Reanalysis
Systems
There
are
fewer
than
10
well-known
reanalysis
systems.
Summaries
of
each
can
be
found
in
the
links
below. Three of them that are important users of ACRE-facilitated data are outlined below:
ERA Series
The European Reanalysis of Global Climate Observations works under the auspices of
The
European
Centre
for
Medium-Range
Weather
Forecasts
(ECMWF)
and
now
involves
15
partner
agencies.
Like
many
reanalysis
systems,
it
has
several
iterations
that
occur
as
the
model
is
refined
and as more historical observations become available. In reverse order, these iterations have been:
ERA-CLIM and ERA-CLIM2
ERA-20C
and ERA-20CM (covering 1900-2010)
ERA-Interim (1979-present)
ERA-40 (1957-2010)
ERA-15 (1979-1993)
Key objectives for ERA projects include:
•
Improving the available observational record for the early 20th century
•
Preparing data sets and assimilation tools needed for global reanalysis
•
Providing information about data quality by means of pilot reanalyses
•
Developing an Observation Feedback Archive facility for users
•
Assessing and reducing uncertainties in reanalysis data
•
Developing a sustainable capability for data recovery and reanalysis
Taking
ERA-20C
as
an
example,
the
system
assimilates
observations
of
surface
pressure
and
surface
marine
winds
only.
The
observations
include
surface
and
mean
sea
level
pressures
from
ISPDv3.2.6
and ICOADSv2.5.1, and surface marine winds from ICOADSv2.5.1.
A
coupled
Atmosphere/Land-surface/Ocean-waves
model
is
used
to
reanalyse
the
weather,
resulting
in
products
that
describe
the
spatio-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.
Atmospheric
data
are
also
available
on
37
pressure
levels,
16
potential
temperature
levels,
and
the
2
PVU
potential
vorticity
level.
The
model
products
are
available
for
3
or
6
hourly
times
while
monthly
mean
data
will
be
made available later.
As reanalysis systems are based on computer models, there is uncertainty in their outputs.
ERA
CLIM2
communicates
this
uncertainty
to
users
by
using
ensemble
Using
weather
observations
as
inputs,
algorithms
generate
hypothetical
drivers
of
climate
scenarios.
Since
these
are
only
informed
‘guesses’,
ensemble
techniques
attempt
to
reduce
the
uncertainty
of
individual
scenarios
by
generating
multiple
hypothetical
drivers
which
are
combined
to
form
a
(hopefully)
better
scenario. (wikipedia)
techniques
Using
weather
observations
as
inputs,
algorithms
generate
hypothetical
drivers
of
climate
scenarios.
Since
these
are
only
informed
‘guesses’,
ensemble
techniques
attempt
to
reduce
the
uncertainty
of
individual
scenarios
by
generating
multiple
hypothetical
drivers
which
are
combined
to
form
a
(hopefully)
better
scenario. (wikipedia)
in
reanalysis
production
by
conducting
a
variety
of
detailed
quality
assessments
of
input
observations
as
well
as
reanalysis output, and by providing open and complete user access to data.
20CR
Though
digitised
data
from
ACRE
is
freely
available
to
the
global
climate
community
for
use
in
any
reanalysis
model
or
system,
the
project
maintains
a
special
relationship
with
the
20CR
Climate
Reanalysis
system.
Conceived
and
run
by
the
Earth
Systems
Research
Laboratory
(CIRES,
University
of
Colorado)
with
the
support
of
the
Earth
System
Research
Laboratory
(ESRL,
of
NOAA)
it
aims
to
reanalyse
climate
stretching
back
200
years.
It
is
this
long-term
time
perspective
that
correlates
it
with
the data rescue activities of ACRE. It has several iterations, marked by version numbers from 1 to 4.
