN ;
	swh_mean:units = "m" ;
	swh_mean:long_name = "mean of median significant wave height values" ;
	swh_mean:coverage_content_type = "physicalMeasurement" ;
	swh_mean:grid_mapping = "crs" ;
	swh_mean:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: mean over days" ;

4.8.1.2. swh_max#

The maximum, within 1°x1° grid cells, of all the (per cell) median significant wave height from all the satellite passes within a given month, in meters.

The swh_max variable in a L4 product follows the format shown in table Table 4.70.

Table 4.70 CDL example description of swh_max variable#

Storage type

Name

Unit

float

swh_max

m (meter)

 
double swh_max(time, lat, lon) ;
	swh_max:_FillValue = NaN ;
	swh_max:units = "m" ;
	swh_max:long_name = "maximum median significant wave height value" ;
	swh_max:coverage_content_type = "physicalMeasurement" ;
	swh_max:grid_mapping = "crs" ;
	swh_max:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: maximum over days" ;

4.8.1.3. swh_rms#

The root mean square, within 1°x1° grid cells, of all the (per cell) median significant wave height from all the satellite passes within a given month, in meters.

The swh_rms variable in a L4 product follows the format shown in table Table 4.71.

Table 4.71 CDL example description of swh_rms variable#

Storage type

Name

Unit

float

swh_rms

m (meter)

 
double swh_rms(time, lat, lon) ;
	swh_rms:_FillValue = NaN ;
	swh_rms:units = "m" ;
	swh_rms:long_name = "rms of median significant wave height values" ;
	swh_rms:coverage_content_type = "physicalMeasurement" ;
	swh_rms:grid_mapping = "crs" ;
	swh_rms:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: root_mean_square over days" ;

4.8.1.4. swh_count#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height from all the satellite passes within a given month.

The swh_count variable in a L4 product follows the format shown in table Table 4.72.

Table 4.72 CDL example description of swh_count variable#

Storage type

Name

Unit

float

swh_count

1 (count)

 
int64 swh_count(time, lat, lon) ;
	swh_count:units = "1" ;
	swh_count:long_name = "number of median significant wave height values" ;
	swh_count:coverage_content_type = "physicalMeasurement" ;
	swh_count:grid_mapping = "crs" ;
	swh_count:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.5. swh_sum#

The sum, within 1°x1° grid cells, of all the (per cell) median significant wave height from all the satellite passes within a given month, in meters.

The swh_sum variable in a L4 product follows the format shown in table Table 4.73.

Table 4.73 CDL example description of swh_sum variable#

Storage type

Name

Unit

float

swh_sum

m (meter)

 
double swh_sum(time, lat, lon) ;
	swh_sum:_FillValue = NaN ;
	swh_sum:units = "m" ;
	swh_sum:long_name = "total of median significant wave height values" ;
	swh_sum:coverage_content_type = "physicalMeasurement" ;
	swh_sum:grid_mapping = "crs" ;
	swh_sum:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: sum over days" ;

4.8.1.6. swh_squared_sum#

The sum, within 1°x1° grid cells, of all the (per cell) squared median significant wave height from all the satellite passes within a given month, in square meters.

The swh_squared_sum variable in a L4 product follows the format shown in table Table 4.74.

Table 4.74 CDL example description of swh_squared_sum variable#

Storage type

Name

Unit

float

swh_squared_sum

\(m^2\) (square meter)

 
double swh_squared_sum(time, lat, lon) ;
	swh_squared_sum:_FillValue = NaN ;
	swh_squared_sum:units = "m2" ;
	swh_squared_sum:long_name = "total of median significant wave height squared values" ;
	swh_squared_sum:coverage_content_type = "physicalMeasurement" ;
	swh_squared_sum:grid_mapping = "crs" ;
	swh_squared_sum:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: sum_of_squares over days" ;

4.8.1.7. swh_log_sum#

The sum, within 1°x1° grid cells, of all the (per cell) log of median significant wave height from all the satellite passes within a given month, in meters.

The swh_log_sum variable in a L4 product follows the format shown in table Table 4.75.

Table 4.75 CDL example description of swh_log_sum variable#

Storage type

Name

Unit

float

swh_log_sum

m (meter)

 
double swh_log_sum(time, lat, lon) ;
	swh_log_sum:_FillValue = NaN ;
	swh_log_sum:units = "m" ;
	swh_log_sum:long_name = "total of median significant wave height log values" ;
	swh_log_sum:coverage_content_type = "physicalMeasurement" ;
	swh_log_sum:grid_mapping = "crs" ;
	swh_log_sum:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: sum_of_log over days" ;

4.8.1.8. swh_log_squared_sum#

The sum, within 1°x1° grid cells, of all the (per cell) squared log of median significant wave height from all the satellite passes within a given month, in meters.

The swh_log_squared_sum variable in a L4 product follows the format shown in table Table 4.76.

