A pathological problem with NCEP reanalyses in the stratosphere
Kevin E. Trenberth and David P. Stepaniak
National Center for Atmospheric Research, Boulder, Colorado
Manuscript received 6 July 2001, in press
A pathological problem has been discovered in the NCEP reanalyses in the stratosphere.
It is manifested most strongly as a two-delta vertical wave in the divergence of the
wind field above steep topography especially where the wind increases with altitude in
the stratosphere. It is present primarily above 50 mb at the topmost four levels in the
NCEP model used for data assimilation and appears to be directly related to the use of
the sigma (terrain-following) coordinate system and the upper boundary condition in the
assimilating model. Recommendations suggested for addressing the problem include
switching to a hybrid coordinate system that transitions to a pressure coordinate in the
stratosphere, and with a damping upper boundary condition. Certain climate diagnostics
are greatly impacted by these pathologies.
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Hongjun Zhang:
zhangho@ucar.edu