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Daily downward longwave 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 5 and 10km OSISAF HL grids (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.

The algorithm is the same as the algorithm used by Meteo France/CMS for GOES/MSG data (see the product manual for details). It is essentially a hybrid method combining a bulk parameterization for the clear sky contribution and satellite derived cloud information according to the formula:

DLI = (e0 + (1 - e0) C ) S Ta4

e0: clear sky emissivity, Prata's formulation + predicted surface air temperature and humidity (NWP model)
S: Stefan-Boltzmann constant
C: infrared cloud amount which is estimated as
during daytime: C = 1 - SSI / SSIclear
SSI, SSIclear surface solar irradiance and clear sky surface solar irradiance.
during nighttime: C = ni Ci
ni: the fractional sky cover by cloud type i. For the AVHRR data this is normally either clear, overcast or contaminated.
Ci: contribution coefficient of this cloud type. The cloud types correspond to a simplified classification deduced from the detailed NWCSAF PPS cloudtype classification (by merging some types). The Ci coefficients have been adjusted on a learning file with pyrgeometer measurements and observed air temperature and humidity.
The OSISAF HL algorithm is of course adapted and tuned to the input of AVHRR data and the NWCSAF PPS output.

AVHRR data both from NOAA and EUMETSAT satellites are used in the processing.

A sample product is provided below. The white dots in the image represents potential validation stations. All of these observe shortwave incoming irradiance, but not all observe longwave incoming irradiance. Please check the map of validation stations on the main information page for OSISAF radiative fluxes. Recent validation results are presented at the bottom of the main product page.

Sample OSISAF DLI daily product