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artefacts on radar scans

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hotnbothered
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artefacts on radar scans

Post by hotnbothered »

Everyone knows that radar scans are prone to radial artefactual lines - but exactly why? Would welcome any good links about this.
And after watching the BoM radar maps over a long period, my impression is that the main Melbourne weather radar at Laverton is prone to showing an artefactual line towards the SSE. If consistent, it must have a technical rather than a meteorological explanation. Does anyone know why?
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Gordon
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Re: artefacts on radar scans

Post by Gordon »

No, but good question!
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JasmineStorm
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Re: artefacts on radar scans

Post by JasmineStorm »

Is there grassland, creek or swamps SSE of the radar in Laverton? Insects or birds can create echoes on the BoM radar and favour these types of spots.
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hotnbothered
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Re: artefacts on radar scans

Post by hotnbothered »

Yes, Jasmine, good thought. The radar is on the RAAF base and there is open flat country to the N, E, and SE, with a small lake to the SSE.
South of the radar, there is also an infrastructure boundary where the edge of a housing development gives way to the open land of the RAAF base. This line of houses also runs roughly SSE.
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JasmineStorm
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Re: artefacts on radar scans

Post by JasmineStorm »

Interesting HNB. From your description of the surrounds, it could be birds. On clear days across the bay, I see small arrow shaped light rain blobs that head SE to NW. Almost certain these are high flying flocks heading into grasslands and wetlands north of the bay.
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hotnbothered
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Re: artefacts on radar scans

Post by hotnbothered »

The radial artefact I'm talking about extends for hundreds of km (see attached recent example, still visible days later), on the National radar scan, which is a composite image. It consistently extends from Melbourne to the NE corner of Tasmania. This cannot be explained by birds or insects. I've done a bit of reading on the web and dug up these points:

1. Modern weather radars are pulsed Doppler radars operated at several standard angles of elevation, the lowest usually being 0.5deg, and this is so low it is often blocked by trees, hills and man-made structures. This gives a low return signal ("no dbZ") and for reasons not explained but presumably due to the heavy post-acquisition signal processing, this often causes long radial spikes as clearly shown in Fig 7 of this PDF:
http://www.weather.gov/media/btv/research/Radar Artifacts and Associated Signatures.pdf
and also in the figure in the "Blocking Beam" section of this site:
http://www.ec.gc.ca/meteo-weather/defau ... 2B931828-1
These will be seen even in the base, non-composite scans. It seems to me that the beam-blocking is a physical event, but the generation of radial lines on the scan is not.

2. Next possibility is EM interference, and C-band EMI from wireless devices can cause sharp radial spikes: see the figure in the final section on EMI in the 2nd reference.

3. Next possibility is artifact due to the program that stitches together the scans at different elevations to produce the composite mode scans often displayed. This is a 3D modelling artifact.

4. Final possibility is artifacts in the stitching program which combines and interpolates images from different geographical sites to produce the 512km composite and the National scans. I feel this might be the cause here, in part because consistent artifact lines in these 2 larger scale scans are not seen in the 64, 128 and 256km scans on the same day.

Most of these (except the EMI case) appear to be due to software artifacts rather than physical effects on the radar hardware. However I still haven't found any info that explains how a low signal, or database inconsistencies between sources of a composite image, creates a linear software artifact.
Attachments
radar 15-03.png
radar 15-03.png (346.79 KiB) Viewed 12205 times
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flyfisher
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Re: artefacts on radar scans

Post by flyfisher »

Another radar question. The new Melbourne radar shows big gaps where mountains are. In particular.

1. Dandenongs
2. Yarra ranges
3. Mount Macedon

So we cannot see any rain at all over these locations. Surely the radar beam goes over the mountain top as it able to resolve rain behind the mountain? Looks like it's deliberately blocked out.

Is the radar still being calibrated?
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