Weather radar image artifacts

16.06.2015 21:49

The Slovenian environment agency (ARSO) operates a 5 GHz Doppler weather radar on a hill in the eastern part of the country (Lisca). They publish a map with rate of rainfall estimate based on radar returns each 10 minutes.

Gašper of the opendata.si group has been archiving these rainfall maps for several years now as part of the Slovenian weather API project. A few days ago I've been playing with the part of this archive that I happened to have on my disk. I've noticed some interesting properties of the radar image that show up when you look at statistics generated from many individual images.

This is a sum of approximately 17676 individual rainfall maps, taken between December 2012 and April 2013:

Sum of 17676 weather radar images from ARSO.

Technically what this image shows is how often each pixel indicated non-zero rainfall during this span of time. In other words, the rainiest parts, shown in red, had rain around 18% of the time according to the radar. I ignored the information about the rate of rainfall.

More interesting is how some artifacts become visible:

  • There is a blind spot around the actual location of the radar. The radar doesn't scan above a certain elevation angle and is blind for weather directly above it.

  • There are triangular shadows centered on the radar location. These are probably due to physical obstructions of the radar beam by terrain. I've also heard that some of these radars automatically decrease sensitivity in directions where interference is detected (weather radars share the 5 GHz band with wireless LAN). So it is also possible that those directions have higher noise levels.

  • Concentric bands are clearly visible. The image is probably composed of several 360° sweeps at different elevation angles of the antenna. Although it is curious that their spacing decreases with distance. They might also be result of some processing algorithm that adjusts for the distance in discrete steps.

  • It's obvious that sensitivity is falling with distance. Some parts of the map in the north-western corner never show any rain.

  • Some moiré-like patterns are visible in the darker parts of the image. They are probably the result of interpolation. The map has rectangular pixels aligned with latitude and longitude while the radar produces an image in polar coordinates.

I should mention that ARSO's weather radar system got upgraded in 2014 with a second radar. Newer rainfall maps are calculated from combined data from both radars. They likely have much fewer artifacts. Unfortunately, the format of the images published on the web changed slightly several times during that year, which makes them a bit less convenient to process and compare.

Posted by Tomaž | Categories: Life

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