The ACORN2 average December temperature for Australia is 25.9 C

I claimed in the last post that average temperatures in Australia for December  was 24C and not the 27C as stated by BOM, even though everyone else seems to agrees with them (CRU,GHCN,Berkeley) – See for example  this graphic. So apparently I must be wrong and should beg for forgiveness on twitter. However if  you actually use just the 112 quality controlled ACORN2 station data to calculate the average temperatures in exactly the same way as CRU (rather than triangulation) then you get 25.9C, provided you first exclude off-shore stations.  The result therefore depends on your definition of the average.  I calculated  the spatial distribution for ACORN2 stations binned and averaged in exactly the same way as CRU (5 degree bins).

Fig1. ACORN2 average December temperatures (1961-1990). Stars show station locations. Values shown are the average temperature within each 5×5 bin.

Figure 1  shows the 30y average temperatures for December using just ACORN2 stations. The station normals that I calculated for each month and for overlapping ACORN station agree exactly with those calculated by CRU. For instance the Merredin 12 monthly normals are:

CRU: 25.8 25.5 23.1 18.9 14.5 12.1 10.8 11.2 13.6 16.9 20.6 23.9

Clive: 25.8 25.5 23.2 19.0 14.6 12.2 10.8 11.2 13.6 16.9 20.7 24.0

So why do others find even warmer temperatures? Well one difference to the ACORN analysis is  that CRU, GHCN and Berkeley Earth are using about 400 Australian weather stations, three quarters of which do not directly appear in ACORN2 although some overlap. Perhaps including these extra stations then boosts the resultant spatial average to 27.1 C, or does it?

This paper describes how BOM selected the stations for ACORN. To quote:

Only some of the stations in a network are suitable for use in long term climate change analyses. Most have too little data (less than 30 years), and some have excessive missing data, poor site or observation quality, or are otherwise unsuitable.

So the Australian average temperature depends on whether you include these “unsuitable” stations in the calculation for Australia, including those rejected from the ACORN series for data quality reasons. Apparently BOM itself then quotes the CRU average temperature (27.1C for December)  resulting from using all stations and not those just those from the quality controlled ACORN set.

None of this makes any difference to the temperature anomaly results which are fine, but it demonstrates just how tricky it can be to convert these anomalies to absolute temperatures.

I then looked at the CRUTEM4 results themselves. I simply used the CRU calculated normals from all the station files within Australia. These have the Country codes  94 and 95. There are 360 of them with normals defined between 1961 and 1990. I likewise also restricted all these to be lie within the continental land mass of Australia  (one is actually located in Antarctica).  Here is the result.

The average December temperature now works out at 26.1C, which is a bit nearer to the quoted 27.1 but still not quite there. If you now exclude Tasmania then we reach a value of 26.4C. This is the highest value I can get using CRUs explicit 1961-1990 normalised values, but this is still 0.8C short of ACORN2’s quoted average December temperature.

All  data and results can be found here:

ACORN2

CRU-Normals

P.S. I am actually in Katoomba, Australia  right now (6 Feb). The temperature today reached just 14C but at least brought some much needed rain!

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5 Responses to The ACORN2 average December temperature for Australia is 25.9 C

1. Nick Stokes says:

“So why do others find even warmer temperatures?”

I think that in these circumstances, they probably use a land mask. That is, they work out which proportion of coastal cells are land, and reweight (downward) accordingly. Otherwise coastal stations are spuriously upweighted by the amount of sea in the cells. Reweighting gives higher weights to inland stations, which would boost DEcember.

“The temperature today reached just 14C”
Katoomba, at 1000 m altitude, is often a good place to be in summer.

2. Clive Best says:

Thanks Nick, maybe that explains it.
I suspect they might also reject light-house stations and off-shore Island stations as well. So it looks to be basically a definition problem.

We drove down today to “The Entrance”. The rain both here and on the way down was incredible – almost monsoonal. I hope that this downpour extinguishes all remaining smouldering bush fires.

We eventually end up in Brisbane where my daughter lives. Would be nice to see a but of sun !

3. Olof R says:

I checked quickly using the ESRL/WRIT interface with the two reanalyses going back that far.
The mask was land only within 10S-45S, 110E-160E.
According to NCEP/NCAR the 1961-1990 average was 28.0 C, according to JRA-55 25.8 C.
Quite a discrepancy, maybe a mask and gridcell size issue, but the most modern (generation 3) reanalysis gives you right about the 1961-1990 base period, Clive.

If we look at all reanalyses with data in so far, the Australian average temp in Dec 2019 was:
29.5 C according to JRA-55
30.0 C according to JRA-55 screen
30.5 C according to NCEP/NCAR
30.6 C according to NCEP/DOE
31.2 C according to NCEP/CFSR
30.8 C according to MERRA2

JRA-55 is clearly the coolest here

• Clive Best says:

Thank you for that analysis!

The monthly baseline temperature is key to use any absolute derived temperatures from ACORN2 or CRUTEM4. So it is encouraging to get a similar result from an independent source.