Within the Copernicus Climate Change Service (C3S), ECMWF is producing the ERA5 reanalysis which, once completed, will embody a detailed record of the global atmosphere, land surface and ocean waves from 1950 onwards. This new reanalysis replaces the ERA-Interim reanalysis (spanning 1979 onwards) which was started in 2006. ERA5 is based on the Integrated Forecasting System (IFS) Cy41r2 which was operational in 2016. ERA5 thus benefits from a decade of developments in model physics, core dynamics and data assimilation. In addition to a significantly enhanced horizontal resolution of 31 km, compared to 80 km for ERA-Interim, ERA5 has hourly output throughout, and an uncertainty estimate from an ensemble (3-hourly at half the horizontal resolution). This paper describes the general setup of ERA5, as well as a basic evaluation of characteristics and performance, with a focus on the dataset from 1979 onwards which is currently publicly available. Re-forecasts from ERA5 analyses show a gain of up to one day in skill with respect to ERA-Interim. Comparison with radiosonde and PILOT data prior to assimilation shows an improved fit for temperature, wind and humidity in the troposphere, but not the stratosphere. A comparison with independent buoy data shows a much improved fit for ocean wave height. The uncertainty estimate reflects the evolution of the observing systems used in ERA5. The enhanced temporal and spatial resolution allows for a detailed evolution of weather systems. For precipitation, global-mean correlation with monthly-mean GPCP data is increased from 67% This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

International audienceA new diagnostic convective closure, which is dependent on convective available potential energy (CAPE), is derived under the quasi-equilibrium assumption for the free troposphere subject to boundary layer forcing. The closure involves a convective adjustment time scale for the free troposphere and a coupling coefficient between the free troposphere and the boundary layer based on different time scales over land and ocean. Earlier studies with the ECMWF Integrated Forecasting System (IFS) have already demonstrated the model's ability to realistically represent tropical convectively coupled waves and synoptic variability with use of the "standard" CAPE closure, given realistic entrainment rates. A comparison of low-resolution seasonal integrations and high-resolution short-range forecasts against complementary satellite and radar data shows that with the extended CAPE closure it is also possible, independent of model resolution and time step, to realistically represent nonequilibrium convection such as the diurnal cycle of convection and the convection tied to advective boundary layers, although representing the late night convection over land remains a challenge. A more in-depth regional analysis of the diurnal cycle and the closure is provided for the continental United States and particularly Africa, including comparison with data from satellites and a cloud-resolving model (CRM). Consequences for global numerical weather prediction (NWP) are not only a better phase representation of convection, but also better forecasts of its spatial distribution and local intensity

A relationship between busted European forecasts, a Rockies trough, and storms over eastern North America suggests the importance of improving quality and use of observations, model depiction of convective systems, and representation of uncertainties.

Because of careful quality control and relatively large observation errors, the all-sky system produces a weaker observational constraint on moisture analysis than the previous system. However, in single-observation experiments in precipitating areas, using the same observation errors as in the previous 1D-Var retrieval approach, the all-sky system is able to produce 4D-Var analyses that are slightly closer to the observations than before. Despite the nonlinearity of rain and cloud processes, 4D-Var minimizes successfully through the use of an incremental technique. Overall, the quality of the 4D-Var minimization, in terms of number of iterations and conditioning, is unaffected by the new approach.

ECMWF's preparations for cloud and rain assimilation encompass development of linearized physics, improved satellite data utilization, a new humidity analysis, and another look at the "spindown" problem. European, American, and Japanese satellite agencies have a number of Earth-observation missions with the objective of providing improved measurements of components of the global hydrological cycle-clouds, precipitation, soil moisture, and water vapor-from a range of operational platforms in both polar and geostationary orbits. Significant development of data assimilation methods will be necessary to make full use of both the existing and new types of observations of the water cycle. The small-scale

