Paul Matthews has recently reported on the highly unstable versions of temperatures at Alice Springs (and elsewhere) in GHCN:
Several years ago Roger Andrews raised doubts about GHCN temperature homogenisation at Alice Springs:
These blog posts and ones related to them (apologies for not mentioning everyone) have inspired me to fire up once again my own temperature analysis tools. This post will develop over the next several weeks, but initial results and thoughts are as follows.
Besides the question of GHCN algorithm stability, there is also the question of what the right answer is, and whether or not one of the GHCN versions has come close. Step one is always to plot the raw data, and the data for close neighbours, as shown in the following figure for annual averages of daily maximum temperature (Tmax), downloaded from BoM Climate Data Online:
The figure above shows the temperatures as they are (not anomalies), with Alice Springs Post Office in black, and data from nearest neighbours in various colours. Several tentative conclusions can be drawn just by eye-balling the data as follows:
- Alice Springs is cooler than its neighbours (possibly due to higher elevation and differences in vegetation and cloud cover), which immediately creates an obstacle to easy homogenisation
- The neighbours share a great deal of consistency in their temperature fluctuations, and in their gentle cooling trend to around 1960, seen elsewhere in Eastern Australia. It should be relatively easy to detect inhomogeneities amongst the neighbours, and correct them at the level of annual averages.
- The elevated temperatures at Alice before around 1900 (marked A on the figure above) suggests that non-standard or damaged exposures were used in that period. Documentary evidence for a Stevenson screen in use at a certain date only provides evidence for its use from that date, not before.
- Alice is missing a rise in temperature in 1931 (marked B on the figure above), which would trigger some algorithms to shift its temperatures at that time, especially as the site moved in 1932 from the Telegraph Office to the Post Office. But, the overall temperature trend of Alice is already consistent with the neighbours, and a major shift in its temperatures would create an inconsistent warming trend.
- It is possible that Alice at the Post Office (after 1932) had urban heating, if so that would make homogenisation extremely difficult. The problem of urban heating at a Post Office site, prior to a shift to an airport site, is not obviously dealt with properly by govt. homogenisations, such as ACORN-SAT. Cobar may be a clear example of the Post Office heating problem. Simply splicing together airport and urban-warmed Post Office data gives an exaggerated warming trend, even if there is no urban warming at the airport.
The bottom line is that I don’t fancy trying to homegenise Alice Springs at all, it can be safely ignored, as it only covers the small (relative to the whole of Australia) area of its local range of hills. Beyond the hills the climate histories of the neighbours can be used.
More to follow later …