In 2017 I purchased from the Bureau of Meteorology (BOM) a slab of one minute data from 16 country Queensland stations with Automatic Weather Stations (AWS). One minute data is the temperature of the final second of every minute- 1,440 of them each day. I posted a few times about this, and now I return to it to check on some recent claims by the BOM.
They repeatedly assert that the difference between AWS temperatures and those measured by mercury thermometers (LIG) is less than 0.1 degree Celsius.
The one minute data, infuriatingly, is NOT published by the BOM for more than 72 hours, and is NOT used for any daily temperature recording. The AWS reads the temperature every second in each minute, but only the highest, lowest and final second temperatures are kept. The highest of those highest one second values, from 9:00 a.m. to 8:59 a.m. next day, becomes the maximum (Tmax) of the day, and the lowest (also 9:00 a.m. to 8:59 p.m.) becomes the minimum (Tmin). Tmax and Tmin are freely available, published at Climate Data Online (CDO). One minute data is available at a cost, and at the time of my purchase did not include one minute high and low values. Therefore, I can only compare daily data for final seconds of 1,440 minutes with the one highest and one lowest seconds, and can only estimate their time of recording. Grrr!
A further source of frustration is that daily temperatures at CDO for many places have not passed Quality Assurance checks more than six years later- but that doesn’t stop them from calculating monthly means for them, claiming the monthly means are quality controlled.
Therefore in this series of analyses I only use daily data that is quality controlled.
Thangool is a very small town about 120km south-west of Gladstone and has the airport for Biloela. Figure 1 shows the difference of the daily Tmin (one second value) minus the lowest one minute (final one second value) for February 2017.
Figure 1: Daily Minimum Difference
Note that no daily minimum value is more than 0.1C below the lowest one minute value on any day in February. No apparent issue there.
Figure 2: Daily Maximum Difference
Clearly the difference is greater for maximum temperatures. On 11 out of 28 days (39.3%) the difference between the maximum temperature and the highest temperature in the final second of any minute was greater than +0.1C. The greatest difference was on 19 February when Tmax was +0.7C higher. And that is at least, as I will show.
That is not comparing AWS readings with the old mercury LIG thermometers- we need parallel data for that, which the BOM is extremely reluctant to release.
However, we can draw some inferences.
Figure 3 is a plot of 1-minute temperature at Thangool Airport between 11:00 a.m. and 2:00 p.m. on 19 February 2017 as measured by the AWS, the maximum recorded by the AWS, and an illustration of what an LIG thermometer might have recorded. If we assume the AWS accurately simulates a mercury thermometer, I have shown how the mercury would have risen in steps: it would not have fallen after these steps until reset at 9:00 a.m. next day. The maximum was reached after 1:00 p.m. and was recorded by the AWS as 35.7C.
Figure 3: One minute and Maximum Temperature at Thangool
Note I show the “theoretical” temperature a mercury thermometer might have recorded as following the peaks of the one minute values. It may well have been higher than these steps, but below 35.7C- but we don’t know because those previous Tmax values were discarded. It is most likely near one of the two spikes between 1:30 and 2:00 p.m. In any case, Tmax of 35.7C is 0.7C above the highest one minute temperature of the day. But the change is supposed to have been up by 0.7C (at least) and back down again in one minute- it is not just one step up.
By the way, the BOM do quality checks on 1 second data, discarding any value that differs from those either side of it by more than 0.4C. So the AWS could record a temperature increase of 4 degrees in 10 seconds without causing any alert.
Figure 4 shows the likely times when the AWS would have measured 35.7C.
Figure 4: One minute and Maximum Temperature at Thangool, 1:30 p.m. to 2:00 p.m.
Figure 5 shows temperatures from 1:30 p.m. to 1:35 p.m.- the time when the minute to minute temperature change is less..
Figure 5: One minute and Maximum Temperature at Thangool, 1:30 p.m. to 1:35 p.m.
Tmax was probably in either of the minutes indicated. If it was at B (between 1:33 and 1:34) the difference was 0.7C. If it was at A (between 1:32 and 1:33) the difference was 0.8C. That’s why I say the real difference between highest 1 minute temperature and Tmax on any day is a minimum estimate. At any previous or later minute (such as the second peak at 1:52 p.m. in Figure 4) the difference would be much greater. The important difference is between Tmax and the next highest 1 minute temperature: that is in this case the previous minute.
BOM apologists assert that the difference between LIG and AWS is negligible. They also assert that each 1-second reading, because of the probe design, is really an average of the previous 40 to 80 seconds.
If that is true, then for the minute from 1:32:01 p.m. to 1:33:00 p.m. the running smoothed average of all the fluctuations between 1:31:01 and 1:33:00 rose from 34.9C to 35.7C then fell to 35C. Therefore the real (unsmoothed) temperature must have fluctuated very rapidly to values much higher and much lower in that 120 second period.
Further, could any human or animal detect such changes in less than one minute, and would it matter to anyone? For example, would aircraft preparing for take-off need such precision?
That is why we say that AWS temperature data is over-precise and inaccurate.
However, only parallel observations will prove whether AWS simulates LIG to within +/- 0.1C.
The next post will look at Sunshine Coast Airport.
Tags: adjustments, Australia, bom, Bureau of Meteorology, extremes, temperature
kenskingdom 
Leave a comment