Journal of Climate:
Vol. 14, No. 7, pp. 1499–1510.
Quality of Reanalyses in the Tropics
Kevin E. Trenberth, David P. Stepaniak and James W. Hurrell
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
Michael Fiorino
Lawrence Livermore National Laboratory, University of California,
Livermore, California
(Manuscript received 4 February 2000, in final form 12 May 2000)
Broad vertical layer-averaged temperatures from the microwave sounder unit (MSU) are used as
a quasi-independent validation of temperature fields from the U.S. National Centers for
Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR)
and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses. While the
MSU and NCEP–NCAR temperatures show fairly good agreement overall, large discrepancies
with ECMWF temperatures indicate that changes in the satellite observing system may have
adversely affected the ECMWF reanalyses, especially in the Tropics. Two spurious
discontinuities are present in tropical temperatures with jumps to warmer values throughout
the Tropics below 500 mb in late 1986 and early 1989, and further spurious interannual
variability is also present. These features are also reflected in the specific humidity
fields. The temperature discrepancies have a complex vertical structure with height that is
not fully understood, although it seems that the problems partly arise from positive
reinforcement of biases in satellite radiances with those of the assimilating model first
guess. Changes in the observing system provide a limit to the usefulness of the reanalyses
in some climate studies.
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Hongjun Zhang:
zhangho@ucar.edu