Atlantic High Latitude Downward Longwave Irradiance OSI-301-c
The high latitude radiative flux products developed at the Norwegian Meteorological Institute contains shortwave (SSI) and longwave (DLI) irradiance received at the surface. The Ocean and Sea Ice SAF radiative flux products are currently produced at 5km spatial resolution once daily. Input data are AVHRR and VIIRS data from NOAA POES and Eumetsat Polar System (EPS). Although each satellite passage is processed, the official radiative flux products from the EUMETSAT OSISAF are daily products which represents the mean daily irradiance received at the surface in Watt per square meter.
This page describes the DLI product.
The processing of OSISAF HL radiative flux products starts with processing the AVHRR and VIIRS data to Level 1C on the satellite swath and then generating cloud type products using the NWCSAF PPS v2021 processor. 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 the satellite swath by the OSISAF HL FLUX processor. These product areas are then, for each satellite passage, resampled onto 5km 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 swaths at the OSISAF HL grid are averaged to get a daily estimate.
The algorithm is the same as the algorithm used by Meteo France/CMS for GOES/MSG data and 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*Ta^4
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 and VIIRS data and the NWCSAF PPS output.
SSI and DLI validation
The estimates based on remote sensing data are routinely validated against available in situ measurements. Much effort has been dedicated to validation and gaining access to relevant validation data in recent years. In the map below validation stations where data are available for usage in the OSISAF validation scheme are indicated.