The principal source of observations has been the ECMWF real-time data collection from the Meteorological Archive and Retrieval System (MARS). Additional sources include:

250 km cloud cleared radiance (CCR) data, augmented by NESDIS I-b data where there are gaps;
Ship and buoy observations from the Comprehensive Ocean Atmosphere Data Set (COADS);
First GARP Global Experiment (FGGE) and Alpine Experiment (ALPEX) II-b data;
GMS satellite cloud winds was made available by the JMA;
PAOBS data from NMC Melbourne;
The JMA also provided supplementary radiosonde and aircraft data;
Subduction and TOGA buoy data.

The ERA assimilation system is based on:

spectral T106 resolution with 31 vertical hybrid levels;
intermittent statistical (Optimum Interpolation) analysis with 6 hour cycling and no FGAT;
one dimensional variational (1D-VAR) physical retrieval of TOVS cloud cleared radiances;
diabatic, non-linear normal mode initialisation (5 vertical modes);
ECMWF's Integrated Forecast System (IFS) forecast model with a fully 3D semi-Lagrangian advection scheme;
mean orography with a compatible parametrization of the effects of sub-grid scale orography;
a four layer prognostic soil scheme, with "nudging" of the soil moisture from boundary layer atmospheric humidity;
ECMWF mass-flux convection;
prognostic equations for cloud water content and cloud cover;
ECMWF operational planetary boundary layer parametrization;
ECMWF operational radiation parametrization with prescribed concentrations of aerosols, carbon dioxide and ozone, with the ozone varying geographically and seasonally, the aerosols varying geographically and vertically, and carbon dioxide held constant.

The only other externally prescribed forcing comes from the sea surface temperatures (SST) and sea ice cover. These were obtained from:

for December 1978, and from January 1980 to October 1981 the UK Meteorological Office (GISST) 1 degree monthly SST analyses;
for the final re-run of 1979 the EOF-enhanced SST from the combined UK Meteorological Office (GISST) and NCEP effort;
NCEP (Reynolds) 1 degree weekly OI SST analyses (November 1981 to 1993);
ice limits derived by A. Nomura of the ERA group from SMMR and SSM/I data.

Special arrangements have been made with NCEP and DAO to enable each of the three re-analyses to use an identical set of observations, sea surface temperatures, and ice limits for 1979.

Apart from the resolution, the ERA assimilation system is identical to that used for ECMWF operations from April 1995 until February 1996.

16.11.2001