Level 4 (L4) Product Specification

8. Level 4 (L4) Product Specification#

8.1. Overview description of the GHRSST L4 data product#

L4 products are the analyzed SST products, usually derived from GHRSST L2P products. L4 data products should ideally be made available within the GHRSST R/GTS framework to the user community within 24 hours.

L4 products include gap-free analyzed SST data together with a number of ancillary fields that simplify interpretation and application of the SST data. Data providers are responsible for providing documentation on their analysis procedure. The common format of L4 products allows data users to code with the security that as new SST data products are brought on-line, very minimal code changes are required to make full use of new L4 product. Time previously spent on coding different I/O routines for each satellite data set can be spent working with the data to produce results.

The GHRSST Science Team determined that there will be 4 mandatory fields that form the core data content of a GHRSST L4 data file. In addition to global attributes and geo-location information, RDACs must produce the following within a L4 file:

Sea Surface temperature data (SST)
Error estimates for SST data
Sea ice fraction
Land/sea/ice flag

In addition there are a number of optional fields that may be used at the data provider’s discretion.

Table 8.1 Summary description of the contents within a GHRSST L4 data product#
Description	Required	Relevant section of this document
Dimensions (e.g., i x j x k)	Mandatory	Section
Global attributes	Mandatory	Section
[i x j x k] geolocation data	Mandatory	Section
[i x j x k] array of SST data	Mandatory	Section 8.3
[i x j x k] array of error estimates	Mandatory	Section 8.4
[i x j x k] array of sea ice fraction	Mandatory	Section 8.5
[i x j x k] array of land/sea/ice mask	Mandatory	Section 8.6
[i x j x k] array of optional fields	Optional	Section 8.7

8.2. L4 data record format specification#

L4 analysed data products are derived from an analysis procedure implemented at regular intervals (daily, six-hourly or other time periods). L4 data products include SST, error statistics, sea ice fraction, land/sea/ice mask, and other optional data for each grid-cell. A six-byte experimental block is available for data providers to test new aspects of the file or information specific to the analysis system that will eventually transition into a GHRSST standard L4 analysis field once tested.

Table 8.2 describes the format of GDS L4 grid cell ancillary data that should be created for each L4 grid cell. In the following sections, each variable within the L4 data file is described in detail.

Table 8.2 L4 SST product data fields#
Name	Description	Units
analysed_sst	SST from analysis system	K, scaled, short
analysis_error	Error standard deviation estimate	K, scaled, short
sea_ice_fraction	Fractional Sea Ice area concentration.	0-1 scaled byte
mask	land/ice/lake mask	Flag [8bits]
sea_ice_fraction_error	If the error estimates on the sea ice field are available, it is recommended to provide this information as an experimental field called sea_ice_fraction_error variable.	0-1 scaled byte
Experimental fields	Each grid cell has a 6 byte storage space available for RDACs and other users to include specific information. The policy for use of these fields is that they should make a useful contribution to the data sets and to GHRSST. Ideally experimental fields should transition into full fields once stable and agreed by the GHRSST Science Team. Use of these fields requires that a description of the content and specification is agreed with the GHRSST Data Assembly and Systems Technical Advisory Group and that GDS-2.1 variable attributes are included in the variable. See XXX.	6 bytes (maximum). Defined by data providers.

8.3. Variable analysed_sst#

The variable analysed_sst will be included with the format requirements shown in Table 8.3.

The data provider is responsible for providing GHRSST with documentation on how the analysed SST is determined. Note that the RDAC should place _FillValue in pixels that fall on land.

Table 8.3 CDL example description of **analysed_sst** variable#
Storage type	Name	Description	Unit
short	analysed_sst	SST values from analysis systems	kelvin

	short analysed_sst(time, lat, lon) ;
		analysed_sst:_FillValue = -32768s ;
		analysed_sst:long_name = "analysed sea surface temperature" ;
		analysed_sst:standard_name = "sea_surface_foundation_temperature" ;
		analysed_sst:units = "kelvin" ;
		analysed_sst:add_offset = 273.15 ;
		analysed_sst:scale_factor = 0.01 ;
		analysed_sst:valid_min = -300s ;
		analysed_sst:valid_max = 4500s ;

8.4. Variable analysis_error#

The variable analysis_error will be included with the format requirements shown in Table 8.4.

The data provider is responsible for providing GHRSST with documentation on how analysis_error is determined.

Table 8.4 CDL example description of analysis_error variable#
Storage type	Name	Description	Unit
short	analysis_error	Error estimate from analysis system	kelvin

	short analysis_error(time, lat, lon) ;
		analysis_error:_FillValue = -32768s ;
		analysis_error:long_name = "estimated error standard deviation of analysed_sst" ;
		analysis_error:units = "kelvin" ;
		analysis_error:add_offset = 0. ;
		analysis_error:scale_factor = 0.01 ;
		analysis_error:valid_min = 0s ;
		analysis_error:valid_max = 32767s ;

8.5. Variable sea_ice_fraction#

The variable sea_ice_fraction will be included with the format requirements shown in Table 8.5.

Some SST data are contaminated in part or wholly by sea ice and the L4 variable sea_ice_fraction is used to quantify the fraction of an area contaminated with sea ice. If the error estimates on the sea ice field are available, it is recommended to provide this information as an experimental field called sea_ice_fraction variable.

Table 8.5 CDL example description of **sea_ice_fraction** variable#
Storage type	Name	Description	Unit
byte	sea_ice_fraction	Fractional sea ice area concentration	Fraction

	byte sea_ice_fraction(time, lat, lon) ;
		sea_ice_fraction:_FillValue = -128b ;
		sea_ice_fraction:long_name = "sea ice area fraction" ;
		sea_ice_fraction:standard_name = "sea_ice_area_fraction" ;
		sea_ice_fraction:units = "1" ;
		sea_ice_fraction:add_offset = 0. ;
		sea_ice_fraction:scale_factor = 0.01 ;
		sea_ice_fraction:valid_min = 0b ;
		sea_ice_fraction:valid_max = 100b ;
		sea_ice_fraction:source_data = "EUMETSAT OSI-SAF" ;

8.6. Variable mask#

The variable mask will be included with the format requirements shown in Table 8.6.

Table 8.6 CDL example description of **mask** variable#
Storage type	Name	Description	Unit
byte	mask	land/sea/ice/lake mask	none

	byte mask(time, lat, lon) ;
		mask:long_name = "land sea ice lake bit mask" ;
		mask:flag_masks = 1b, 2b, 4b, 8b ;
		mask:flag_meanings = "sea land lake ice" ;

Comments:

Comments This is a land/sea/ice mask with the following bit values:

Bit 0:1 = water in grid
Bit 1:1 = land in grid
Bit 2:1 = optional, lake surface in grid
Bit 3:1 = sea ice
Bit 4:1 = optional, river surface in grid
Bits [5-7] spare

Note that the lake and river surface bit values are optional.

The source attribute should list any data products used in creating this mask. List: <provider>_<type_of_mask>_<version_mask>.

8.7. Optional Variable sea_ice_fraction_error#

If the error estimates on the sea ice field are available, it is recommended to provide this information as an experimental field called sea_ice_fraction_error variable. The data provider is responsible for providing GHRSST with documentation on how sea_ice_fraction_error is determined.

Table 8.7 CDL example description of **sea_ice_fraction_error** variable#
Storage type	Name	Description	Unit
byte	sea_ice_fraction_error	Fractional sea ice area concentration	Fraction