SUMMARY A large-scale condensation scheme, able to treat separately both atmospheric cloud condensate and precipitation content in a prognostic way, has been implemented and validated in Météo-France's operational global model, ARPEGE. The proposed scheme can be used for climate simulations and short-range numerical weather prediction, although it was originally designed for the future variational assimilation of cloud and precipitation observations.The main originalities of the scheme, compared with other existing schemes having a similar moderate level of complexity, lie in the inclusion of a prognostic variable for rain and snow content, and in the use of a simple semi-Lagrangian treatment of the fall of precipitation. The calculations of large-scale condensation/evapora tion and cloud fraction are based on probability-density functions, and the parametrized microphysical processes that involve precipitation are autoconversion, collection, and evaporation/sublimation.Various observations, which include satellite data from METEOSAT and from the Defense Meteorological Satellite Program's Special Sensor Microwave Imager, have been used for validating the cloud scheme within three-dimensional ARPEGE simulations at operational resolution for cases from the Fronts and Atlantic StormTrack Experiment. These ARPEGE simulations have also been compared with 10 km runs obtained with the Met Of ce's Uni ed Model and with the French Méso-NH research model. In addition, cloud radar, ceilometer, and lidar observations from the Atmospheric Radiation Measurement project have been utilized for validating the simulation of a synoptic winter cloud system over the southern Great Plains in the USA. The behaviour of the scheme was then assessed at a coarser resolution, with a particular focus on the zonal-mean radiative budget of the earth and the zonal-mean cloud cover.Finally, the question of the sensitivity of the results from the new scheme to various parameters has been addressed, including the time step and the speci cation of the fall velocities for rain and snow.

Abstract. The first simulation experiment and output archives of the Project to Intercompare Regional Climate Simulations (PIRCS) is described. Initial results from simulations of the summer 1988 drought over the central United States indicate that limited-area models forced by large-scale information at the lateral boundaries reproduce bulk temporal and spatial characteristics of meteorological fields. In particular, the 500 hPa height field time average and temporal variability are generally well simulated by all participating models. Model simulations of precipitation episodes vary depending on the scale of the dynamical forcing. Organized synoptic-scale precipitation systems are simulated deterministically in that precipitation occurs at close to the same time and location as observed (although amounts may vary from observations). Episodes of mesoscale and convective precipitation are represented in a more stochastic sense, with less precise agreement in temporal and spatial patterns. Simulated surface energy fluxes show broad similarity with the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) observations in their temporal evolution and time average diurnal cycle. Intermodel differences in midday Bowen ratio tend to be closely associated with precipitation differences. Differences in daily maximum temperatures also are linked to Bowen ratio differences, indicating strong local, surface influence on this field. Although some models have bias with respect to FIFE observations, all tend to reproduce the synoptic variability of observed daily maximum and minimum temperatures. Results also reveal the advantage of an intercomparison in exposing common tendencies of models despite their differences in convective and surface parameterizations and different methods of assimilating lateral boundary conditions.

Direct four-dimensional variational data assimilation (4D-Var) of NCEP stage IV radar and gauge precipitation observations over the eastern United States has been developed and tested in ECMWF’s global Integrated Forecasting System. This is the natural extension of earlier work using a two-step 1D+4D-Var approach. Major aspects of the implementation are described and discussed in this paper. In particular, it is found that assimilating 6-h precipitation accumulations instead of the original hourly data substantially improves the behavior of 4D-Var, especially as regards the validity of the tangent-linear assumption. The comparison of background and analysis precipitation departures demonstrates that most of the information contained in the new precipitation observations is properly assimilated. Experiments run over the periods April–May and September–October 2009 also show that local precipitation forecasts become significantly better for ranges up to 12 h, which indicates that a genuine precipitation analysis can now be obtained over the eastern United States. Geopotential, temperature, moisture, and wind forecast scores are generally neutral or slightly positive for all regions of the globe and at all ranges, which is consistent with previous 1D+4D-Var results. The most crucial issue that remains unsolved is the treatment of nonprecipitating model background occurrences because of the corresponding absence of sensitivity in the linearized moist physics. For the moment, only points where both model background and observations are rainy are assimilated. Operational implementation using U.S. data is planned in 2011 and one can hope that new networks of radars (and maybe rain gauges) can be added in the 4D-Var assimilation process in the future.