What is the mean temperature of Australia ?

ACORN2 is the latest BOM version of station data in Australia. You can plot the average temperature anomaly data for December using their trend tracker. Shown below the graph is the average temperature for the baseline period 1961-1990. The value is 27.2C. Ed Hawkins used the anomaly  data and simply added on this average temperature to produce this plot.

Ed’s alarmist plot

I have analysed ACORN1 data before so I downloaded the latest ACORN2 version and expected to be able to use the same software to check the result. However the format has changed dramatically. ACORN1 had daily Tmax and Tmin values in a single file, one for each station covering the full measurement period. These have now been split into two separate files Tmax and Tmin and the metadata is no longer directly available ( I found it in their Javascript!). Other changes which caused me difficulties were:

  1. Different timescales for Tmax and Tmin
  2. Missing values are now simply left blank whereas in ACORN1 they were set to -9999.9
  3. Sometimes ( eg. Merriden) there are up to 17 days of consecutive data simply missing .
  4. The start dates for the two files are quite often different which means for example we have only a tmin and a missing tmax .

Having resolved all these problems I then calculated the anomalies for December and the area averaged temperatures for Australia. I got good agreement for the temperature anomalies.

Compare ACORN2 with my calculation

However I got wildly different values for the Average December temperature. I got 23.8C as the area averaged temperature for Australia between for 1961-1990 instead of their 27.2C.

So obviously I must be wrong – or am I? I take the daily average  temperature to be (Tmax+Tmin)/2 and then average this value in each of the 12 months between 1961 and 1990 and for each of the 112 stations. My values then agree almost perfectly with various travel/tourist website averages for expected monthly temperatures. For example my monthly average values for Sydney are:

22.965, 23.093,  21.857,  19.548,  16.531,  14.016,  13.068,  14.176,  16.347,  18.671,  20.297,  22.175

Compared with https://www.holiday-weather.com/sydney/averages/

My area averaged value for Australia uses a triangulation method so I thought that maybe I had screwed that up, so to check I recalculated everything using the CRU 5×5 grid method, and I basically  got exactly the same result. So I think the difference is instead the following.

Everyone uses monthly average temperatures  – GHCN, Berkeley, CRU etc to calculate anomalies whereas I am using the daily values. So my 1961-1990 averages are based on the 30 year average daily temperature for each station during any particular month. These are then spatially integrated to give the normal  climatology. So why are they not the same as ACORN or GHCN?

My suspicion is that ACORN, GHCN, Berkeley etc. uses the monthly values over the 30 years period. So the minimum temperature is the lowest Average  temperature and NOT the lowest Minimum (night-time) temperature for a given month. This could explain why they get higher values.

 

My average of daily (Tmax+Tmin)/2
ACORN2 monthly averages

Finally here are my trends for December. The increased temperatures in December are mostly because the minimum (night-time) temperatures have been rising much faster. There is not much change in the average maximum temperatures up until 2018.

December temperatures from 1910 to 2018. Note how the minimum night-time temperatures have has risen fastest,

P.S. I have no time to check all this as I will soon be on a plane to Australia !

Posted in AGW, Australia, Climate Change | Tagged , | 5 Comments

Differences in 2019 temperatures

HadCRUT4.6 has released their annual temperature result for 2019 as 0.74C. They grid the station data (CRUTEM4) with SST data (HADSST3) in 5×5 degree bins and perform an area weighted: \cos (lat) global average. CRUTEM4 has 7680 stations which contribute to calculating this average. Although GHCN-V4 has more stations (17280) it is not clear to me that the coverage is really all that much better. CRUTEM4 is similar to the coverage of V3 but with some additional stations.

I downloaded the CRUTEM4 station data and calculated the global average with HadSST3 using my 3D averaging method (Spherical Triangulation). This is exactly the same data as that used by HadCRUT4.  Both results are compared below together with those using GHCN-V4 and HadSST3..

Comparison of methodology and station data used to calculate annual global temperatures.

The largest difference is that between spatial integration techniques. Exactly the same data produces a difference of 0.07C in the result for 2019. The reason for this is simply because spherical triangulation makes an implicit interpolation over both poles, whereas the traditional 5×5 lat/lon grid averages over just the occupied cells. This difference will  depend from year by year on just how much warmer high latitude stations warm as compared to those at lower latitudes. So in 2004 and 2015 there was little difference between the two as compared to say 2016.

Here is the 3D grid used to calculate the anomaly for December 2019.

The spherical triangulation grid for December 2019.

This shows how the triangulation connects together all station locations covering all the earth’s surface and as a result interpolates the average temperature from the 3 vertices across each triangular area. The coloured triangles show the relative increase in temperature anomalies relative to 1961-1990..

This procedure also gives very similar results to those of Cowtan & Way who instead use a kriging technique to interpolate into polar regions.

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2019 Annual Temperature

The December 2019 temperature was up by 0.12C from November at  1.01C relative to 1961-1990. This completes the annual global average temperature for 2019 making it the second warmest year at 0.86C. This is  just 0.015C cooler than 2016. All these values are calculated using GHCN-V4 and HadSST3 and spherical triangulation.

Here is the temperature distribution for December

Notice in particular the warmer than average temperatures across Australia, parts of the US and Northern Europe plus the ocean hot spot west of New Zealand.

Here are the final trend results for 2019

Recent Monthly temperature trends

 

and the Annual trends showing 2019 slightly cooler than 2016.

Annual average temperature anomalies

So 2019 ended with a warm December and Australia suffering terrible bush fires. I will be there in 3 weeks time and was planning to first visit the Blue Mountains.

Posted in Australia, climate science, Hadley, NOAA | Tagged , | 5 Comments