18.104.22.168.6.1 Water Confidence Checks (step 2.6.5)
Water confidence checks include processing of Sun glint, and flagging of low pressure and whitecaps. These steps use the wind and pressure provided in the L1B product annotations. All water confidence checks apply to water pixels within a 4x4 pixels window.
22.214.171.124.6.1.1 Glint processing (step 126.96.36.199)
Glint processing applies only to water pixels
· Glint estimation
The Sun glint reflectance rg is calculated (step 188.8.131.52.1) by interpolation in LUT produced using the Cox and Munk model (1954) as a function of geometry, wind speed modulus and direction. An estimate of glint reflectance is produced. Wind speed modulus and wind direction are computed from the W_u and W_v annotations.
Note: Sun glint reflectance is now an input to current step as its computation a been moved to section “Pixel identification” (step 2.1c).
· Glint classification (low, medium or high glint ?)
This glint reflectance is compared to a low glint threshold (step 184.108.40.206.3). If the glint reflectance is below this low glint threshold then no glint correction for this pixel is applied. If the pixel is not bright then it is further processed by step 220.127.116.11. If bright, it is flagged as ice or high aerosol load.
If the glint reflectance is above the low glint threshold then the glint reflectance is compared to a medium glint threshold (step 18.104.22.168.4).
If the glint reflectance is below the medium glint threshold then a medium glint flag is raised and the pixel is corrected for glint reflectance in step 22.214.171.124.7.
If the glint reflectance is above the medium glint threshold then no correction is applied and the pixel is flagged as uncorrected sun glint (step 126.96.36.199.5).
In addition, a specific glint threshold is used to derive a second “high glint” flag dedicated to Water Vapour Processing over water (step 188.8.131.52.1.9): As a matter of fact, when the Glint reflectance is very high, total water vapour over water surface is best determined using the Water Vapour Over Land algorithm, best suited for bright underlying surfaces.
· Glint correction in case of medium glint (step 184.108.40.206.7)
The glint reflectance at surface level is transferred to the Top Of Atmosphere by applying a direct atmospheric transmission term (including Rayleigh scattering on both sun-surface and surface sensor paths). The Top Of Atmosphere glint reflectance rg* is then subtracted from the TOA reflectance.
Note : step 220.127.116.11.8, because of its simplicity, is only shown in the corresponding equation section.
18.104.22.168.6.1.2 Low pressure water flagging (step 22.214.171.124)
Surface pressure (from ECMWF annotation) is compared with a threshold in order to flag low pressure, typically high altitude inland waters. Above such waters, the results of the atmosphere correction are disturbed.
126.96.36.199.6.1.3 Whitecaps Flagging (step 188.8.131.52)
The modulus of the wind speed is compared to a threshold. Above that threshold a whitecap flag is raised because white caps are likely to disturb the performance of the atmosphere correction.
184.108.40.206.6.1.4 Reflectance threshold on reflectance at 412 nm (step 220.127.116.11)
An additional “bright” pixels screening, specific to water pixels, is performed by comparison of the Glint corrected reflectance at 412 nm with a pre-computed threshold. It is intended to further sort out and flag those pixels affected by sea-ice, partial clouds or very high aerosol load.