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A long term global atmospheric reanalysis Japanese 25-year Reanalysis (JRA-25) which covers from 1979 to 2004 was
completed using the Japan Meteorological Agency (JMA) numerical assimilation and forecast system. This is the first
long term reanalysis undertaken in Asia. JMA's latest numerical assimilation system, and observational data collected as
much as possible, were used in JRA-25 to generate a consistent and high quality reanalysis dataset to contribute to
climate research and operational work. One purpose of JRA-25 is to enhance to a high quality the analysis in the Asian
region.
6-hourly data assimilation cycles were performed and produced 6-hourly atmospheric analysis and forecast fields with
various kinds of physical variables. The global model used in JRA-25 has a spectral resolution of T106 (equivalent to a
horizontal grid size of around 120km) and 40 vertical layers with the top level at 0.4hPa. For observational data, a great
deal of satellite data was used in addition to conventional surface and upper air data. Atmospheric Motion Vector
(AMV) data retrieved from geostationary satellites, brightness temperature (TBB) data from TIROS Operational Vertical
Sounder (TOVS), precipitable water retrieved from radiance of microwave radiometer from orbital satellites and some
other satellite data were assimilated with 3-dimensional variational method (3DVAR).
Many advantages have been found in the JRA-25 reanalysis. Firstly, forecast 6-hour global total precipitation in JRA-25
performs well, distribution and amount are properly represented both in space and time. JRA-25 has the best
performance compared to other reanalysis with respect to time series of global precipitation over many years, with few
unrealistic variations caused by degraded quality of satellite data due to volcanic eruptions. Secondly, JRA-25 is the first
reanalysis which assimilated wind profiles surrounding tropical cyclones retrieved from historical best track information;
tropical cyclones were analyzed correctly in all the global regions. Additionally, low-level cloud along the subtropical
western coast of continents is forecast very accurately, and snow depth analysis is also good.
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