Use of Observations

Satellite Observations

The first ECMWF reanalysis, ERA-15, covered the period 1979-1993, during which the observing system was relatively homogeneous. There were two sources of satellite data in ERA-15, data from the TOVS instruments in the form of Cloud Cleared Radiances (CCR) from NESDIS and the Cloud Motion Winds from geostationary satellites, both available throughout the period. The CCR data had gone through many preprocessing steps. A more direct way and the way ERA-40 will use these data is to use the Level-1c radiances calibrated in-house from Level-1b counts. A full TOVS-1b dataset, 1979-1998, has been obtained from NCAR, NASA and LMD. The TOVS and ATOVS data from 1998 onwards will be taken from ECMWF archives. Vertical Temperature Profiler Radiometer (VTPR) data (8-channel infrared instrument), 1972-1979, will be used for the first time in a data assimilation as radiances. All earlier data assimilations of VTPR data have been based on the old temperature and humidity retrievals. The direct use of radiances should reveal interesting potential in this old data.

Data from Special Sensor Microwave/ Imager (SSM/I) data are used in ERA-40. The first satellite, F08, was launched in June 1987. The data, from one satellite at any time, originates from F. Wentz in the form of navigated antenna temperatures, which are then calibrated into brightness temperatures (Wentz, 1991). In contrast to the use of TOVS and VTPR radiances, the use of SSM/I radiances involves a one-dimensional variational analysis of the Total Column Water Content and surface wind speed. Scatterometer winds over oceans and altimeter wave height data from ESA/ ERS satellites will be used from 1991 onwards in the wave analysis. Cloud Motion Wind processing techniques have been improved throughout the period, and EUMETSAT has initiated a project to reprocess the wind information from Meteosat-2, at least.

The ozone observations to be used in the ERA-40 are TOMS total ozone and SBUV ozone layer measurements and TOVS/ HIRS channel 9 radiances. All these observations are available from 1978 to the present time. HIRS channel 9 radiances give information on ozone in the upper troposphere and lower stratosphere, where ozone absorbs infrared radiation. The SBUV (and the SBUV/2) instrument measures solar backscattered radiation and gives information on ozone in 7-8 layers in the middle stratosphere and above. The TOMS instrument also measures solar backscattered radiation and gives accurate limits on the total ozone content, which may possibly lead to more accurate estimates of lower tropospheric ozone.

Conventional Observations

Conventional data for ERA-40 come from a much wider selection of sources. The ERA-40 period begins with the International Geophysical Year of 1958 when the foundation for the current conventional network was established. Most of these data were collected by NCAR and will be used in ERA-40. The datasets used in ERA-15, such as the ECMWF observation archive, FGGE, ALPEX and PAOBs will also be included. A fully, newly updated Comprehensive Ocean-Atmosphere Data Set (COADS) has been provided by NCAR. Separate additional datasets have been received from JMA and US Navy archives. Software has been developed to extract a unique stream of observations for the analysis Observation Data Base (ODB). Conventional observations from all sources are included in the database. Prior knowledge of the quality of each dataset is needed in order to flag identified duplicates using information from the reports. Satellites allow for a more observation-driven stratospheric analysis during the second half of the ERA40 period. The conventional network experienced large changes, e.g., disappearance of Ocean Weather Ships, improvement of the radiosonde network in the southern hemisphere and the gradually improving WWW system during the ERA-15 period. Thus, there are several "epochs" with similarities in data sources and observation distribution. The main epochs are: