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182 The Hydrological Cycle in Atmospheric Reanalysis

Part 15. Global Hydrology

  1. Anton Beljaars,
  2. Ulf Andrae,
  3. Per Kallberg,
  4. Adrian Simmons,
  5. Sakari Uppala,
  6. Pedro Viterbo

Published Online: 15 APR 2006

DOI: 10.1002/0470848944.hsa189

Encyclopedia of Hydrological Sciences

Encyclopedia of Hydrological Sciences

How to Cite

Beljaars, A., Andrae, U., Kallberg, P., Simmons, A., Uppala, S. and Viterbo, P. 2006. The Hydrological Cycle in Atmospheric Reanalysis. Encyclopedia of Hydrological Sciences. 15:182.

Author Information

  1. European Centre for Medium-Range Weather Forecasts, Research Department, Reading, UK

Publication History

  1. Published Online: 15 APR 2006


Atmospheric (re-)analysis is a powerful tool to create data sets that provide a good description of the atmosphere including the hydrological cycle. Atmospheric analysis systems use a numerical weather prediction model to propagate the state of the atmosphere in time. Observations, which are irregular in space and time, are inserted into the system in such a way that an optimal blend is obtained between the background fields (forecast from e.g. 6 hours before) and the observations. Observations range from traditional surface observations and radiosondes, to commercial aircraft observations, cloud track winds from satellites and profiling measurements from infrared and microwave instruments in space. Re-analyses are performed to benefit from the most recent model improvements, data assimilation improvements and increased computer power and to obtain a time series that does not suffer from system changes as in operational numerical weather prediction. Current re-analyses have a spatial resolution of the order of 1 degree with a temporal sampling interval of 3 to 6 hours. Re-analysis products have the advantage of being projected on a global numerical grid and have no gaps.

The ECMWF 40-year Re-Analysis is described as an example. Its realism is discussed by comparing with observations of precipitation, vertically integrated water vapor, moisture convergence and surface fluxes over land and ocean. It is concluded that re-analysis products are very useful to study the hydrological cycle in the atmosphere. The synoptic variability is particularly good although biases may exist in some of the parameters.


  • re-analysis;
  • atmospheric analysis;
  • hydrological cycle;
  • evaporation;
  • precipitation;
  • atmospheric moisture;
  • clouds