Table 4.76 CDL example description of swh_log_squared_sum variable#

Storage type

Name

Unit

float

swh_log_squared_sum

\(m^2\) (square meter)

 
double swh_log_squared_sum(time, lat, lon) ;
	swh_log_squared_sum:_FillValue = NaN ;
	swh_log_squared_sum:units = "m2" ;
	swh_log_squared_sum:long_name = "total of median significant wave height log squared values" ;
	swh_log_squared_sum:coverage_content_type = "physicalMeasurement" ;
	swh_log_squared_sum:grid_mapping = "crs" ;
	swh_log_squared_sum:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: sum_of_log_squares over days" ;

4.8.1.9. swh_count_greater_than_0.50#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 0.5 meter. passes within a given month, in meters.

The swh_count_greater_than_0.50 variable in a L4 product follows the format shown in table Table 4.77.

Table 4.77 CDL example description of swh_count_greater_than_0.50 variable#

Storage type

Name

Unit

float

swh_count_greater_than_0.50

1 (count)

 
double swh_count_greater_than_0.50(time, lat, lon) ;
	swh_count_greater_than_0.50:_FillValue = NaN ;
	swh_count_greater_than_0.50:units = "1" ;
	swh_count_greater_than_0.50:long_name = "number of median significant wave height values greater than 0.5m" ;
	swh_count_greater_than_0.50:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_0.50:grid_mapping = "crs" ;
	swh_count_greater_than_0.50:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count (interval:  over days" ;

4.8.1.10. swh_count_greater_than_1.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 1 meter. passes within a given month, in meters.

The swh_count_greater_than_1.00 variable in a L4 product follows the format shown in table Table 4.78.

Table 4.78 CDL example description of swh_count_greater_than_1.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_1.00

1 (count)

 
double swh_count_greater_than_1.00(time, lat, lon) ;
	swh_count_greater_than_1.00:_FillValue = NaN ;
	swh_count_greater_than_1.00:units = "1" ;
	swh_count_greater_than_1.00:long_name = "number of median significant wave height values greater than 1.0m" ;
	swh_count_greater_than_1.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_1.00:grid_mapping = "crs" ;
	swh_count_greater_than_1.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.11. swh_count_greater_than_1.50#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 1.5 meters. passes within a given month, in meters.

The swh_count_greater_than_1.50 variable in a L4 product follows the format shown in table Table 4.79.

Table 4.79 CDL example description of swh_count_greater_than_1.50 variable#

Storage type

Name

Unit

float

swh_count_greater_than_1.50

1 (count)

 
double swh_count_greater_than_1.50(time, lat, lon) ;
	swh_count_greater_than_1.50:_FillValue = NaN ;
	swh_count_greater_than_1.50:units = "1" ;
	swh_count_greater_than_1.50:long_name = "number of median significant wave height values greater than 1.5m" ;
	swh_count_greater_than_1.50:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_1.50:grid_mapping = "crs" ;
	swh_count_greater_than_1.50:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.12. swh_count_greater_than_2.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 2 meter. passes within a given month, in meters.

The swh_count_greater_than_2.00 variable in a L4 product follows the format shown in table Table 4.80.

Table 4.80 CDL example description of swh_count_greater_than_2.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_2.00

1 (count)

 
double swh_count_greater_than_2.00(time, lat, lon) ;
	swh_count_greater_than_2.00:_FillValue = NaN ;
	swh_count_greater_than_2.00:units = "1" ;
	swh_count_greater_than_2.00:long_name = "number of median significant wave height values greater than 2.0m" ;
	swh_count_greater_than_2.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_2.00:grid_mapping = "crs" ;
	swh_count_greater_than_2.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.13. swh_count_greater_than_2.50#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 2.5 meter. passes within a given month, in meters.

The swh_count_greater_than_2.50 variable in a L4 product follows the format shown in table Table 4.81.

Table 4.81 CDL example description of swh_count_greater_than_2.50 variable#

Storage type

Name

Unit

float

swh_count_greater_than_2.50

1 (count)

 
double swh_count_greater_than_2.50(time, lat, lon) ;
	swh_count_greater_than_2.50:_FillValue = NaN ;
	swh_count_greater_than_2.50:units = "1" ;
	swh_count_greater_than_2.50:long_name = "number of median significant wave height values greater than 2.5m" ;
	swh_count_greater_than_2.50:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_2.50:grid_mapping = "crs" ;
	swh_count_greater_than_2.50:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.14. swh_count_greater_than_3.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 3 meter. passes within a given month, in meters.

The swh_count_greater_than_3.00 variable in a L4 product follows the format shown in table Table 4.82.

