There   are   fewer   than   10   well-known   reanalysis   systems.      Summaries   of   each   can   be   found   here    and here .  Two of them that are important users of ACRE-facilitated data are outlined below: ERA Series The 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-CLIM2 ERA-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 reanalysis Taking   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   techniques   in   reanalysis   production (include   mouse   over   df.      as   below) ,   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. df’s   Ensemble   techniques   –   Using   weather   observations   as   inputs,   algorithms   generate   hypothetical drivers   of   climate   scenarios.   Since   these   are   only   informed   ‘guesses’,   ensemble   techniques   attempt   to reduce   the   uncertainty   of   individual   scenarios   by   generating   multiple   hypothetical   drivers   which   are combined to form a (hopefully) better scenario. http://en.wikipedia.org/wiki/Ensemble_learning 20CR Though   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    20CR    Climate 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 1958 Version 2: 1871 to 2012 (current version) Version 3: 1850-2015 (in 2016) Version 4: 1816-2019 (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   video    shows   Gilbert   Compo,   Research   Scientist   and   Leader   of   the      NOAA   Series   20th   century Reanalysis   explaining   how   the   20CR   Reanalysis   System   works.      Using   a   series   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. or (embed??) two shorter versions at C:\Users\Mac\Documents\Videos\Magix\Compo… 20CR Output Historical   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) Temperature u wind (east west) v wind (north south) Pressure vertical velocity Specific humidity Relative humidity At 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:

surface pressure tropopause height precipitable water convective available potential energy convective inhibition potential temp. total ozone cloud water sensible heat flux latent heat flux volumetric soil moisture accumulated snow downward long wave flux upward long wave flux upward short wave flux

downward short wave flux precipitation rate convective precipitation rate ground heat flux land cover ice concentration water runoff potential evaporation rate planetary boundary layer height albedo total cloud cover zonal momentum flux meridional momentum flux.

LINKS Download 20CR products HERE or HERE Article on how reanalysis systems create ‘new’ data Layman’s description of renanalysis systems Description of the PRECIS regional climate model that downscales atmospheric and land surface data to a high resolution for a limited area of the globe Latest activity in ACRE-facilitated reanalysis systems