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SEVIRI

Background

The Spinning Enhanced Visible and InfraRed Imager (SEVIRI) is MSG's primary instrument and has the capacity to observe the Earth in 12 spectral channels. Eight of the channels take measurements in the infrared band of the spectra, the rest for the visible channels (one of them in high resolution). The horizontal resolution is 3 km, except for the high resolution visible channel that is 1 km. Time resolution is one image every 15 minutes.

SEVIRI ChannelsMain characteristics
IR 3.9Used at night to detect fog and very low clouds. Window channel of CO2
WV 6.2Water Vapour Channel
WV 7.3Water Vapour Channel
IR 8.7Window channel of H2O
IR 9.7Ozone absorption channel. Not suitable to data assimilation
IR 10.8Window channel of H2O
IR 12.0Window channel of H2O
IR 13.4CO2 absorption band

SEVIRI channels and their characteristics

Normalised Weighting Functions

Figure of normalised weighting functions

See Eumetsat seviri for more detail on the instrument.

Radiance data from geostationary satellites have been assimilated in global numerical weather prediction models for some time (Köpken et al., 2003; Szyndel et al., 2004). Their high spatial and temporal resolution are particularly beneficial on global scales and in regions with a paucity of ground based measurements. For example, the assimilation of radiance data from Meteosat-8 and Meteosat-11 continue to be of value to the ECMWF's IFS data assimilation system (Burrows, 2020). Similarly, limited area models can benefit from these radiances.

Preparing data

This describes the work flow for preparing SEVIRI observations suitable for assimilation.

Radiances are processed in order to reduce the size and change the format of the files using the NWC-SAF products to discriminate between clear-sky and cloudy pixels.

Links to NWC-SAF software and description:

Model settings

Enable assimilation

  • Set SEVIRI_OBS=1 in scr/include.ass
  • Ensure seviri${DTG}.nc files are available in $OBDIR (defined in ecf/config_exp.h )

Preparing ODB

The processing (NetCDF –> ODB) by Bator is controlled by namelist entries. A brief some of the more relevant namelist entries is provided here.

NADIRS

Namelist entryTypeDescription
InbTypeNetcdfIntegerNumber of NETCDF templates to read in the param.cfg file. Default value : 0
MinSeviriSatidIntegerLowest SID waited in the SEVIRI data (NETCDF format). It is used as lower bound when allocating NSEVIRI type. Default value : 0
MaxSeviriSatidIntegerHighest SID waited in the SEVIRI data (NETCDF format). It is used as upper bound when allocating NSEVIRI type. Default value : 0
MinMtvzaSatidIntegerLowest SID waited in the MTVZA data (HDF5 format). It is used as lower bound when allocating HMTVZA type. Default value : 0
MaxMtvzaSatidIntegerHighest SID waited in the MTVZA data (HDF5 format). It is used as upper bound when allocating HMTVZA type. Default value : 0

e.g.

&NADIRS
  InbTypeBufr = 200,
  InbTypeNetcdf = 1,
  InbTypeHdf5 = 2,
  MinSeviriSatid = 54,
  MaxSeviriSatid = 70,
  MinMtvzaSatid = 320,
  MaxMtvzaSatid = 320,
  SIGMAO_COEF(7) = 1.15,
  SIGMAO_COEF(9) = 1.,
  ECTERO(9,6,125,3) = 1.24,
  ECTERO(9,6,124,3) = 1.34,
  LATMS_MANDATORY_AVG = .TRUE.,
  LVARBC_APD = .FALSE.,
/

NetCDF

Main key%ComponentTypeDescription
NSEVIRI(:)%SautintegerArray of SEVIRI data type. The indexes match SIDs.
NSEVIRI(:)%NbChannelsintegerObservations sampling (1/n). Default value : 1.
NSEVIRI(:)%Channels(:)integerNumber of wished channels. Default value : 0.
NSEVIRI(:)%NbSuppintegerArray of wished channels number. Default value : -9
NSEVIRI(:)%NamChannels(:)stringNumber of meta-data per observation ( ZENTSUP). Default value : 0.
NSEVIRI(:)%NcmlNamestringOne dimension array of variable labels containing Tb data. Each label index must be the same as the index of its channel number used in channels(:) component. Default value : ‘’.
NSEVIRI(:)%NwcSafNamestringLabel of “NetCDF multicanal source” general attribute. Default value : ‘’.
NSEVIRI(:)%NamLatstringLabel of “nwcsafalgorithm “ general attribute. Default value : ‘’.
NSEVIRI(:)%NamLonstringLabel of the “lat” variable. Default value : ‘’.
NSEVIRI(:)%NamTimestringLabel of the “lon” variable. Default value : ‘’.
NSEVIRI(:)%NamSatAzimuthstringLabel of the variable which gives observation data in second since 01/01/1970 00h. Default value : ‘’.
NSEVIRI(:)%NamSatZenithstringLabel of the variable containing satellite azimuth angles. Default value : ‘’.
NSEVIRI(:)%NamCTstringLabel of the variable contaning satellite zenithal angles. Default value : ‘’.
NSEVIRI(:)%NamCTQstringLabel of the variable containing the clouds types (CT). Default value : ‘’.
NSEVIRI(:)%NamCTPstringLabel of the variable containing quality flags associated to CT. Default value : ‘’.
NSEVIRI(:)%NamCTPQstringLabel of the variable containing top clouds pressures (CTP). Default value : ‘’.
NSEVIRI(:)%LPrintbooleanPrint

A sample NetCDF namelist entry for SEVIRI data

&NETCDF
  NSEVIRI(57)%Sensor=29,
  NSEVIRI(57)%NcmlName='ncml_version',
  NSEVIRI(57)%NwcSafName='nwc_saf_algorith_version',
  NSEVIRI(57)%NamLat='lat',
  NSEVIRI(57)%NamLon='lon',
  NSEVIRI(57)%NamTime='time',
  NSEVIRI(57)%NamSatAzimuth='sat_azi_ang',
  NSEVIRI(57)%NamSatZenith='sat_zen_ang',
  NSEVIRI(57)%NamSolAzimuth='sol_azi_ang',
  NSEVIRI(57)%NamSolZenith='sol_zen_ang',
  NSEVIRI(57)%NamCT='CT',
  NSEVIRI(57)%NamCTQ='CT_QUALITY',
  NSEVIRI(57)%NamCTP='CTP',
  NSEVIRI(57)%NamCTPQ='CTP_QUALITY',
  NSEVIRI(57)%SAUT=5,
  NSEVIRI(57)%NbSupp=12,
  NSEVIRI(57)%NbChannels= 8,
  NSEVIRI(57)%Channels(1:8)= 1,2,3,4,5,6,7,8,
  NSEVIRI(57)%NamChannels(1:8)='IR_039','WV_062','WV_073','IR_087','IR_097','IR_108','IR_120','IR_134',
/

Model settings (Screening and Minimisation)

  • Thinning:
    • One pixel in every five is selected in Bator.
    • In Screening the thinning distance is defined as 0.65°.
  • Observation error:
    • Set in Bator namelist
    • 1.7x0.9(SIGMAO_COEF)=1.54 for channels 2,3 & 8
    • 1.05x0.9(SIGMAO_COEF)=0.945 for the channels 4,6,7
  • VarBC:
    • AEMET use p0 for all channels with 24 h cycling.
  • Anything else:
    • "Safe" active channels are WV6.2 and WV7.3 (channels 2 and 3) over sea.
    • AEMET only use Infrared channels so in Bator there is a conversion of the channels list. ch 4 -> ch 1

References

Technical stuff:

Further reading and links to reports/presentations: