Editing General circulation model (section)

==Relation to weather forecasting==
The global climate models used for climate projections are similar in structure to (and often share computer code with) [[numerical weather prediction|numerical models for weather prediction]], but are nonetheless logically distinct.

Most [[weather forecasting]] is done on the basis of interpreting numerical model results. Since forecasts are typically a few days or a week and sea surface temperatures change relatively slowly, such models do not usually contain an ocean model but rely on imposed SSTs. They also require accurate initial conditions to begin the forecast{{snd}} typically these are taken from the output of a previous forecast, blended with observations. Weather predictions are required at higher temporal resolutions than climate projections, often sub-hourly compared to monthly or yearly averages for climate. However, because weather forecasts only cover around 10 days the models can also be run at higher vertical and horizontal resolutions than climate mode. Currently the [[ECMWF]] runs at {{convert|9|km|abbr=on|adj=on}} resolution<ref>{{cite web|url=http://www.ecmwf.int/index_forecasts.html |title=ECMWF |access-date=2016-02-07 |url-status=dead |archive-url=https://web.archive.org/web/20080503014123/http://www.ecmwf.int/index_forecasts.html |archive-date=3 May 2008 }} [http://www.ecmwf.int/sites/default/files/elibrary/2016/16299-newsletter-no147-spring-2016.pdf ECMWF-Newsletter spring 2016]</ref> as opposed to the {{convert|100|to|200|km|abbr=on|adj=on}} scale used by typical climate model runs. Often local models are run using global model results for boundary conditions, to achieve higher local resolution: for example, the [[Met Office]] runs a mesoscale model with an {{convert|11|km|abbr=on|adj=on}} resolution<ref>{{Cite web |title=Operational Numerical Modelling |url=http://www.metoffice.gov.uk/research/nwp/numerical/operational/index.html |archive-url=https://web.archive.org/web/20050307010834/http://www.metoffice.gov.uk/research/nwp/numerical/operational/index.html |archive-date=7 March 2005 |website=[[Met Office]] |access-date=28 March 2005 |url-status=bot: unknown }}</ref> covering the UK, and various agencies in the US employ models such as the NGM and NAM models. Like most global numerical weather prediction models such as the [[Global Forecast System|GFS]], global climate models are often spectral models<ref>{{cite web|url=http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/nwp_gcm.html |title=What are general circulation models (GCM)? |publisher=Das.uwyo.edu |access-date=2012-02-18}}</ref> instead of grid models. Spectral models are often used for global models because some computations in modeling can be performed faster, thus reducing run times.