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Information in brief
Type Surface Solar Irradiance (SSI)
Donwward Longwave Irradiance (DLI)
Input data AVHRR
Auxiliary data Climatological maps of albedo
Climatological maps of ozone
NWP humidity simulations
Resolution 5 kilometer
Frequency Daily
Map projection Polar stereographic
File formats WMO Grib
NCSA HDF5
Dissemination channels FTP server
EUMETCAST
WMO GTS (upon request from a user community)
Documentation Product manual
Contact email osisaf-manager[at]met.no
Responsible scientist ěystein God°y

Surface radiative flux products

Introduction

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 5 and 10 km spatial resolution once daily. Input data are AVHRR data from both NOAA POES and Eumetsat Polar System (EPS).

Currently data are delivered in a Polar Stereographic map projection, but work is ongoing to deliver in satellite projection. 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.

Status

Daily quicklook products are available through the specific product description pages for SSI and DLI respectively. WMO GRIB and HDF5 files including quality flag etc are available for FTP download through the main page. Work is ongoing to deliver data in NetCDF/CF as well as covering the whole area indicated in the illustrations below.

Products

Shortwave irradiance Longwave irradiance
Surface Solar Insolation sample product Downward Longwave Insolation sample product
Shortwave radiation is in this project defined as radiation in the range 0.3-3 micrometer. Further information on the SSI processing and access to quicklook imagery of the products is available through the menu at the left or by clicking on the image presented here. Longwave radiation is in this project defined as radiation in the range 4-50 micrometer. For further information on the DLI processing and access to quicklook imagery of the products is available through the menu at the left or by clicking on the image presented here.

Validation

Downward Longwave Insolation sample product 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 sample products provided here and on the individual product pages white circles indicate validation stations where data are available for usage in the OSISAF validation scheme. Not all these are used all the time and some of them only cover shortwave products. Extracts from the validation report will be presented on these pages twice yearly. The normal validation report covers periods of 6 months, but longer validation reports will be provided here soon.

In the illustration provided, the following keywords are used:
Keyword Meaning
QLI Incoming longwave irradiance at the surface
QLO Outgoing longwave irradiance at the surface
QSI Incoming shortwave irradiance at the surface
QSO Outgoing shortwave irradiance at the surface
OT Sunshine duration (normally when global irradiance exceeds 120 W/m▓)

The most recent validation results, covering a period of 6 months is presented below. Not all stations listed in the map above is used. For some of the stations, extensive shadow effects cause problems validating SSI, work is ongoing to create adjustment factors for these. The illustration presented here is the relative bias error of the product. The left panel shows validation results for SSI and the right panel for DLI. This ilustration is a merged version of the validation results for several validation periods.

Validation results for DAILY SSI and DLI in 2013/2014

Typically the quality of the SSI products is poorer during winter than during summer due to the snow covered ground at high latitudes. Maintenance of the in situ equipment may also affect results, but usually stations affected by such issues are removed from the validation until they are working properly again (this is why the stations used may differ between validation periods). The quality of DLI is usually slightly poorer during summer than during winter due to more scattered cloud cover.

Details SSI

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 (see product manual for details).

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

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 are at any time used in the validation as maintenance, other technical problems or characteristics of the station prevents it from being used at times.

Sample OSISAF SSI daily product

Details DLI

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

where
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.

Sample OSISAF DLI daily product
   
       
     

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