ARC - Algorithm Specification

Algorithm Specification of the science processors of the SNAP Toolbox

Saharan Dust Detection

Dust aerosol can be a problem for some satellite SST products as infrared SST retrievals are biased cold in the presence of dust. While very high dust loadings are often (incorrectly) flagged as "cloud", more moderate amounts are passed as "clear" and impact the resulting SST fields. This is a particular problem over the Atlantic in the summer months when desert dust is lifted from the Saharan desert and transported west over the ocean.

Desert dust is detected using the ATSR Saharan Dust Index (ASDI) method described in . This functions as a dual-view retrieval of dust index using the 11 and 12 micron channels which can therefore be used both day and night.

SST Retrieval

ARC SSTs are estimated using a coefficient-based retrieval scheme ( ) which is robust to the presence of stratospheric aerosol from the Mount Pinatubo eruption in 1991. The coefficients are banded by: total column water vapour (TCWV) to reduce the effects of atmospheric variability on the nadir and day-time retrievals; satellite zenith angle to reduce viewing angle dependent biases from the dual-view geometry.

SNAP implementation

In the SNAP implementation the dependence on TCWV is not fully implemented. The coefficients include the dependence, but current ATSR L1b files do not include the required data. As a result the user must select a constant value to use for processing the scene.
In future versions of the SNAP ARC processor it will be possible to specify TCWV as a per-pixel field, use the TCWV supplied with SLSTR data, or make use of globally defined coefficients (globally defined coefficients will be less accurate).