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DS126.2 represents a dataset implemented and computed by NCAR's Data
Support Section, and forms an essential part of efforts undertaken
in late 2004, early 2005, to produce an archive of selected segments
of ERA-40 on a standard transformation grid. In this case, ERA-40
monthly mean upper air variables on 60 model levels were transformed
from either spherical harmonics (surface geopotential, temperature,
vertical pressure velocity, vorticity, logarithm of surface
pressure, divergence), or a reduced N80 Gaussian grid (specific
humidity, ozone mass mixing ratio, cloud liquid water content, cloud
ice water content, cloud cover), to a 256 by 128 regular Gaussian
grid at T85 spectral truncation. In addition, horizontal wind
components were derived from spectral vorticity and divergence and
also archived on a T85 Gaussian grid. All scalar fields were
transformed using routines from the ECMWF EMOS library, whereas the
horizontal winds were obtained using NCAR's SPHEREPACK library. All
corresponding 00Z, 06Z, 12Z, and 18Z monthly mean upper air
variables on 60 model levels were also transformed to a T85 Gaussian
grid. The choice of a T85 Gaussian grid was based on considerations
of limiting the volume of new data generated to a moderate level,
and to match the horizontal resolution of the Community Atmosphere
Model (CAM) [https://www.cesm.ucar.edu/models/atm-cam/] component of
NCAR's Community Climate System Model (CCSM).
The ERA-Interim data from ECMWF is an update to the ERA-40 project.
The ERA-Interim data starts in 1989 and has a higher horizontal
resolution (T255, N128 nominally 0.703125 degrees) than the ERA-40
data (T159, N80 nominally 1.125 degrees). ERA-Interim is based on a
more current model than ERA-40 and uses 4DVAR (as opposed to 3DVAR
in ERA-40). ECMWF will continue to run the ERA-Interim model in near
real time through at least 2010, and possibly longer. This data is
available in ds627.0 [https://rda.ucar.edu/datasets/ds627.0/].
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