The calculation of standard deviations of background errors is unfortunately an area where the use of inaccurate nomenclature is widespread. For example, standard deviations of background error are almost universally referred to as `background errors'. Likewise, standard deviations of analysis and forecast error are referred to as `analysis errors' and `forecast errors'. Although inaccurate, this nomenclature has been adopted in the following for the sake of brevity.

A second source of confusion is that terms `background error' and `forecast error' are often used interchangeably. This confusion has even crept into the code, where the buffer which contains the standard deviations of background error is called FCEBUF. Such confusion is clearly unwise when discussing the calculation of forecast errors. The following sections will describe the processing of error variances during a single analysis cycle. The term `background error' will refer exclusively to the standard deviations of background error which are used in the background cost function. The background errors are an input to the analysis. The term `forecast error' will refer to an estimate of the standard deviation of error in a short-term forecast made from the current analysis. The forecast errors are calculated by inflating an estimate of the standard deviation of analysis error, and are an output from the analysis system.

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20.03.2002

DATA ASSIMILATION

IFS documentation Front Page

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7.1 Nomenclature