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SeaWiFS Surface Solar Irradiance

DX Data Methods

High resolution photosynthetically active surface irradiance (400-700 nm) and full-spectral surface irradiance were computed from DX data from the International Satellite Cloud Climatology Project (ISCCP). The results from five test days in March 1991 done on a 0.5° latitude and longitude grid are available. The scheme for the high resolution calculation is a modification of the SeaWiFS scheme based on the Bishop and Rossow (1991) algorithm, complete documentation for which is available as a 1.6 MB PDF file.

The input DX data consist of pixel data for clouds (values 0 or 1), cloud albedo, cloud optical thickness, surface albedo, and ice.

Ozone, water vapor and surface pressure were taken from the C1 data (and will be taken from the D1 data). The filling scheme for ozone, water vapor and surface pressure is the same as in the third version of the SeaWiFS production.

No calibration factors similar to what were used for versions 2 and 3 of the C1 data were applied.

The filling scheme for the high resolution data is:

  • Points in the high latitudes (where there is no data in the equal lat-long grid) and in the coasts:
    • fill clouds (3 hourly), first by filling east-west up to a maximum distance (dependent on latitude) and then by filling with neighboring points: east-west or north-south. The latter fills non-existing coastal pixels in the DX data.
    • fill cloud albedo and cloud optical thickness: E-W up to a maximum distance (dependent on latitude) for points where the filled value is cloud (or take zero otherwise)
  • Fill clouds for remaining points: during day, fill with the cloud fraction averaged during day if present, otherwise with the averaged during 24 hours of cloud fraction; during night, fill with the 24 hour average of cloud fraction. The 24-hour cloud fraction was filled successively with the averaged E-W value, with the averaged N-S value, from yesterday, and from a filled zonal cloud fraction array (dependent on latitude and surface type).
  • Fill cloud albedo: A 2.5x2.5° resolution cloud albedo is computed for each hour. If there is cloud in the point to be filled, fill with the low resolution albedo at the same hour. If it is not present, fill from the day time average albedo, or from a binned zonal albedo for the corresponding cloud fraction (10 cloud fraction intervals), and for land or ocean.
  • Fill cloud optical thickness: obtain the missing optical thickness by applying the tau conversion table to the filled cloud albedo.
  • Ice is filled for the high latitudes and E-W N-S by two points.
  • Surface reflectance is calculated with the same scheme as in the SeaWiFS production, in which the reflectance over ocean is calculated from the theoretical models of Cox and Munk (1956) and Morel and Gentili (1991). Filling is done for each of 6 different surface types (land, coast, waters: non-ocean, Atlantic, Indian, Pacific) and taking into account the ice-snow value.

With these basic variables filled, a calculation as in version 3, based in Frouin et al. (1989) as done in Bishop and Rossow (1991) is applied.

References

Bishop, J.K.B., and W.B. Rossow 1991. Spatial and temporal variability of global surface solar irradiance. J. Geophys. Res. 96, 16839-16858.

Cox, C., and W. Munk 1956. Slopes of the sea surface deduced from photographs of sun glitter. Bull. Scripps Inst. Oceanogr., Univ. Calif. 6, 401-488.

Frouin, R., D.W. Lingner, C. Gautier, K.S. Baker, and R.C. Smith 1989. A simple analytical formula to compute clear sky total and photosynthetically available solar irradiance at the ocean surface. J. Geophys. Res. 94, 9731-9742.

Morel, A., and B. Gentili 1991. Diffuse reflectance of oceanic waters: Its dependence on sun angle as influenced by the molecular scattering contribution. Appl. Optics 30, 4427-4438.

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