L4 Data File Description
GHRSST-PP L4 products are designed to provide the best available estimate of the SST
from a combined analysis of all available SST data. In situ data form an important component of the L4 process as these data
are used to correct for biases between the satellite data sets. GHRSST-PP bias and standard deviation errors are provided
as Single Sensor Error Statistics (SSES). L4 products capoitalisae on the synergy benefits of using in situ, microwave satellite
SST and infra-red satellite SST.
The GHRSST-PP L4 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
Several methods are available for the generation of complete SST
fields from incomplete L2P input data (e.g., Reynolds and Smith, 1994; Reynolds et al, 2002; Guan and Kawamura, 2003;
Murray et al. 2002; Murray et al. 1994; Fieguth et al., 1998;2000, Menemenelis et al. 1997; Lorenc, 1981
although none of these methods has been fully developed for use of an operational high resolution processor
such as the GDS demands. Bias correction of all input data to the analysis procedure is critical to
obtaining a valid output (see for example Reynolds et al, 2002). The GDS will use the L2P SSES derived bias as a
measure of the overall uncertainty associated with each input data stream. In addition, bias due to
diurnal stratification and cool skin effects must also be accounted for using additional data.
Some satellite sensors provide a direct estimate of the SSTskin (AATSR) that must be adjusted to the SSTfnd.
These data can then be used in an optimal analysis procedure to derive complete global fields.
As the GHRSST-PP Science Team and RDAC project teams gain experience, it is expected that the bias
correction strategy will evolve.
There are a variety of operational L4 SST analysis products available
from different agencies. Some systems use only a limited number of satellite inputs (e.g.,1º global
uses only AVHRR data) whereas others try to blend data from complementary sources (e.g.,
JMA MGDSST operational 25km global SST based on AVHHRR and AMSR-E).
Not all products make use of GHRSST-PP data at this time (e.g., NOAA 0.5º global
RTG_SST) whereas others use GHRSST-PP
data almost exclusively (e.g., 1/20 º
Ultra-high Resolution SSTfnd products
In addition to global L4 products (typically 10 -50km spatial resolution),
several Ultra-high resolution (UHR, <5km resolution) regional SST analysis products are available
for example, the Mediterranean UHR SST shown here is
a 2km analysis. UHR products are dificult to generate as the analysis system must capture the local dynamics of the
region in a way that preserves structure and minimuses noise.