DS124.0 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, forty six ERA-40 6-hourly surface and single level
analysis variables were transformed from a reduced N80 Gaussian grid
to a 256 by 128 regular Gaussian grid. All fields were transformed
using routines from the ECMWF EMOS library, including 10 meter winds
which were treated as scalars because of a lack of 10 meter spectral
vorticity and divergence. A missing value occurs in the sea surface
temperature and sea ice fields to mask grid points occurring over
land. Fields formerly archived as whole integers, such as vegetation
indices and cloud cover, occur as integers plus a fractional part in
the T85 version due to interpolation.
In addition, twenty seven ERA-40 6-hourly surface and single level
6-hour forecast variables were transformed from a reduced N80
Gaussian grid to a 256 by 128 regular Gaussian grid. All variables
are valid 6 hours after the forecast was initiated. Thus, 00Z 6-hour
forecast evaporation is valid at 06Z. Four of the variables are
"instantaneous" variables, and the remaining twenty three variables
are "accumulated" over the 6-hour forecast time. Divide the
accumulated variables by 21600 seconds to obtain instantaneous
values. (Multiplication by minus one may also be necessary to match
the sign convention one is accustomed to.) All fields were
transformed using routines from the ECMWF EMOS library, including
three pairs of stresses which were treated as scalars because of a
lack of spectral precursors.
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/].