Atmospheric Circulation Reconstructions  over the Earth
Weather  Hindcasting - Using models to look backwards ACRE   and   its   partners   undertake   the   recovery   and   digitisation   of   historical   surface   terrestrial   and marine   weather   data   from   around   the   globe.   The   data   they   rescue   is   stored   in   international   databanks   which   are   used   by   a   host   of   climate   reanalysis   systems    to   estimate   past   weather   and   its   variations. Generally   speaking,   climate   reanalysis   systems   take   the   concepts   of   weather   forecasting   models   and use   them   to   reconstruct   the   past   weather   instead   of   forecasting   tomorrow’s   weather.      This   weather hindcasting ’ recreates what past weather probably looked like. A   more   nuanced   description   of      the   difference   between   climate   models   and   weather   forecasting models can be found here . Products of Reanalysis Systems ACRE    data    are    described    as    “observational”    as    it    originates    from    human    or    machine-recorded observations   of   weather   instruments   such   as   thermometers,   barometers,   anemometers,   etc.      Using limited   sets   of   these   observational   data   as   a   starting   point,   climate   reanalysis   systems   simulate   the   full array   of   the   Earth’s   weather   patterns.   They   do   this   over   several   decades   or   longer,   and   can   cover   the entire   globe   from   the   Earth’s   surface   to   well   above   the   stratosphere.      By   calculating   the   overall historical   state   of   the   climate,   they   generate   estimates   of   many   more   weather   variables   beyond   the original   limited   set   of   observational   data   started   with.   These   interpolated   variables,   or   reanalysis “products”,   can   cover   50   or   more   weather   conditions   including   atmospheric   temperature,   pressure, wind   and   humidity   at   different   altitudes,   surface   rainfall   and   soil   moisture   content,   and   even   ocean- surface   temperature   and   salinity.   They   also   create   an   array   of   derived   variables,   such   as   fluxes.   Thus, limited historical observations are used to create a much richer perspective of our climate heritage. With   this   perspective,   climate   scientists   use   reanalysis   products   to   assess   climate   variability   over   a long   period   of   time,   so   gaining   an   insight   into   how   it   may   be   changing.   Specific   outputs   from   the systems    are    also    useful    in    other    fields    of    study    including    ecology    (climate    impact    on    species), commercial   risk   analysis   (insuring   for   storm/flood   damage)   and   the   social   sciences   (human   reactions to   climate   extremes).      A   more   comprehensive   review   of   how   reanalyses   products   are   used   can   be found in the Applications section of this website. Differences Between Systems Reanalysis   systems   can   be   distinguished   by   the   geographical   span   and   resolution,   temporal   coverage and   individual   characteristics   of   their   output   products.      Most   systems   have   a   global   reach,   they   cover at   least   the   last   30-50   years   of   weather.   Some   systems   produce   50+   estimates   to   gauge   uncertainty   in the   ground,   air   and   water   weather   products.      An   important   feature   is   the   level   of   accuracy   in   each   of the   products,   considering   that   they   combine   uncertain   observations   with   an   uncertain   forecast.      For estimates   of   the   weather   100   years   ago,   reanalysis   fields   in   the   northern   hemisphere   can   be   expected to have an accuracy similar to current three to four day weather forecasts. Those   systems   focused   on   reanalyses   after   the   1940s   generally   use   large   amounts   of   automatically recorded    data    by    radiosondes,    ocean    buoys,    automatic    ground    weather    stations,    and    eventually satellites.      These   late   20th   Century   measurement   devices   have   automated   data-logging   capabilities that   generate   a   very   rich   set   of   observational   weather   data.      However,   before   the   1940s,   weather observations   are   generally   only   available   from   physical   recording   media   like   paper.      Some   of   the observations   were   made   by   machines   onto   paper,   while   others   were   recorded   by   humans,   resulting   in far   less   data   being   available   for   reanalysis.      What   data   does   exist   (and   has   been   discovered)   has   to   be digitised   before   it   can   be   assimilated   by   the   reanalysis   systems.      This   is   done   either   by   humans   or   by character    recognition    software.    Both    are    laborious    processes    resulting    in    a    further    reduction    in available data.  Addressing this deficiency is one of ACRE’s core activities. .
“Most systems have a global reach”
“Climate scientists use reanalysis products to assess climate variability…”