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.
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 closest 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 may create 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, but only back to around 1905, the date from which there may be enough neighbours to provide reasonable confidence.
- 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. See below for documentary evidence that may explain the 1931 discrepancy, a change to either a new or a replacement Stevenson screen.
- It is possible that Alice at the Post Office (after 1932) had urban heating, if so that would make accurate homogenisation difficult. The problem of urban heating at a Post Office (or similar urban) site, prior to a shift to an airport site, is not obviously dealt with properly by govt. homogenisations, such as ACORN-SAT. 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.
Here is some documentary evidence of station history, firstly an extract from the PhD of Simon Torok, an invaluable source of site history information, especially for stations that are not part of the ACORN-SAT network:
A link to the PhD thesis of Simon Torok is given as ref. 5 in the CLIMATE HISTORY OF SE AUS page of this blog (the appendix contains the station history summary). The extract above indicates a Stevenson screen supplied in November 1931, but it is unclear whether it is the first one used, or a replacement for a degraded one.
Information via ACORN-SAT adds to the above, with details of the area in use:
Finally, Alice Springs was used as a case study by Blair Trewin in describing the techniques used in the development of ACORN-SAT, see page 86 of his report here:
The ACORN-SAT report mentions considerable local temperature variations due to heavy rainfall.
I am developing another line of attack on Alice Springs, approaching from North West NSW, and will update this post when sufficient surrounding climate history is available to give confidence in any answer for Alice Springs.