Model Assimilation

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Model Assimilation
Details for Today DATE 18th November
2004 BY Mark Cresswell FOLLOWED
BY Literature exercise

• 69EG3137 Impacts Models of Climate Change

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Lecture Topics

• What is assimilation?
• Reanalysis what is it and why is it?
• Meteorological stations
• Ship and buoy instruments
• Radiosonde, dropsonde and Aireps
• Climate model assimilation
• Spin-up

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What is Assimilation?
Assimilation is a process which involves the
collation and analysis of global meteorological
observations into a digital georeferenced
format Once global weather data is stored
electronically it is used to teach a climate
model the current state of the world ocean,
atmosphere and land conditions Once the model
learns the current global state of the weather
it can calculate future change based on these
initial conditions
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Reanalysis form and function

• In order to efficiently calculate forecast fields
  for each vertical level and global gridpoint
• Climate models dont see individual points (like
  meteorological stations) they see grids of
  regularly spaced squares
• Point data (typically derived from meteorological
  stations) must be transformed into a smooth
  gridded surface

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Reanalysis form and function

• Once all observations are known (station, ship,
  buoy, aircraft, satellite, balloon and
  radiosonde) the data are blended to generate a
  smooth grid
• This observational grid is known as reanalysis
• Reanalysis represents the most objective record
  of what the atmosphere and oceans were like for a
  specific date and time

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Reanalysis form and function
Reanalysis fields are generated for different
pressure levelsfrom surface to 31 or so levels
up to the top of the atmosphere
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Reanalysis form and function

• Reanalysis may be used as a gold standard
  against which model hindcasts can be compared (to
  assess model skill, reliability and bias)
• Reanalysis may also be used to spin-up global
  climate change models
• European modellers make use of the ECMWF
  reanalysis covering 15 years (1979-1993) known as
  ERA-15. We now have ERA-40 (covering the last 40
  years)

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Meteorological Stations

• Around the world are a network of stations where
  standard meteorological observations are made
  coded as SYNOP
• Observations are recorded at main synoptic hours
  only for minor stations or main and intermediate
  hours for major stations
• Major stations are manned
• Minor stations may make use of automatic
  instruments recording to magnetic tape or
  relaying data via radio

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Meteorological Stations

• 00
• 03
• 06
• 09
• 12
• 15
• 18
• 21

MAIN synoptic hours shown in RED INTERMEDIATE
synoptic hours shown in BLACK Data is relayed to
the World Meteorological Organization in Geneva,
Switzerland via the GTS Global Telecommunication
System Currently there are about 10,000
stations globally (WMO, 2002)
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Meteorological Stations

• 00
• 03
• 06
• 09
• 12
• 15
• 18
• 21

• Data recorded are
• Air pressure
• Temperature
• Relative humidity
• Precipitation
• Visibility
• Cloud parameters

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Meteorological Stations
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Meteorological Stations
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Typical Meteorological Station Layout
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Ship and Buoy

• As well as land-based observations, the GTS
  collects measurements of meteorological
  conditions over the sea coded as SHIP
• These data are collected by
• Ships
• Moored buoys
• Drifting buoys
• Measurements made by ships recruited under the
  WMO Voluntary Observing Ship Programme. Drifter
  data is coded as DRIFTER

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Ship and Buoy
ABOVE moored buoy LEFT drifting buoy
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Upper Air Measurements
As well as land and oceanic surface measurements
the GTS collates upper air data. Radiosondes
instruments attached to ascent balloons are used
to generate vertical profiles of the
atmosphere Aircraft observations are reported
from aircraft as well as specific pilot reports.
Data from aerodromes and airfields comprise the
METAR encoded data Rockets and radar are also
used for vertical data collection
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Upper Air Measurements
LEFT radiosonde
LEFT launch of radiosonde balloon RIGHT soundin
g rocket
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Satellite Data
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Climate Model Assimilation
Following the collation of ALL available data
into a uniform gridded reanalysis dataset, this
is read in by the climate model The computers
memory is used to store the values for each field
(temperature, pressure, windspeed etc) for each
level (surface, 850hPa, 500hPa etc) When ALL of
this data is stored in the computers memory and
the model initialises all of the variables it
uses for each forecast field, the data is said to
have been assimilated.
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Climate Model Assimilation
Specific assimilation schemes may be based on a
single snapshot of conditions known as
3D-Variational Assimilationor it may be based on
several days of reanalysis so uses a
4D-Variational Assimilation scheme (the 4th
dimension being time!) 4D-Var. is a standard
data assimilation method and is essentially a
means of estimating the initial conditions of a
model by optimising the fit between real
observations and predicted 'observations' found
from a projection of the model forward in
time After assimilation is complete, the model
can be run forward in time to generate the actual
forecast
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Spin-up
When global weather observations have been
successfully assimilated into a climate model
and it is initialised, the start of the forecast
run (the initial period of time being forecast)
may produce erratic results The initial period
of a forecast run (say the first few days of a 1
month forecast run) is known as the spin-up
period. This period varies according to the model
used and the period being forecast More
specifically, Spin-up is the time taken for a
model to reach a state of statistical equilibrium
under the applied forcing
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Spin-up
Cold start Usually occurs when a model is first
initialised and needs to be spun up. For example,
if a model is configured in a new domain, it
would need to be started in this manner. A cold
start could be from climatology rather than
reanalysis. The model is then run until a
statistical equilibrium is achieved. Warm
start A warm start is a restart of a model,
which is used to eliminate or reduce the model
spin up time. The saved fields from a recent
forecast of the same model can be used to
initialise a new simulation, or continue the
previous simulation. The saved fields may be used
as a first guess for an analysis including new
data, and then that field is used to initialise
the new forecast.
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Practical

• TODAYUsing library and online literature
  resources, find paper references for the
  following topics
• Natural climate forcing AND sea-level rise
• Anthropogenic climate forcing AND human health
• Dynamical climate modelling AND hydrology/water
  resources
• Statistical climate modelling AND
  politics/policies e.g Kyoto
• In this practical slot, look at Science Direct
  and Web of Knowledge
• (http//www.sciencedirect.com and
  http//wok.mimas.ac.uk)
• EACH GROUP WILL BE EXPECTED TO SHOW EVIDENCE OF
  HAVING READ AND UNDERSTOOD PAPERS DONT JUST
  READ THE ABSTRACT!!!! Tutorial NEXT WEEK based
  on literature exercise in groups