I posted recently on flutter in GHCN adjustment. This is the tendency of the Pairwise Homogenisation Algorithm (PHA) to produce short-term fluctuations in monthly adjustments. It arose on a recent discussion of the kerfuffle of John Bates and the Karl 2015 paper, and has been investigated by Peter O'Neill, who is currently posting on the topic. In my earlier post, I looked at the distribution of individual month adjustments, and noted that with generally zero mean, they would be heavily damped on averaging.
But I was curious about the mechanics, so here I compare the same two adjusted files (June 2015 and Feb 9 2017) collected by station. I'll show a histogram, but more interesting is the spatial distribution shown on a trackball sphere map. The histogram shows a distribution of station RMS values tapering rather rapidly toward 1°C. The map shows the flutter is strongly associated with remoteness, especially islands.
Update: I have now enabled clicking to show not only the name of the nearest station, but the RMS adjustment change there in °C. I have also adopted William's suggestion about the color scheme (white for zero, red for large).
Here is the histogram. There were some outliers, of which the greatest is Ayacucho in Peru, at about 3.3°C. I thought that was extreme, but when I looked it up, I found that almost all years got no adjusted value at all; there were 11 years with adjusted values, all differing by the same 3.3 years. I don't know how that happened, but the station would not have got into any indices, since no normal can be assigned. As you see from the plot, these extremes are rare:
So I made a WebGL globe map of the results. This is in the style of the GHCN monthly stations page. It has a triangular mesh with stations colored according to their RMS difference value, and color linearly shaded in between. I haven't shown a scale, since the colors are rather non-linear with RMS, but generally green is zero (no flutter) and red is high. The stations are shown as dots, and you can click to bring up names. You can rotate by dragging, and expand by mousing vertically with right button down.
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