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The goal of the radar declutter project is to remove unwanted radar pixels in a radar image. This clutter remains even after the KNMI applies their (ground-) declutter algorithm [Holleman et.al., 2005]. According to Holleman this algorithm works best above land and has problems above water.
It’s important that the declutter algorithm does only remove pixels that are 100% clearly clutter, it’s better to keep some clutter, than to remove some real precipitation.
In the figure below an example of a case with clutter in the radar. In this case there is clutter above the North Sea and there are some real showers in the south west.
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Clutter can have multiple causes, like inversions in the boundary layer, insects, but also technical reasons, mostly caused by the military. Clutter usually looks like noise on the radar image, therefore it is easy to detect clutter with the human eye. This is the property used by the MRD declutter algorithm.
The declutter algorithm has been inspired by the 2D entropy theory [Yang et.al., 1996]. Using the assumption that clutter is chaotic, it can be filtered by excluding all pixels that are marked as random noise. When all bad pixels are removed, a declutter radar image remains. In this case that would give the image below. Not all clutter has been removed. Because some pixels or group of pixels can look like both clutter or precipitation therefore with the current method they will never be removed.
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Holleman, I. en Beekhuis, H., 2005: Review of the KNMI clutter removal scheme, Technical Report, KNMI TR-284.
Yang, C.W. en Chung, P.C., 1996: Hierarchical fast two-dimensional entropic thresholding algorithm using a histogram pyramid, Optical Engineering, Vol. 35, nr. 11.
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