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Weather regimes

The large-scale air-flow over a region of the earth, which is governed by the height contour patterns, can be considered as inducing or controlling typical weather conditions in an area.   The patterns are spatially fairly large, they affect the weather over regions as large as continents and have a persistence of typically ten days or more.   These patterns over the Euro-Atlantic region may be described by a few weather regime definitions:  

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• 4-regime scheme is used to produce Regime Frequency Histograms in Extended Range Regime Charts.
• Mirrored 2-regime scheme is used to produce phase space diagrams in Medium Range Regime Charts and Extended Range Regime Charts. 

Regime assignment of forecast data is in each case computed in Empirical Orthogonal Function (EOF) space using "calibrated" anomalies of 500mb height/geopotential.  These

In order to achieve regime assignment:

• first the anomalies are computed with respect to the re-forecast 20-year climate (ER-M-climate)

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• .
• then those anomalies are compared with reference points for the regime definitions. The regime definitions

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• are themselves based on re-analysis data.

) in order to achieve regime assignment.   Fig7.4-1 and Fig7.4-2 show what these regime reference points look like in 500mb anomaly space for the two schemes.  These are for illustrative purposes only – the user should not expect a model forecast assigned to a particular regime to look exactly like these; ordinarily there will just be some vague resemblance.  

For the 4-regime scheme, whilst two sets of anomalies are shown, in reality the reference points against which forecast anomalies are compared actually vary smoothly through the year.  This is done by giving different weights to these patterns depending upon the month.  So  The reference points for forecasts for the main months of winter and of summer the reference points will be almost summer months are almost identical to that shown in , respectively, the left and right sets in Fig7.4-1, whilst for .  For intermediate month forecasts they will be a weighted combination.

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The use of regimes can be explored at medium range regime charts and extended range regime charts.

Read more about the use of "Weather regimes" at ECMWF (2020 article)

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