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The general circulation of the atmosphere
By S. Tibaldi* and R
Mureau
* Current address: University of Bologna,
Department of Physics, Via Imerio 46, 40126 Bologna, Italy
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1 Introduction
The main goal of this module of the ECMWF training course is to give some information about the ability of General Circulation Models (GCMs) in general (and of the ECMWF global models in particular) to represent the main characteristics of the Atmospheric General Circulation (AGC). It is well known that GCMs show systematic deficiencies in representing the earth's atmospheric climate. Such deficiencies are often referred to as Systematic Errors (SEs) or as the models' Climate Drift. This last term expresses clearly the main essence of the problem: when a GCM integration is initiated from real-data initial conditions (that is from a single realisation of the observed distribution of atmospheric states, the `climate'), a progressive drift takes place from that single realisation of the real climate towards the model's own climatic distribution. Not only climate modellers suffer from this, but also numerical forecasters. We will see how the climate drift (or the progressive onset of the systematic error) is also an important source of forecast error.
In order to be able to diagnose and describe model deficiencies in representing the observed circulation, we need first to describe the real atmosphere, albeit in a synthetic way. The purpose of these first two lectures will indeed be: describing the observed mid-latitude AGC and the main physical processes responsible for it. We will then try to describe model deficiencies in representing it and this will put us in a position to make hypotheses on the possible causes for SEs, depending upon their structure, characteristics and evolution properties.
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07.06.2002 |
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