Version 1: 1908 to 1958
Version 2:
1871 to 2012 (current version)
Version 2c
: 1851 to 2012
Version 3: 1850 to 2015 (in 2016)
Version 4: 1816 to 2019 (in 2019)
20CR
uses
surface
synoptic
pressure
observations
and
monthly
sea
surface
temperature
and
sea-ice
distribution
to
generate
a
six-hourly
global
atmospheric
dataset
at
a
resolution
of
2
degree
latitude/longitude.
Much
of
the
surface
pressure
data
comes
from
the
International
Surface
Pressure
Databank
version
2
(ISPD).
20CR
offers
three
unique
features:
It
covers
a
period
of
nearly
150
years,
it
provides
the
first
estimates
of
global
tropospheric
variability
and
it
supplies
estimates
of
uncertainty
in its analysis.
Comparisons
with
independent
radiosonde
data
indicate
that
the
reanalysis
products
are
generally
of
high
quality.
To
further
enhance
the
value
of
20CR
products,
the
UK
Met
Office
has
used
dynamical
downscaling to take 20CR output down to finer resolution of 25 km to 100 metres.
This
video
shows
Gilbert
Compo,
Research
Scientist
and
Leader
of
the
NOAA
Series
20th
century
Reanalysis
explaining
how
the
20CR
Reanalysis
System
works.
Using
reanalysis
outputs
to
analyse
three
weather
environments
of
the
past,
he
illustrates
the
value
of
rescuing
our
weather
heritage
to
enhance our understanding of past weather.
20CR Output
Historical
weather
data
that
has
been
rescued
and
digitised
by
ACRE
partners
is
used
in
reanalysis
models
to
produce
extensive
data
sets
that
describe
past
weather.
For
instance,
20CR
is
capable
of
using
sparse
input
of
surface
presure
and
sea
temperature
data
to
re-create
the
full
scope
of
metaorological data including:
Geopotential Height (air pressure)
Temperature
u wind (east west)
v wind (north south)
Pressure vertical velocity
Specific humidity
Relative humidity
At 19 Levels: (1000 hPa–100 hPa every 50 hpa)
Others at specific levels (eg, sigma) or over the depth of the atmospheric column include:
include:
“…resulting in products
that describe the
atmosphere on 91 vertical
levels.”
“ACRE maintains a special
relationship with the 20CR
Climate Reanalysis
system”
surface pressure
tropopause height
precipitable water
convective available potential energy
convective inhibition
potential temp. total ozone
cloud water
sensible heat flux
latent heat flux
volumetric soil moisture
accumulated snow
downward long wave flux
upward long wave flux
upward short wave flux
downward short wave flux
precipitation rate
convective precipitation rate
ground heat flux
land cover
ice concentration
water runoff
potential evaporation rate
planetary boundary layer height
albedo
total cloud cover
zonal momentum flux
meridional momentum flux.
LINKS
ACRE Facilitated Datasets -
Atmospheric Reanalyses Comparison Table
Overview of current ocean reanalyses
General Notes, Questions and Discussion about Reanalysis Datasets
Download 20CR products HERE or HERE
Data access and plotting tools
HERE
and
HERE
PRECIS - introduction and user notes
“Improving the available
observational record for
the early 20th century”
PRECIS
(REgional Climates for Impacts Studies)
PRECIS
is
a
useful
adjunct
to
reanalyses
systems.
It
is
a
regional
climate
modelling
tool
that
takes
large
scale
atmospheric
and
ocean
conditions
from
reanalysis
systems,
where
horizontal
resolutions
vary
from
100
to
300kms,
and
downscales
it
over
a
region
of
interest
to
resolutions
of
25
or
50km.
This
allows
for
a
more
realistic
representation
of
the
climate
over
the
region
of
interest,
accounting
for
complex
surface
features
such
as
mountains,
coastlines
and
islands
which
are
not
resolved
in
the
global models.
PRECIS
has
been
utilised
in
a
wide
range
of
climate-related
studies
and
impacts
research
projects
across
the
world,
including
analysis
of
droughts
in
the
Horn
of
Africa,
climate
change
impacts
in
Bangladesh, and building resilience to climate extremes in the Philippines.