Table 4.82 CDL example description of swh_count_greater_than_3.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_3.00

1 (count)

 
double swh_count_greater_than_3.00(time, lat, lon) ;
	swh_count_greater_than_3.00:_FillValue = NaN ;
	swh_count_greater_than_3.00:units = "1" ;
	swh_count_greater_than_3.00:long_name = "number of median significant wave height values greater than 3.0m" ;
	swh_count_greater_than_3.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_3.00:grid_mapping = "crs" ;
	swh_count_greater_than_3.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.15. swh_count_greater_than_3.50#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 3.5 meter. passes within a given month, in meters.

The swh_count_greater_than_3.50 variable in a L4 product follows the format shown in table Table 4.83.

Table 4.83 CDL example description of swh_count_greater_than_3.50 variable#

Storage type

Name

Unit

float

swh_count_greater_than_3.50

1 (count)

 
double swh_count_greater_than_3.50(time, lat, lon) ;
	swh_count_greater_than_3.50:_FillValue = NaN ;
	swh_count_greater_than_3.50:units = "1" ;
	swh_count_greater_than_3.50:long_name = "number of median significant wave height values greater than 3.5m" ;
	swh_count_greater_than_3.50:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_3.50:grid_mapping = "crs" ;
	swh_count_greater_than_3.50:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.16. swh_count_greater_than_4.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 4 meter. passes within a given month, in meters.

The swh_count_greater_than_4.00 variable in a L4 product follows the format shown in table Table 4.84.

Table 4.84 CDL example description of swh_count_greater_than_4.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_4.00

1 (count)

 
double swh_count_greater_than_4.00(time, lat, lon) ;
	swh_count_greater_than_4.00:_FillValue = NaN ;
	swh_count_greater_than_4.00:units = "1" ;
	swh_count_greater_than_4.00:long_name = "number of median significant wave height values greater than 4.0m" ;
	swh_count_greater_than_4.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_4.00:grid_mapping = "crs" ;
	swh_count_greater_than_4.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.17. swh_count_greater_than_5.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 5 meter. passes within a given month, in meters.

The swh_count_greater_than_5.00 variable in a L4 product follows the format shown in table Table 4.85.

Table 4.85 CDL example description of swh_count_greater_than_5.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_5.00

1 (count)

 
double swh_count_greater_than_5.00(time, lat, lon) ;
	swh_count_greater_than_5.00:_FillValue = NaN ;
	swh_count_greater_than_5.00:units = "1" ;
	swh_count_greater_than_5.00:long_name = "number of median significant wave height values greater than 5.0m" ;
	swh_count_greater_than_5.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_5.00:grid_mapping = "crs" ;
	swh_count_greater_than_5.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.18. swh_count_greater_than_6.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 6 meter. passes within a given month, in meters.

The swh_count_greater_than_6.00 variable in a L4 product follows the format shown in table Table 4.86.

Table 4.86 CDL example description of swh_count_greater_than_6.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_6.00

1 (count)

 
double swh_count_greater_than_6.00(time, lat, lon) ;
	swh_count_greater_than_6.00:_FillValue = NaN ;
	swh_count_greater_than_6.00:units = "1" ;
	swh_count_greater_than_6.00:long_name = "number of median significant wave height values greater than 6.0m" ;
	swh_count_greater_than_6.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_6.00:grid_mapping = "crs" ;
	swh_count_greater_than_6.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.19. swh_count_greater_than_8.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 8 meter. passes within a given month, in meters.

The swh_count_greater_than_8.00 variable in a L4 product follows the format shown in table Table 4.87.

Table 4.87 CDL example description of swh_count_greater_than_8.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_8.00

1 (count)

 
double swh_count_greater_than_8.00(time, lat, lon) ;
	swh_count_greater_than_8.00:_FillValue = NaN ;
	swh_count_greater_than_8.00:units = "1" ;
	swh_count_greater_than_8.00:long_name = "number of median significant wave height values greater than 8.0m" ;
	swh_count_greater_than_8.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_8.00:grid_mapping = "crs" ;
	swh_count_greater_than_8.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;

4.8.1.20. swh_count_greater_than_10.00#

The total number, within 1°x1° grid cells, of all the (per cell) median significant wave height values from all the satellite greater than 10 meter. passes within a given month, in meters.

The swh_count_greater_than_10.00 variable in a L4 product follows the format shown in table Table 4.88.

Table 4.88 CDL example description of swh_count_greater_than_10.00 variable#

Storage type

Name

Unit

float

swh_count_greater_than_10.00

1 (count)

 
double swh_count_greater_than_10.00(time, lat, lon) ;
	swh_count_greater_than_10.00:_FillValue = NaN ;
	swh_count_greater_than_10.00:units = "1" ;
	swh_count_greater_than_10.00:long_name = "number of median significant wave height values greater than 10.0m" ;
	swh_count_greater_than_10.00:coverage_content_type = "physicalMeasurement" ;
	swh_count_greater_than_10.00:grid_mapping = "crs" ;
	swh_count_greater_than_10.00:cell_method = "lat: lon: median (interval: 0.1 degree_N interval: 0.1 degree_E comment: median per bin and satellite pass) time: count over days" ;
Contents