What are those flashing, oscillating spots or blobs on radar animations? Short answer: noise. NOAA has the long answer. In part:
Echoes from surface targets appear in almost all radar reflectivity images. In the immediate area of the radar, “ground clutter” generally appears within a radius of 20 nm. This appears as a roughly circular region with echoes that show little spatial continuity. It results from radio energy reflected back to the radar from outside the central radar beam, from the earth’s surface or buildings.
Under highly stable atmospheric conditions (typically on calm, clear nights), the radar beam can be refracted almost directly into the ground at some distance from the radar, resulting in an area of intense-looking echoes. This “anomalous propagation” phenomenon (commonly known as AP) is much less common than ground clutter. Certain sites situated at low elevations on coastlines regularly detect “sea return”, a phenomenon similar to ground clutter except that the echoes come from ocean waves.
Radar on Blue Marble image
In many national radar animations you can see these blobs of ground clutter reflections sweep across the US from east to west, and the timestamp shows that happens mostly at night. I guess that must be what they talk about in the second paragraph quoted above. Follow the link for more information about NOAA’s radars and what you see.
This noise is annoying when displayed on the white map of the US. It is very annoying when displayed on the Blue Marble map because the noise contrasts so much more with that background. It’s really too bad that NOAA does not provide radar images with the noise removed.
The ground clutter reflections can be effectively removed, a fact demonstrated by many (most? all?) commercial weather services. I have made some really amazing looking time-lapse videos with radar images from Weather Underground. Unfortunately, those images are copyrighted, so I cannot in good faith redistribute the videos made from them.
Could I remove the clutter? Playing with an image in GIMP makes me think that eliminating all pixels at or close to white would help a lot. I don’t think I can do that with the command line tools I use (mostly ImageMagick). Writing a program to do so might happen some day. It would probably look for pixels with a high value and very low saturation, i.e., very close to white. That algorithm wouldn’t get the blue that shows up close to the radar, but just removing the surges of white would help a lot.
Update 1-Sept-09:
Actually, it looks like it might be pretty easy to remove the noise by looking at hue and saturation. The problem is getting the right headers and libraries on my main development systems. Right now I’m in dependency hell, it would appear.
Update 8-Sept-09:
See this post about the solution.
