Archive for December, 2020

Surface and Satellite Temperatures: 2020 Update

December 19, 2020

What’s gone wrong?

In November 2015 in my post “Why are Surface and Satellite temperatures Different?” and two follow up posts I showed that the difference is very largely due to rainfall.  You are urged to read these posts in full.

I repeat a key paragraph:

Firstly, surface temperatures are supposed to be different from atmospheric temperatures. Both are useful, both have limitations. The TLT is a metric of the temperature of the bulk of the atmosphere from the surface to several kilometres above the whole continent, in the realm of the greenhouse gases- useful for analysing any greenhouse signals and regional and global climate change. Surface temperature is a metric of temperature 1.5 metres above the ground at 104 ACORN-SAT locations around Australia, area averaged across the continent- useful for describing and predicting weather conditions as they relate to such things as human comfort, crop and stock needs, and bushfire behaviour.

Here are three plots from my 2015 post.

Fig.1:  Tmax and Scaled, Inverted Rain (from Figure 7 from my 2015 post)

Dry periods are hotter, wet periods are cooler.

Fig. 2:  Surface maxima minus atmospheric temperatures and inverted rain (Figure 10 from my 2015 post)

Fig. 3:  Temperature difference compared with rainfall (from Figure 12)

The difference between Australian surface and satellite temperatures was very largely explained by rainfall. However, after five more years of satellite and surface data there is a problem (and I thank Chris Gillham for alerting me to this.)

Fig. 4:  Surface maxima minus atmospheric temperatures and inverted rain

Since about 2013 the difference between surface Tmax and satellite data has visibly increased above rainfall.

Now I have a confession to make.

In previous analyses I used running 12 month means in calculating correlation.  This can lead to inaccuracy as the means can be highly auto-correlated.  From now on I will use annual data, either with calendar years or, as in this post, annual means from December to November (so that summer months and most of the northern Wet season are included in the one datapoint).

I downloaded data from:

Monthly maxima

Monthly rainfall

Temperature of the Lower Troposphere- Australia Land

As with my 2015 post, I have recalculated rainfall and maxima from 1981-2010 means to match UAH.

In the past five years there have been changes:  the Australian maximum temperature record is now based on ACORN-SAT Version 2 instead of Version 1, including new adjustments and some station changes.  No doubt UAH has been tweaked a little as well.

However correlation between the difference between the surface maxima as recorded by Acorn and temperature of the lower troposphere (TLT) as recorded by UAH, and rainfall, has decreased.

Fig. 5:  Temperature difference compared with rainfall

The close connection between the temperature differences and rainfall became broken from about 2005, as can be seen in Figure 4.  Another step up occurred in 2013.

So there appear to be three distinct periods: 1979 to 2004, 2005 to 2012, and after 2013.  Plotting temperature differences against rainfall allows us to compare each period.

 Fig. 6:  Temperature difference compared with rainfall

From 1979 to 2004 and from 2005 to 2012 slopes are identical at 0.4 degrees lower temperature for each 10 mm of rain, with 76% and 93% of temperature variance explained by rainfall. The trend lines are parallel but offset by 0.26 degrees indicating either atmospheric temperatures have reduced or surface maxima have increased in the middle period.  From 2013 the relationship is different with closer to 0.5 degrees lower temperature per 10mm of rainfall, with rainfall explaining 78% of the variance.  Again, the offset shows either UAH has suddenly decreased or Acorn has suddenly increased.

Conclusion:  Something has gone wrong with the relationship between rainfall and temperature in Australia.  In recent years, and certainly since 2013, the surface- atmospheric temperature difference has rapidly increased relative to rainfall.  That should not have happened.

My suspicion is that Acorn’s maxima are to blame.   Figure 1 showed Acorn appeared to step up relative to rainfall in 2001 or 2002, or perhaps earlier in 1997, and again in 2013.  There can be no meteorological explanation for this.

The accuracy, and therefore usefulness, of the ACORN-SAT adjusted temperature record will be the topic of my next post.

Stay tuned.

Acorn Mish-Mash Part 2: Scone

December 13, 2020

In Part 1 we saw that Scone in NSW has the fastest increase in 120 month mean maximum temperatures of all 112 Acorn stations.  The Station Catalogue shows a recent photo of the site with long grass at least 60cm high surrounding the screen- not a very good advertisement for compliance with siting specifications.

Fig. 1:  BOM photograph of Scone site

However the Metadata for this site reveals how much the site has changed.  Before 2005 the screen was close to the runway and a service road, and there was considerable earthworks nearby in 2001.  By April 2005 the screen had been moved to its current location.  In 2012 the grass was 60cm high as in the above photo, and was whipper-snipped during the annual inspection.  In 2015 and 2019 the grass around the instruments was “sparse” as weed control had been used i.e. it had been sprayed out with herbicide.  Temperature data for the airport may be questionable based solely on site information.

The Acorn record has been created by merging data from 01/01/1995 to 31/12/1995 from the present site at the airport with that of a Soil Conservation Research Station (SCS) 10 km away from 1965 to 1994.

Data before 1975 were adjusted downwards because of a change or repair to the screen.

Fig. 2:  Adjustments to annual data at Scone

This resulted in an increase in trend of +0.43C per 100 years.

Fig. 3:  Scone raw and Acorn annual data

However, comparison with the average of the Bureau’s nominated neighbouring stations used to make this adjustment shows the adjustment was much too great.  While the raw record from 1965 to 1973 shows Scone warming 0.29C per 100 years faster than the neighbours, the Acorn record is warming at 1.46C per 100 years- much faster than the neighbours.

Fig. 4:  Difference between Scone and average of neighbours, 1965 – 1973

While that alone is enough to cast doubt on the Acorn adjustments, an analysis of the relationship between maxima and rainfall shows that little reliance can be placed on temperature data before 1974, and after 1995.

At every well maintained site there is a relationship between maximum temperature and rainfall: periods of dry weather are hotter and periods of wet weather are cooler, because of the effects of more or less cloud cover, evaporation and transpiration.  (Wind direction will also have an influence, especially in dry seasons.)  At a well maintained station much more than half of temperature variation is due to rainfall. Therefore, if this relationship varies markedly we can deduce that either temperature or rainfall data are questionable.  This is shown by Dr Bill Johnston at his website, BomWatch, which I urge you to visit, and my analysis is loosely based on his methods.

I calculated 12 month running means of temperature and rainfall for the Airport and the Soil Conservation (SCS) sites.  Figures 5 to 7 show 12 month average temperature plotted against 12 month average rainfall for the three periods, 1965 – 1973 (in which Acorn temperatures are adjusted), 1974 – 1994 (when Acorn and raw are the same), and 1995 – 2018 (when the temperature record switches from the SCS to the airport).

Figure 5:  Scone adjusted maxima plotted against local rainfall

That is a very poor relationship: either temperature data or rainfall data are unreliable.

Figure 6:  Scone unadjusted maxima plotted against local rainfall

Here, more than half of temperature variation can be explained by rainfall.  It is not brilliant, but much better than what comes before and after.

Figure 7:  Scone Airport maxima plotted against local rainfall

While not as bad as pre-1974, less than 30% of temperature variation is explained by rainfall.  Either temperature or rainfall data, or both, are dubious.  Considering the site history and varying vegetation, this is not surprising.

It is unlikely that Acorn is a true record of temperatures at this location. Scone Acorn data are not reliable and should not be included in regional and national climate analysis.