The ECMWF Re-Analysis (ERA40) is a global atmospheric analysis of
many conventional observations and satellite data streams for the
period September 1957 to August 2002. There are numerous data
products that are separated into dataset series based on resolution,
vertical coordinate reference, and likely research applications.
DS124.1 represents a new data set implemented and computed by
NCAR's Data Support Section, and forms a substantial part of ongoing
efforts to produce an archive of selected segments of ERA-40 on a
standard transformation grid. In this case, ERA-40 upper air
variables on 23 pressure levels have been transformed from either
spherical harmonics (geopotential, temperature, vertical pressure
velocity, vorticity, divergence, relative humidity), or a reduced
N80 Gaussian grid (potential vorticity, specific humidity, ozone
mass mixing ratio), to a 256 by 128 regular Gaussian grid at T85
spectral truncation. In addition, horizontal wind components have
been derived from spectral vorticity and divergence and also
archived on a T85 Gaussian grid. All fields are available four times
per day (00,06,12 and 18 UTC). All scalar fields have been
transformed using routines from the ECMWF EMOS library, whereas the
horizontal winds have been 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/].