Stickler, A., S. Brönnimann, M. A. Valente, J. Bethke, A. Sterin,
S. Jourdain, E. Roucaute, M. V. Vasquez, D. A. Reyes, R. Allan, and
D. Dee, 2014: ERA-CLIM: Historical Surface and Upper-Air Data for
Future Reanalyses. Bull. Amer. Meteor. Soc., 95(9), 1419-1430 (DOI:
ERA-20C is ECMWF's first atmospheric reanalysis of the 20th century,
from 1900-2010. It assimilates observations of surface pressure and
surface marine winds only. It is an outcome of the ERA-CLIM project.
ERA-20C was produced with IFS version CY38r1 and the same surface
and atmospheric forcings as the final version of the atmospheric
model integration ERA-20CM. A coupled atmosphere land surface and
ocean wave model is used to reanalyze the weather, by assimilating
surface observations. The ERA-20C products describe the
spatial-temporal evolution of the atmosphere (on 91 vertical levels,
between the surface and 0.01 hPa), the land-surface (on 4 soil
layers), and the ocean waves (on 25 frequencies and 12 directions).
The horizontal resolution is approximately 125 km (spectral
truncation T159). Note that atmospheric data are not only available
on the native 91 model levels, but also on 37 pressure levels (as in
ERA-Interim), 16 potential temperature levels, and the 2 potential
vorticity unit level. NCAR's Data Support Section (DSS) is
performing and supplying a grid transformed version of ERA-20C, in
which variables originally represented as spectral coefficients or
archived on a reduced Gaussian grid are transformed to a regular 320
longitude by 160 latitude N80 Gaussian grid. In addition, DSS is
also computing horizontal winds (u-component, v-component) from
spectral vorticity and divergence where these are available. The
data is being organized into single parameter time series.
The assimilation methodology is 24-hour 4D-Var analysis, with
variational bias correction of surface pressure observations.
Analysis increments are at T95 horizontal resolution (approximately
210 km). The analyses provide the initial conditions for subsequent
forecasts that serve as backgrounds to the next analyses. The
spatial temporal evolution of background errors was provided by a
10-member ensemble produced a priori.
The observations assimilated in ERA-20C include surface and mean sea
level pressures from ISPD v3.2.6 and ICOADS v2.5.1, and surface
marine winds from ICOADS v2.5.1. The observation feedback from
ERA-20C, including, for example, departures before and after
assimilation and usage flags, will be released at a later stage.
ERA-20C was produced in 2014 in about 6 weeks on ECMWF's IBM POWER7
high-performance computer. This production employed 22 parallel
streams: January 1899-December 1904, January 1904-December 1909,
etc. The first year of each stream was only used for spin-up and
discarded; the final product is made of the last years of each
The temporal resolution of the daily products is usually 3-hourly.
Exceptions are in the analysis products, where the following surface
parameters are only available 6-hourly: Sea-ice cover, Snow density,
Sea surface temperature, Volumetric soil water layer 1, Volumetric
soil water layer 2, Volumetric soil water layer 3, Volumetric soil
water layer 4, Soil temperature level 1, Snow depth, 2 meter
temperature, 2 meter dew point temperature, Soil temperature level
2, Soil temperature level 3, Temperature of snow layer.