A Flexible Bandpass Filter Design Procedure Applied to Midlatitude Intraseasonal Variability

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Abstract
A general digital bandpass filtering procedure is presented whose advantages over other simple filtering methods are 1) versatility in its design, because only a set of three parameters is needed to calculate the filter weights via a simple analytic expression, 2) good performance at the transition band depending on the number of weights considered, and 3) the reduction of the Gibbs oscillations in the pass band of a given raw filter by convolving it with a convergence window. In order to illustrate the method, the filter has been first used to assess the ability of the Météo-France general circulation model ARPEGE to simulate the midtropospheric low-frequency intraseasonal variability in the Northern Hemisphere. The filter examined here allows one to assess the model drawbacks in different frequency bands. As a second example, the synoptic-scale baroclinic fluctuations in midlatitudes have also been studied. It is shown that the horizontal and vertical structure of these fluctuations does not depend very much on the frequency band up to a period of 10 days, but shows an increase in zonal wavelength as lower-frequency fluctuations are considered.

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