Wednesday, September 19, 2007

Our stats vs the Australian Bureau's

Previously, we had a look at Australia's maximum and minimum temperatures and concluded and found that why there were some irregularities before the second world war, both maximum and minimum temperatures in Australia had increased since.

We expected this similar pattern to go with what the Australian Bureau of Meteorology report. Keeping in mind, that we have not chosen the exact same weather stations as the Bureau in our analysis, so our results will not be exactly the same, we do however expect that similar trends will be shown.

Australia's maximum and minimum temperature anomalies over time according to the ABOM are shown here and here, and bot display similar increases in temperature.

We decided to look at the difference in these anomalies since 1942. We specifically chose 1942 because our weather station data is more complete since then and temperature records before this time are more variable and haphazard and might not be good for strong statistical analysis.

Differences between Gust of Hot Air maximum temperatures over time and the ABOM maximum temperatures over time produced no trend as shown below. In other words, our records for maximum temperature are backed up by the ABOM.

However when looking at the differences in minimum temperature for ABOM and GOHA, a surprising and strongly significant upward trend occurs:

So strong is the trend, that the difference between ABOM and GOHA is recorded at 0.0106 degrees per year or 0.7 degrees greater since 1942. This surprising result is shown on the graph below which shows the actual minimum anomalies that the ABOM and GOHA record:

Clearly both show an increasing trend, but note that in years prior to 1970 ABOM's minimum anomalies are less than our recorded minimums whilst after 1970 ABOM's minimum anomalies are greater than ours.

Once again I must stress, that both results have used different weather stations and we do not expect to get the same results. That being said it is strange to see such a strong and obvious contradictory trend between the two minimum temperature anomalies. Hence it is hard for one to say that the difference occurs just purely due to different weather stations. Keep in mind that our (GOHA) has only used non urban stations that have accurate time based temperature readings going back to at least 1970, and hence one could argue that these are more reliable stations.

I will shortly attempt to replicate ABOM's findings using the exact staions that they use. But next up we will look at some time based results.


philip said...

Jonathan, does either data set contain adjustments? Or are they raw data?

Jonathan Lowe said...

raw data, i believe

philip said...

I've come across references to BOM adjusting their data, but it doesn't seem to be documented anywhere. It would be strange if they don't given everyone else seems to, NOAA, HDCRU.