SUMMARY 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.A number of results indicative of the overall performance of the data assimilation system, and implicitly of the observing system, are presented and discussed. The comparison of background (short-range) forecasts and analyses with observations, the consistency of the global mass budget, the magnitude of differences between analysis and background fields and the accuracy of medium-range forecasts run from the ERA-40 analyses are illustrated. Several results demonstrate the marked improvement that was made to the observing system for the * Corresponding author: European Centre for Medium-Range Weather Forecasts, Shinfield Park, Reading RG2 9AX, UK. e-mail: adrian.simmons@ecmwf. southern hemisphere in the 1970s, particularly towards the end of the decade. In contrast, the synoptic quality of the analysis for the northern hemisphere is sufficient to provide forecasts that remain skilful well into the medium range for all years. Two particular problems are also examined: excessive precipitation over tropical oceans and a too strong Brewer-Dobson circulation, both of which are pronounced in later years. Several other aspects of the quality of the re-analyses revealed by monitoring and validation studies are summarized. Expectations that the 'second-generation' ERA-40 re-analysis would provide products that are better than those from the firstgeneration ERA-15 and NCEP/NCAR re-analyses are found to have been met in most cases.

SUMMARYIn the first of this set of three papers, the formulation of the European Centre for Medium-Range Weather Forecasts (ECMWF) implementation of 3D-Var is described. In the second, the specification of the structure function is presented, and the last is devoted to the results of the extensive numerical experimentation programme which was conducted. The 3D-Var formulation uses a spherical-harmonic expansion, much as the ECMWF optimal interpolation (01) scheme used an expansion of Bessel functions. This formulation is introduced using a convolution algebra over the sphere expressed directly in spectral space. It is shown that all features of the 0 1 statistical model can be implemented within 3D-Var. Furthermore, a non-separable statistical model is described. In the present formulation, geostrophy is accounted for through a Hough-modes separation of the gravity and Rossby components of the analysis increments. As in 01, the tropical analysis remains essentially non-divergent and with a weak mass-wind coupling. The observations used, as well as their specified statistics of errors, are presented, together with some implementation details. In the light of the results, 3D-Var was implemented operationally at the end of January 1996.

A new land surface analysis system based on a simplified point-wise Extended Kalman Filter (EKF) was implemented at the European Centre for MediumRange Weather Forecasts in the global operational Integrated Forecasting System (IFS) in November 2010. This system will allow consistent and optimal analyses of land surface parameters like soil moisture, surface temperatures, snow and vegetation properties. As part of the system implementation, the surface analysis structure has been revised to permit an independent and parallel computation with the upper-air 4D-Var analysis. The new analysis system is used for the soil moisture analysis, replacing the previous Optimal Interpolation (OI) scheme. Similar to the OI system, the simplified EKF uses 2 m air temperature and relative humidity observations from the SYNOP (land surface synoptic report) groundbased networks to analyse soil moisture. This paper describes the new land surface analysis, its application for analysing soil moisture, and initial verification results which supported its operational implementation at ECMWF. The performance is evaluated based on a set of one-year analysis experiments. The simplified EKF is compared to the OI, on soil moisture, 2 m temperature and relative humidity, showing a consistent improvement on screen-level parameters and soil moisture forecasts. To demonstrate the potential of the new analysis scheme, soil moisture derived from ASCAT (Advanced Scatterometer) has been assimilated through the simplified EKF.

SUMMARYWithin the Atmospheric Dynamics Mission Aeolus (ADM-Aeolus), the European Space Agency (ESA) has approved a Doppler wind lidar (DWL) to fly on a dedicated platform orbiting dawn to dusk at 400 km altitude, planned for launch in 2008. Rigorous design trade-offs have resulted in a lidar concept capable of delivering high-quality wind component profiles, but with a limited coverage. A companion paper describes the realistic simulation of this DWL, whereas this paper sets out to assess the impact of such a lidar in meteorological analyses and forecasts. To this end, an Observing System Simulation Experiment (OSSE) is run. The superior conventional observation coverage of 1993 is used to simulate all conventional observations, although a limited set of satellite observations is simulated. As a consequence, only the northern hemisphere DWL impact in the OSSE is assumed realistic. Here, over a 15-day period with variable weather, out of 15 daily forecasts, 14 show beneficial impact of the DWL. Although the experiment is limited, it corroborates other practical and theoretical evidence that the ADM DWL will demonstrate a beneficial impact in meteorological analyses and forecasts.

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During the African Monsoon Multidisciplinary Analysis (AMMA) field experiment in 2006 there was a large increase in the number of radiosonde data over West Africa. This has the potential of improving the numerical weather prediction (NWP) analysis/forecast and the water budget studies over that region. However, it is well known that the humidity from radiosondes can have some errors depending on sonde type, relative humidity (RH), temperature and the age of the sensor and can give rise to dry biases that are typically between 5% and 30% for RH. Three main sonde types were used in the AMMA field experiment: Vaisala RS80A, Vaisala RS92 and MODEM. In this article, a new empirical method is presented by using the operational European Centre for Medium-Range Weather Forecasts (ECMWF) short-range forecast as an intermediary dataset for computing biases. The validation of the correction method using global positioning system (GPS) total columnar water vapour (TCWV) confirms that the method is able to correct for a large part of the dry biases associated with the different sonde types. Results from analysis experiments show how the correction of humidity is particularly important in the West African region due to its impact on the development of convection in NWP models. The proposed radiosonde humidity bias correction has been applied to the special AMMA reanalysis experiment performed at ECMWF for the 2006 West African wet monsoon season. This is expected to benefit a wide number of AMMA-related studies that make use of the reanalysis, in particular those focusing on the water cycle.

SUMMARYThe directional wind turning across the boundary layer in short-range forecasts made as part of the recent ERA-40 reanalysis project is compared with radiosonde data from oceanic stations in the North Atlantic. Significant errors are identified, particularly in cases of warm advection in which it is found that the observed wind turning is systematically underestimated. Similar problems are found in the higher-resolution operational forecasts, and also in forecasts made with the independent Met Office model. Furthermore, comparisons of forecast wind direction with scatterometer data give consistent results over the North Atlantic and indicate that the same problems are present over other oceans.A large fraction of the cases with the largest errors are shown to have stable boundary layers. In these cases the modelled boundary layers are typically too deep, and sensitivity tests have shown that more realistic structures are obtained by switching to a stable boundary-layer parametrization based on Monin-Obukhov similarity. Some possible problems with the convective boundary layer parametrization in highly baroclinic conditions are also identified.

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