This circular rain pattern was observed in Sydney’s rain radar today. It remained for the entire loop, wiggling around a bit but mostly staying in the same place.

The fact that it’s centred around the radar made me suspect it is some kind of artifact. Is that the case? Or this this real? If it’s real, why would it look like that?


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    $\begingroup$ Funny thing about human perception: We want to detect patterns, forms and such in anything - even random natural phenomenoms like rainclouds. In my opinion there is a slightly arched, very long raincloud to the (north-)east and south of sidney. Which by itself is a well known occurence at the front of low pressure systems. The circle is only completed by your brain, which wants to see one. The bit of drizzle to the west and north is not really worth mentioning. $\endgroup$ – Erik Jan 17 at 12:58
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    $\begingroup$ Radar sampling can also occasionally introduce a circular appearance to the precipitation echo structure, particularly at individual scan elevation angles, since (approximately speaking) each scan typically extends outward from the radar at an upward elevation - they may pass through clouds and precipitation only around a particular range, making it appear as if there is a persistent circular structure to the echoes. I couldn't immediately find an archived radar loop for this particular case to see whether this measurement artifact could also be a contributing factor, however. $\endgroup$ – dplmmr Jan 17 at 18:06

A way to check if those clouds and precipitation are real is to check the same area and time with another radar.

This is the image loop of Sidney (Terrey Hills) radar for the same time (the same you used). It shows the same pattern than your image. In the animated images that arch centered in the radar looks even weirder.

This is the image loop of Newcastle radar for January 17th, which in great part overlaps the Terrey Hills radar image. It shows an arched pattern, too, and with similar radius, but this one is centered in Terrey Hills radar instead of the Newcastle radar.

Therefore, since those patterns are different for each radar, I would say that they are artefacts, and that both radars suffered from the same kind of artefacts.

Interestingly, that kind of artifact seems consistent with the situation explained in dplmmr's comment: if both radar were just scanning an individual elevation angle (the same angle for both radars) and precipitation were just in a range of heights, we could expect that kind of pattern.

  • $\begingroup$ At around the same time most of the area above the sea showed us as raining, but almost none above the land. The boundary was clearly the coastline. Could that be related? $\endgroup$ – Gimelist Jan 22 at 22:35
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    $\begingroup$ I could imagine several combinations of real weather, dplmmr's proposed artefact, orography and radar settings that could lead to such pattern. However, I'm afraid I would be just speculating. Checking weather reports from that day or other radars may help. However, from what we can see on those two radars, I suspect that the artefact is caused by a problem in analysis and it could have affected more radars in the area. $\endgroup$ – Pere Jan 23 at 11:10

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