Reanalysis is the method of running climate models to simulate reality by using observations as tight constraints in order to produce a consistent, gridded record of atmospheric, oceanic, and land surface conditions. Pushing the model to be as close as possible to (or even assimilated with) data from sources like satellites, weather stations, and radiosondes, reanalysis fills in gaps and smooths out inconsistencies in the observational record. The result is a comprehensive, long-term dataset that represents the state of the climate over time. Reanalysis is widely used for climate monitoring, model evaluation, and research on past climate variability and trends.
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Reanalyses.org
Reanalyses.org is a community editable website designed as a place to provide researchers with help to obtain, read and analyze reanalysis datasets created by different climate and weather organizations. In addition, users have a place to carry out discussions about these reanalyses, how they compare with each other and with observations. It is a wiki so users are encouraged to provide content and make comments. Pages can be made public or for group viewing.
Important Data links and catalogues
- ECMWF (European Centre for Medium-Range Weather Forecasts) - Provides access to ERA5 and earlier ERA products.
- NOAA Physical Sciences Laboratory (PSL) - Hosts various reanalysis products including NCEP/NCAR and 20th Century Reanalysis.
- Japan Meteorological Agency (JMA) / JRA Reanalysis - Hosts the Japanese 55-year Reanalysis (JRA-55) and earlier datasets.
- Copernicus Climate Data Store (CDS) - Central access point for ERA5 and other Copernicus reanalysis datasets.
- ...
Other resources
Comparison Table of Ocean Reanalyses: Global
| Dataset | Yyears | resolution | levels | model |
| NCEP/GODAS | 1980-present | Horizontal 1°x1° (1/3° meridional at the tropics) | MOMv3 | |
| NASA ECCO | 1993-2017? | 1x1 10 in top 10 | 50 | MITgcm and MOM4 |
| BLUELINK | 1992-2008 | ? | 47 | MOM4 + Chen hybrid mixed layer |
| CMCC C-GLORS | 1980-2015 | Horizontal 1/4°x1/4° (ORCA025 tripolar grid) | 50 | NEMO V3.2 + LIM2 |
| CMCC CHOR (Historical Reanalysis) | 1900-2010 | Horizontal 1/2°x1/2° (ORCA05 tripolar grid) | 75 | NEMO V3.4 + LIM2 |
| NCEP CFSR | 1979-near present |
Horizontal .25 near the equator; 0.5 beyond the tropics |
40 | GODAS w/MOM4 |
| ECMWF ORAS5 | 1979-near present |
Horizontal 1/4°x1/4° about 1m level thickness in upper 200 m. |
75 | NEMO V3.4.1 |
| ECMWF ORAS4 | 1958-? |
Horizontal 1°x1° with equatorial refinement (0.3) about 10m level thickness in upper 200 m. |
42 | NEMO V3.0 |
| GECCO | 1952-2001 | Horizontal 1°x1° | 23 | MITgcm |
| GECCO2 | 1948-2014 | 1x1/3° | 50 | MITgcm |
| GFDL | 1961-2012 | Global Tripolar grid (360 by 200), | 50 | ECDA |
| GLORYS2V4 | 1993-2016 | Global, 1/4° Mercator grid.1m top level, 200m bottom level |
75
|
Mercator Océan / Copernicus Marine Environment Service (CMEMS) |
| GMAO | 1980-present |
1/2° Ocean, 1° Atmosphere |
? |
GEOS AGCM, MOM4p, CICE |
| INCOIS-GODAS | 2004-almost real time | 0.5 deg in the zonal direction. In the meridional direction it is 0.25 between 10S-10N and 0.5 poleward 30 | 40 | MOM4.0 |
| SODA | ?? | Horizontal 0.25°x0.25o at the equator reducing to <10km in the Arctic | 50 | MOM5 |
| TOPAZ | 1992-2016 | Horizontal 12km in the Arctic to <16 km in North Atlantic | 28 | HYCOM v2.2, CICE v3 |
| U of Reading UR025.3 | ?? | 1/4o | 46 | NEMO coupled ice-ocean model v2.3 |
| U of Reading UR025.4 | 1989-2010 | 1/4o | 75 | NEMO coupled ice-ocean model v3.2 |
| BRAN2020 | 1993-2020 |
(from reanalyses.org)
Important reanalysis related terms
These are some concepts that are specifically important in relation to reanalysis:
- Data Assimilation - The process of combining observations with model output to produce a consistent and complete representation of the climate system.
- Background Field - Also called the "first guess," it's the short-term model forecast used as a starting point for assimilation of observations.
- Analysis Increment - The adjustment made to the model state during assimilation to bring it closer to observations.
- Reanalysis Product - A specific output dataset created using a fixed model and assimilation system over a long historical period.
- 4D-Var (Four-Dimensional Variational Assimilation) - A sophisticated assimilation method that optimally fits observations across space and time within a model window.
- Observation Operators - Functions used to compare model variables with observed quantities, especially when the variables are not directly measured.
- Observation Coverage - Refers to the spatial and temporal density of observations available for assimilation (e.g., satellite, radiosonde, surface).
- Bias Correction - The process of adjusting observations or model inputs to remove systematic errors before assimilation.
- Spin-up Period - The initial time segment during which the model and assimilation system stabilize; typically excluded from analysis.