There are fewer than 10 well-known reanalysis systems. Summaries of each can be found in the links below. Three of them that are important users of ACRE-facilitated data are outlined below: ERA SeriesThe European Reanalysis of Global Climate Observations works under the auspices of The European Centre for Medium-Range Weather Forecasts (ECMWF) and now involves 15 partner agencies. Like many reanalysis systems, it has several iterations that occur as the model is refined and as more historical observations become available. In reverse order, these iterations have been:ERA-CLIM and ERA-CLIM2ERA-20C and ERA-20CM (covering 1900-2010)ERA-Interim (1979-present)ERA-40 (1957-2010)ERA-15 (1979-1993)Key objectives for ERA projects include:•Improving the available observational record for the early 20th century•Preparing data sets and assimilation tools needed for global reanalysis•Providing information about data quality by means of pilot reanalyses•Developing an Observation Feedback Archive facility for users•Assessing and reducing uncertainties in reanalysis data•Developing a sustainable capability for data recovery and reanalysisTaking ERA-20C as an example, the system assimilates observations of surface pressure and surface marine winds only. The observations include surface and mean sea level pressures from ISPDv3.2.6 and ICOADSv2.5.1, and surface marine winds from ICOADSv2.5.1. A coupled Atmosphere/Land-surface/Ocean-waves model is used to reanalyse the weather, resulting in products that describe the spatio-temporal evolution of the atmosphere (on 91 vertical levels, between the surface and 0.01 hPa), the land-surface (on 4 soil layers), and the ocean waves (on 25 frequencies and 12 directions). The horizontal resolution is approximately 125 km. Atmospheric data are also available on 37 pressure levels, 16 potential temperature levels, and the 2 PVU potential vorticity level. The model products are available for 3 or 6 hourly times while monthly mean data will be made available later.As reanalysis systems are based on computer models, there is uncertainty in their outputs.ERA CLIM2 communicates this uncertainty to users by using ensemble
in reanalysis production by conducting a variety of detailed quality assessments of input observations as well as reanalysis output, and by providing open and complete user access to data.20CRThough digitised data from ACRE is freely available to the global climate community for use in any reanalysis model or system, the project maintains a special relationship with the 20CRClimate Reanalysis system. Conceived and run by the Earth Systems Research Laboratory (CIRES, University of Colorado) with the support of the Earth System Research Laboratory (ESRL, of NOAA) it aims to reanalyse climate stretching back 200 years. It is this long-term time perspective that correlates it with the data rescue activities of ACRE. It has several iterations, marked by version numbers from 1 to 4.Version 1: 1908 to 1958Version 2: 1871 to 2012 (current version)Version 2c: 1851 to 2012Version 3: 1850 to 2015 (in 2016)Version 4: 1816 to 2019 (in 2019)20CR uses surface synoptic pressure observations and monthly sea surface temperature and sea-ice distribution to generate a six-hourly global atmospheric dataset at a resolution of 2 degree latitude/longitude. Much of the surface pressure data comes from the International Surface Pressure Databank version 2 (ISPD). 20CR offers three unique features: It covers a period of nearly 150 years, it provides the first estimates of global tropospheric variability and it supplies estimates of uncertainty in its analysis.Comparisons with independent radiosonde data indicate that the reanalysis products are generally of high quality. To further enhance the value of 20CR products, the UK Met Office has used dynamical downscaling to take 20CR output down to finer resolution of 25 km to 100 metres.This videoshows Gilbert Compo, Research Scientist and Leader of the NOAA Series 20th century Reanalysis explaining how the 20CR Reanalysis System works. Using reanalysis outputs to analyse three weather environments of the past, he illustrates the value of rescuing our weather heritage to enhance our understanding of past weather.20CR OutputHistorical weather data that has been rescued and digitised by ACRE partners is used in reanalysis models to produce extensive data sets that describe past weather. For instance, 20CR is capable of using sparse input of surface presure and sea temperature data to re-create the full scope of metaorological data including:Geopotential Height (air pressure)Temperatureu wind (east west)v wind (north south)Pressure vertical velocitySpecific humidityRelative humidityAt 19 Levels: (1000 hPa–100 hPa every 50 hpa)Others at specific levels (eg, sigma) or over the depth of the atmospheric column include:include:
“…resulting in products that describe the atmosphere on 91 vertical levels.”
“ACRE maintains a special relationship with the 20CR Climate Reanalysis system”
surface pressuretropopause heightprecipitable waterconvective available potential energyconvective inhibitionpotential temp. total ozonecloud watersensible heat fluxlatent heat fluxvolumetric soil moistureaccumulated snowdownward long wave fluxupward long wave fluxupward short wave flux
“Improving the available observational record for the early 20th century”
PRECIS (REgional Climates for Impacts Studies)PRECIS is a useful adjunct to reanalyses systems. It is a regional climate modelling tool that takes large scale atmospheric and ocean conditions from reanalysis systems, where horizontal resolutions vary from 100 to 300kms, and downscales it over a region of interest to resolutions of 25 or 50km. This allows for a more realistic representation of the climate over the region of interest, accounting for complex surface features such as mountains, coastlines and islands which are not resolved in the global models.PRECIS has been utilised in a wide range of climate-related studies and impacts research projects across the world, including analysis of droughts in the Horn of Africa, climate change impacts in Bangladesh, and building resilience to climate extremes in the Philippines.