AMIP-II Reanalysis -- Global Atmospheric Analyses for 1979-1997
Masao Kanamitsu, Wesley Ebisuzaki, and Jack Woollen, National Centers for Environmental Prediction
Gerald Potter and Michael Fiorino, Lawrence Livermore National Laboratory
January 1980 precipitation rate from the NCEP/NCAR Reanalysis. Note the "spectral snow" problem caused by the poor approximation used in calculating the horizontal diffusion of moisture.
January 1980 precipitation rate from the NCEP/DOE AMIP-II Reanalysis. Note the improvement from using a more accurate horizontal moisture diffusion.
Research Objectives
Use the historical observations to make better analyses of the atmospheric state for the years 1979-1997 and provide analyses for the NCEP/DOE Atmospheric Model Intercomparison Project-II (AMIP-II).
Computational Approach
Describing the three-dimensional state of the atmosphere is difficult, as there are many more degrees of freedom in the atmosphere than observations made on any given day. The data assimilation system works by having an atmospheric general circulation model create a "first guess" and then finding the best analysis which minimizes the sum of the weighted fits to the first guess, the observations, and the divergence tendency.
The atmospheric general circulation model is pseudo-spectral, as some computations are computed in spectral space, others in grid space. In grid space, the atmosphere is discretized into 192 latitudinal x 94 meridional x 28 vertical (505,344) points. The calculations were solved using vectorized Fortran code running on the NERSC C90 computer.
Accomplishments
Analyses from January 1979 to November 1984 have been made. These analyses fix many of the serious problems in the earlier NCEP/NCAR Reanalysis. In addition, the use of more realistic cloud and radiation code has produced better estimates of the radiative fluxes than the NCEP/NCAR Reanalysis, when compared to the available satellite data.
Significance
Analyses of the atmospheric state are an important dataset for understanding the atmosphere. The analyses are used by researchers conducting diagnostic studies as well as those trying to predict future states of the atmosphere. There are three sets of analyses covering the satellite era made with modern data-assimilation systems: ERA-15 from the European Center for Medium Range Weather Forecasts (ECMWF), the NCEP/NCAR Reanalysis, and the DAO Reanalysis from NASA. This new AMIP-II Reanalysis replaces the NCEP/NCAR Reanalysis.