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DS124.2 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
6-hourly upper air variables on 60 model levels were transformed
from either spherical harmonics (surface geopotential, temperature,
vertical 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. 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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