Posts Tagged ‘DTR’

DTR, Cloud, and Rainfall

September 19, 2016

In my last brief post I showed how Diurnal Temperature Range is related to rainfall in Northern and Southern Australia in Northern and Southern wet seasons (which correspond roughly to summer and winter).

In this post I show the relationship between DTR and daytime cloud, and between rainfall and daytime cloud, and something very peculiar about South-Western Australia.

All data are taken straight from the Bureau’s Climate Change Time Series page.

DTR is affected by rainfall through Tmax being cooled by cloud albedo, evaporation and transpiration, and Tmin warmed by night cloud and humidity.  There must be a relationship between clouds and rain, although it is (rarely) possible to have rain falling from a clear sky with no visible cloud.  Rain is easily measured in standard rain gauges.  Cloud is calculated by trained observers, and we only have data for 9 a.m., 3 p.m., and daytime cloud.  The data give no indication of cloud type, thickness, or altitude, just amount of sky covered (in oktas, or eighths).

Here I show scatterplots for Australia as a whole annually, and for Northern, South-Eastern, and South-Western Australia in summer and winter.  I calculate both rainfall and cloud as percentage differences from their means.

Fig. 1:  DTR vs Rain for Australia annually:

dtr-vs-rain-oz-ann

Fig. 2:  DTR vs Cloud for Australia annually:

dtr-vs-cloud-oz-ann

Notice much better correlation between DTR and Cloud.

Now let’s look at the relationship between rainfall and daytime cloud.

Fig. 3:  Percentage difference in Rainfall vs percentage difference in Cloud for Australia annually:

rain-v-cloud-oz-ann

Note a 10% increase in cloud cover could be expected to be associated with a 25% increase in rainfall.

Fig. 4: Percentage difference in Rainfall vs percentage difference in Cloud North Australian summers:

rain-v-cloud-n-oz-summ

Fig. 5: Percentage difference in Rainfall vs percentage difference in Cloud North Australian winters:

Note how rainfall in the North Australian dry season varies proportionally more, but has a slightly lower correlation (>0.8 vs 0.9).

Fig. 6: Percentage difference in Rainfall vs percentage difference in Cloud South-East Australian summers:

rain-v-cloud-se-oz-summ

Note the much greater effect of cloud on rainfall in the southern dry season.

Fig. 7: Percentage difference in Rainfall vs percentage difference in Cloud South-East Australian winters:

rain-v-cloud-se-oz-wint

Now, get ready for a surprise.

Fig. 8: Percentage difference in Rainfall vs percentage difference in Cloud South-West Australian summers:

rain-v-cloud-sw-oz-summ

Fig. 9: Percentage difference in Rainfall vs percentage difference in Cloud South-West Australian winters:

rain-v-cloud-sw-oz-wint

What’s going on in the south-west?

Here’s how DTR compares:

Fig. 10:  DTR vs percentage difference in rainfall: South-west Australia

dtr-vs-rain-sw-oz-ann

Similar relationship to everywhere else.

Fig. 11:  DTR vs percentage difference in cloud cover: South-west Australia

dtr-vs-cloud-sw-oz-ann

And this graph clearly shows the relationship between rain and cloud is closer in the wet seasons, but also clearly shows that South-west Australia is an extreme outlier.

Fig. 12:  R-squared comparison between rain and cloud in wet and dry seasons

chart-seasonal-r2

Why the huge difference?  There is no relationship between cloud and rain in south-west Australia, unlike everywhere else.  The South-West has seen a marked decline in rainfall since the late 1960s, but an increase in cloud cover.  It seems counter intuitive, but there you go.

Any suggestions are welcome.

Australian DTR – the Regional Context

January 12, 2014

I’ve been banging on about DTR in Australia for a while, showing that as an indicator of greenhouse warming, decreasing DTR trend has been lacking from Australian records for some time, such that the trend is flat since 1947.

Update:

DTR is Diurnal Temperature Range, the difference between Minimum and Maximum temperature daily.  Several previous posts discuss this.  Greenhouse gases slow back radiation, and thus night time temperatures are expected to be warmer than normal, and minima are expected to increase faster than maxima, so DTR should decrease.

Fig.1: Australian DTR anomalies, 1947 – 2013dtr1947-2013

I’ll now show what is happening on a regional basis.  This map shows the main meteorological regions of Australia.

Fig. 2: The regions.summer1213  regions

The main difference is between Northern Australia and Southern Australia.

Fig.3:  Northern Australian DTR anomalies, 1971 – 2013dtr nth oz 71-2013

43 years of flat trend in DTR!

Fig.4: Southern Australian DTR anomalies, 1938 – 2013dtr sth oz

76 years!

Fig. 5:  South-Western Australian DTR anomalies, 1941 – 2013dtr sw aus

73 years.  But the real eye opener is South Eastern Australia:

Fig. 6: South-Eastern Australian DTR anomalies, 1934 – 2013dtr se aus

That’s right, in South-East Australia, the DTR trend has been flat for 80 years!

Decreasing DTR as a “fingerprint” of greenhouse warming was championed by the 2004 paper by Dr Karl Braganza et.al,

“Diurnal temperature range as an index of global climate change during the twentieth century” Karl Braganza, School of Mathematical Sciences, Monash University, Clayton, Victoria, Australia; David J. Karoly, School of Meteorology, University of Oklahoma, Norman, Oklahoma, USA; J. M. Arblaster, National Center for Atmospheric Research (NCAR), Boulder, Colorado, USA

Braganza et. al. analysed global DTR from 1951 to 2000, finding a significant decline of ~0.4 degrees C.  If we compare Australian data for the same period we find this is corroborated.

Fig. 7:  Australian DTR anomalies 1951 – 2000dtr oz 51-2000

The observed decrease over this period is ~0.35  – 0.4 C.

With the benefit of an extra 13 years of data, we can check whether this continues to be the case.

Fig. 8:  Australian DTR anomalies 1951 – 2013dtr oz 51-2013

What a difference a few years make.