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Presentation of the Products

Atlantic Downward Longwave Irradiance

Physical definition

The DLI is the longwave irradiance reaching the Earth surface in the 4-100 mm band, the irradiance being the radiant flux received per unit area. This defines an instantaneous DLI value, which corresponds to the definition of the hourly DLI. The 3-hourly and daily DLI values are mean values over the considered period of time, 3 or 24 hours.

Delivered products

The delivered SSI products are :

• The LML DLI, 3-hourly products on LML area, derived from the geostationary satellite data (GOES and MSG), covering the oceanic area from 60 N to 60 S and from 100 W to 45 E, at 0.1 degree resolution,
• The MAP DLI, daily products on MAP area, derived from GOES, MSG and NOAA polar orbiter satellite data, covering the oceanic area from 90 N to 60 S and from 100 W to 45 E, at 0.1 degree resolution.

Fluxes products are accessible to all users on Météo-France and IFREMER ftp servers, via EUMETCAST, and may be delivered on request through the Regional Meteorological Data Communication Network (RMDCN) to the European Meteorological services.

Units and range

DLI values are expressed in tenth of in W/m2 and coded on 16-bit words. The expected range is 100 to 500 W/m2 (whatever the temporal resolution), these minimum and maximum values being approximate. Unprocessed data, for whatever reason, are equal to -32768 in HDF format or flagged as missing in GRIB section 3.

Quality indexes

Each DLI field is associated with a quality index, which includes a confidence level corresponding to the quality of the calculated DLI and information on the processing conditions.

Satellites data sources

The LML Fluxes products are derived from Met-08 and GOES-12.
The MAP Fluxes are derived from Met-08, GOES-12 and NOAA polar satellites.

Processing scheme

• At Met.no :

  At each satellite pass (NOAA and further METOP) received at Met.no, data are processed to produce HL DLI fields.

• At CMS :

  GOES or MSG images are processed every hour at M-F/CMS, Lannion, producing instantaneous DLI fields. These instantaneous fields are called hourly DLI, because of their time sampling.
  The hourly DLI are combined to produce 3-hourly and daily DLI. These tasks are performed separately for each satellite, GOES or MSG.
  Finally, the daily LML and the daily HL, derived from NOAA and produced by Met.no and DMI, are merged into the MAP product.
  The algorithm is a hybrid method combining a bulk parameterization for the clear sky contribution and satellite derived cloud information.
  
  
  DLI = (e₀ + (1 - e₀) C ) s T_a⁴
  
  

  e₀

  clear sky emissivity, Prata's formulation + predicted surface air temperature and humidity (NWP model)

  s

  Stefan-Boltzmann constant

  C

  • daytime cases : C = 1 - SSI / SSI_clear

    SSI, SSI_clear

    surface solar irradiance and clear sky surface solar irradiance.

  • nighttime cases : C = n₁ C₁ + n₂C₂ + ... +n₈C₈

    n_i

    fractional sky cover by cloud type i

    C_i

    contribution coefficient of this cloud type.
    The cloud types correspond to a simplified classification deduced from the detailed SAF Nowcasting classification (by merging some types).
    The C_i coefficients have been adjusted on a learning file with pyrgeometer measurements and observed air temperature and humidity.

Current parameters:

• GOES-E configuration file.
• Meteosat configuration file.