A simple notch filter for removing high frequency noise in atmospheric measurements

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The experimenter occasionally encounters high-frequency (> 1 Hz) contamination in sonic anemometer data that adversely affects higher-order moment calculations. One has the option of applying a low-pass filter (either a simple, equally weighted moving average filter or any one of the sharper cut-off versions available today) to eliminate not only the unwanted frequency, but all frequencies above it as well. A band rejection or notch filter would be a better choice because of its selectivity, but is seldom used because of computational complexity. In many instances the contamination is quasi-sinusoidal, often caused by vibration of the supporting boom at a frequency typically between 1 and 2 Hz. A simple procedure exists for removing such noise without seriously degrading the signal. It is a notch filter, realized by averaging data points spaced τ/2 apart in time, where τ is the period of the noise signal.

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