The Bureau of Meteorology is either incompetent or has knowingly allowed inaccurate data to garble the record.
My colleague Chris Gillham at http://www.waclimate.net/ has alerted me to growing problems with the BOM’s record for Diurnal Temperature Range (DTR). DTR is the difference between daytime temperature (Tmax) and night-time temperature (Tmin).
According to Dr Karl Braganza’s paper at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2004GL019998 , “an index of climate change” is that DTR should decrease as greenhouse gases accumulate. To oversimplify, greenhouse gases will enhance daytime temperature while at night greenhouse gases will slow down cooling. With increasing greenhouse gas concentration, daytime maxima are expected to increase, certainly, but the effect on night-time minima will be relatively greater. Thus, minimum temperatures will increase faster than maxima, and DTR will decrease. While Dr Braganza was referring to global values, Australia is a large dry continent where DTR should show up clearly.
We now have 111 years of temperature data in ACORN-SAT (Australian Climate Observation Reporting Network- Surface Air Temperatures). In this post I only use Acorn temperature data and corresponding rainfall data. Skeptics have been bagging Acorn ever since it was introduced, and for good reasons as you will see.
Figure 1 is straight from the Bureau’s climate time series page, and shows how DTR has varied over the years. There is a centred 15 year running mean overlaid.
Figure 1: Official plot of annual DTR
Melbourne, We Have A Problem… DTR has been increasing recently.
I have used BOM data to make plots that show this more clearly. First, Figure 2 shows annual DTR from 1910 to 2020 has no trend. It should be decreasing.
Figure 2: Annual DTR
There appears to be a distinct step up around 2000-2002.
Figure 3 shows the same data for the last 70 years, broken into two periods, from 1951 to 2000, and 2001 to 2020.
Figure 3: DTR since 1951
From 1951 to 2000, DTR behaves as it should, with a long term decrease. After 2000, DTR steps up well above expected values. The average from 1981-2000 is -0.12 C. From 2001-2020 the average is +0.35C. DTR suddenly increases by nearly 0.5C. Why?
DTR is very much governed by that other greenhouse gas, H2O. Dry days, months and years produce hot days and cooler nights; wet periods result in cooler than average days and warmer than average nights. This relationship is shown in Figure 4.
Figure 4: DTR anomalies plotted against rainfall anomalies- all years 1910-2020
As rainfall increases, DTR decreases. The effect is more marked in very wet (>100mm above average) and very dry (100mm or more below average) years.
Figure 5 shows time series of DTR (as in Figure 2) and rainfall. Rainfall has been inverted and scaled down by a factor of 250.
Figure 5: DTR and Inverted, Scaled Rainfall
There is close match between the two.
Using 10 year averages in Figure 6 makes the change after 2001 much clearer.
Figure 6: Decadal means of DTR and inverted, scaled rainfall
The 10 year average rainfall to 2020 is about the same as the 1961-1990 average (the period the BOM uses for calculating anomalies). The 10 year average DTR should be about the same value- not at a record level.
As DTR decrease due to greenhouse gas accumulation is caused by minimum temperatures increasing faster than maximum temperatures, Figure 7 shows 10 year averages of maxima and minima for all years to 2020.
Figure 7: 10 year running means of Tmax and Tmin
Tmax has clearly accelerated in the last 20 years, increasing much faster than Tmin.
This is NOT what should be happening: indeed it is the exact opposite of what greenhouse theory predicts.
Something happened to Australian maximum temperature recording or reporting early this century. I suspect that the BOM changed from using the highest one-minute average of temperatures recorded in Automatic Weather Systems to the current highest one-second value for the day becoming the reported maximum; or else the design of a significant number of AWS changed, with new, faster-responding probes replacing old ones.
I also suspect I know why this was allowed to happen and continue.
Warmer minimum temperatures at night and in winter are not very scary, but record high temperatures and heatwaves make headlines.
It would suit the Global Warming Enthusiasts in the Bureau for apparently rapidly rising maxima and ever higher records being broken to make headlines, frighten the public, put pressure on governments, and generally support The Narrative.
But someone forgot to tell the left hand what the right hand was doing.
The result is that they are now faced with a contradiction- Diurnal Temperature Range is not decreasing as it should.
The Bureau is either incompetent or has knowingly allowed inaccurate data to garble the record.
kenskingdom 