Evaluating the tropospheric variability in National Centers for Environmental Prediction's climate forecast system reanalysis

The National Centers for Environmental Prediction (NCEP) recently completed the latest, and partially coupled, atmosphere-ocean-sea ice model-based climate forecast system reanalysis (CFSR) for the 1979 to current satellite era. In the reanalysis, the observed CO2 concentration and the volcanic aerosols were also prescribed. This paper provides an initial overview of the tropospheric variability in the CFSR by comparing it against available previous reanalyses. CFSR's monthly mean zonal and meridional component of wind U and V, temperature T, and geopotential height H at pressure levels up to 100 mb are compared against those of three other readily available reanalyses NCEP/R1, NCEP/R2 and ERA40 for the period from 1979 to 2008 (2002 for ERA40) and also against modern reanalyses such as JRA, MERRA, and C20. At any given time (analysis hour, day, or month), for the globe as a whole, CFSR analysis agrees reasonably well with the other reanalyses. The CFSR's new coupled model and assimilation system makes use of the recent advances in these areas and hence is possibly an improvement to NCEP's previous reanalyses R1 and R2, which are 15 and 10 years old, respectively. For these long-term climate variability measures the analysis indicates that the CFSR was generally the outlier, with much stronger easterly trades, cooler tropospheric temperatures, and lower geopotential heights during much of the earlier part of the analysis period (1979-similar to 1998). Consequently, real-time monitoring of many of the ENSO-related climate wind indices in the equatorial Pacific or the wind shear index in the tropical North Atlantic from CFSR may be problematic in the context of historical variability.