L2P and L2Pc Observation data products
Fully formed L2P products
The GHRSST-PP Level-2 Pre-processed (L2P) data format specification is a netCDF file following the Climate Forecast (CF) convention. A full product specification can be found in the GHRSST-PP Data processing Specification (GDS).The common format of L2P products allows data users to code with the security that as new satellite derived SST data sets are brought on-line, very minimal code changes are required to make full use of the data. Time previously spent on coding different i/o routines for each satellite data set can be spent working with the data to produce results.
GHRSST-PP L2P format data products are netCDF files containing SST data, error estimates (bias error and standard deviation) for each pixel and a series of ancillary fields to help interpret and use the SST data themselves. L2P files are the basic building blocks from which all other GHRSST-PP SST data products can be derived. They include SST data as delivered by a data provider in their native format (swath, grid, vector), together with a number of ancillary fields (some shown to the left) that simplify interpretation an application of the SST data. For example, the time of SST observation, geo-location data, land and ice flags are just a few fields included with every GHRSST-PP L2P file. Ancillary data is also included to help interpret the quality and character of the SST data. Ancillary data are referred to as dynamic flags. L2P dynamic flags include estimates of the surface wind field (that can be used to flag low wind speed areas prone to diurnal variability), surface solar irradiance (SSI, that can also be used to flag areas of diurnal variability), aerosol optical depth (used to flag areas contaminated with atmospheric dust, important for infrared SST retrievals from space) and sea ice concentration.
The power of L2P lies in the fact that all satellite SST data are presented in a common easy to use data format so that only a single i/o utility is required to start using data rather than having to re-code and maintain code for ingestion of different satellite data. The ancillary fields provide users with a suite of dynamic flags that can be used to filter data based on criteria tuned to a particular application. For example, if data contaminated by diurnal warming are unwanted then surface wind speed and solar radiation auxiliary fields can be used to flag data at low wind speeds with high solar radiation. For IR SST data, aerosol optical depth can be used to filter data suspect due to contamination.
L2Pcore (L2Pc) data products
For some data providers providing fully-formed L2P data file is an enormous burden so much so that some L2 SST satellite data providers were not interested in participation within the GHRSST-PP R/GTS. This meant that unique L2 SST data was not entering the GHRSST-PP system leading to a potential reduction in L4 SST analysis products. At the 6th GHRSST-PP Science Team meeting (Exeter, United Kingdom), the Science Team agreed the principle that there will be 6 mandatory fields that form the core data content of a GHRSST-PP L2P data file. These fields will be known as L2P ‘core' fields or L2Pc. A full specification of L2Pcore can be found here.
Optional 'Experimental' L2P fields
Data space within a L2P file for each pixel may be used to hold RDAC or GDAC defined experimental fields. These experimental fields must conform to CF-1.0 standards and must be registered with the GHRSST office prior to use so that users are aware of their presence and can find sufficient documentation to either use the data or know to ignore it. Documentation is to be provided by the initiator and explanation provided for publication on the GHRSST website. However, these experimental fields will not appear in the GDS. At present, the total volume should be limited to 6 bytes per pixel. The volume limit should be reviewed in future in the light of the need for optional fields and the opinion of users who download the full L2P product.
(Last Updated: 21-11-2006)