Daily downward shortwave irradiance

The processing of OSISAF HL radiative flux products are based upon processing the AVHRR data to Level 2 and predefined product areas by the EUMETSAT NWPSAF software package AAPP in combination with Meteo France/CMS software ANA and EUMETSAT NWCSAF software AHAMAP. The predefined product areas are also output from the NWCSAF PPS cloudmask/cloudtype processing. Whenever a new satellite passage is received, the NWCSAF PPS cloudtype is generated and OSISAF HL Flux processing is initiated.

All auxiliary data needed in the processing are remapped to these predefined areas which also are the high resolution products (1.5km nominal horizontal resolution) generated by the OSISAF HL FLUX processing. These product areas are, for each satellite passage, resampled onto a 10km OSISAF HL grid (Polar stereographic, correct at 60°N, aligned along the Greenwich meridian. In the resampling only pixels with a confidence level of equal or better than bad are used.

The same procedure is used when the individual satellite passages at the OSISAF HL grid are averaged to get a daily estimate. Due to the poor coverage in time, no 3 hourly product is generated of the OSISAF HL Flux processing.

Basically the following steps are required to generate the surface solar irradiance from AVHRR input at pixel level (see product manual for details):

1. Get input data
2. Decide whether cloudy or cloud free
3. Estimate clear sky irradiance
4. If cloudy
1. Narrowband to Broadband correction
2. Anisotropy correction
3. Estimation of cloud factor
4. Estimation of cloudy sky irradiance

At present only AVHRR data from NOAA POES are used, but EPS METOP data will be used when available.