AN ENSEMBLE of Data Assimilations (EDA) system will be introduced at ECMWF with cycle 36r2 of the Integrated Forecasting System (IFS). The EDA system consists of an ensemble of ten independent lower-resolution 4D-Var assimilations that differ by perturbing observations, sea-surface temperature fields and model physics. The computing cost is significant, similar to running the deterministic analysis suite. The main justification for implementing the EDA is that it quantifies analysis uncertainty. u It is the first system implemented at ECMWF that provides estimates of analysis uncertainty. A properly designed EDA will complement the data assimilation system with important information about the quality of the deterministic analysis. u It can be used to estimate flow-dependent background errors in the deterministic 4D-Var assimilation system; this will potentially improve the medium-range forecast. Flow-dependent background errors will be introduced in the second phase, in the autumn of 2010. u It can improve the representation of initial uncertainties for the Ensemble Prediction System (EPS). When the EDA is introduced in IFS cycle 36r2, EDA-based perturbations will replace evolved singular vectors (SVs) to generate the EPS initial conditions. This change will improve the EPS skill, especially over the tropics. The application of EDA in EPS will be described in an accompanying article by Buizza et al. in this issue of the ECMWF Newsletter (pages 22 to 28). The EDA is expected to become an important part of the ECMWF data assimilation system, with the introduction of a hybrid 4D-Var/EDA system. Also, in the coming years, the EPS and EDA are expected to be further integrated to the benefit of both systems.

ERA‐40 is a re‐analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium‐Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re‐analysis period, with assimilable data provided by a succession of satellite‐borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean‐buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA‐40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA‐40. This benefited from many of the changes introduced into operational forecasting since the mid‐1990s, when the systems used for the 15‐year ECMWF re‐analysis (ERA‐15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re‐analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized.

In this third and final paper of a series, we assess the performance of the three-dimensional variational data assimilation scheme, in the light of the results from the extensive pre-operational programme of numerical experimentation. Its performance is compared with that of the previous operational scheme at the European Centre for Medium-Range Weather Forecasts, which was based on Optimal Interpolation. The main features of the new scheme are illustrated in particular the effects of non-separable structure functions and the improved data usage. TIROS-N Operational Vertical Sounder cloud-cleared radiances, for example, are used directly without a separate retrieval step. Scatterometer data are assimilated in the form of ambiguous winds with the ambiguity removal taking place within the analysis itself. problems encountered during the tests are discussed and the solutions implemented are explained.The overall impact on forecast accuracy in the troposphere of the northern hemisphere extratropics is neutral for geopotential and positive for wind and temperature. The impact is neutral in the tropics, and significantly positive in the southern hemisphere. Analyses and forecasts for the stratosphere have improved in all regions. Other positive results include a clear improvement in analyses of near-surface winds over oceans, particularly in the vicinity of tropical storms. This is predominantly because of the assimilation of scatterometer wind-data.