By optimally combining observations and models, reanalyses indeed provide consistent “maps without gaps" of ECVs and strive to ensure integrity and coherence in the representation of the main Earth system cycles (e.g., water, energy). ERA-Interim and the earlier ERA-40 reanalysis (Uppala et al., 2005) have also been a resource for the production of ECVs (Bojinski et al., 2014)) and Climate Indicators recommended by the GCOS. ECMWF's ERA-Interim reanalysis (Dee et al., 2011) has been routinely used, together with other datasets, as input to the WMO annual assessment of the State of the Climate and in the assessments carried out by the IPCC 1 1 All acronyms are defined in the Glossary (Table 8). The role of reanalyses in climate monitoring applications is now widely recognized. In general, low-frequency variability is found to be well represented and from 10 hPa downwards general patterns of anomalies in temperature match those from the ERA-Interim, MERRA-2 and JRA-55 reanalyses. For precipitation, global-mean correlation with monthly-mean GPCP data is increased from 67% to 77%. The enhanced temporal and spatial resolution allows for a detailed evolution of weather systems. The uncertainty estimate reflects the evolution of the observing systems used in ERA5. A comparison with independent buoy data shows a much improved fit for ocean wave height. 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.
Re-forecasts from ERA5 analyses show a gain of up to one day in skill with respect to ERA-Interim.
This paper describes the general set-up 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. 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). ERA5 thus benefits from a decade of developments in model physics, core dynamics and data assimilation. ERA5 is based on the Integrated Forecasting System (IFS) Cy41r2 which was operational in 2016. This new reanalysis replaces the ERA-Interim reanalysis (spanning 1979 onwards) which was started in 2006. 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.
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