Details for Today:

DATE: 18th November 2004BY: Mark Cresswell

FOLLOWED BY: Literature exercise

Model Assimilation

69EG3137 – Impacts & Models of Climate Change

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

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

• In order to efficiently calculate forecast fields for each vertical level and global gridpoint

• Climate models don’t 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

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

Reanalysis: form and function

Reanalysis: form and functionReanalysis fields are generated for different pressure levels…from surface to 31 or so levels up to the top of the atmosphere

• 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)

Reanalysis: form and function

• 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

Meteorological Stations

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Meteorological StationsMAIN synoptic hours shown in REDINTERMEDIATE synoptic hours shown in BLACK

Data is relayed to the World Meteorological Organization in Geneva, Switzerland via the GTS:

GlobalTelecommunicationSystem

Currently there are about 10,000 stations globally(WMO, 2002)

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Meteorological StationsData recorded are:

•Air pressure•Temperature•Relative humidity•Precipitation•Visibility•Cloud parameters

Meteorological Stations

Meteorological Stations

Typical Meteorological Station Layout

Ship and BuoyAs 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

Ship and Buoy

ABOVE: moored buoyLEFT: drifting buoy

Upper Air MeasurementsAs 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

Upper Air MeasurementsLEFT:radiosonde

LEFT: launchof radiosondeballoon

RIGHT:sounding rocket

Satellite Data

Climate Model AssimilationFollowing the collation of ALL available data into a uniform gridded reanalysis dataset, this is “read in” by the climate model

The computer’s 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 computer’s memory and the model initialises all of the variables it uses for each forecast field, the data is said to have been assimilated.

Climate Model AssimilationSpecific assimilation schemes may be based on a single “snapshot” of conditions – known as 3D-Variational Assimilation…or 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

Spin-upWhen 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

Spin-upCold 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.

PracticalTODAY:Using 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 – DON’T JUST READ THE ABSTRACT!!!! Tutorial NEXT WEEK: based on literature exercise in groups