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DID YOU KNOW? PART 11

Welcome to Week 3 of 'Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks. 3 of this week's 5 'Did You Know's relate to rainfall. We have a Friday subscriber question and we have another cyclone question through the week as well.



Did You Know:

"Nitso I know you guys hate talking about climate change but can you tell me since we started keeping reliable rain records, are we seeing more or less rain in Northern Australia?"


ANSWER

You're right I (and the OCC team) do hate talking about climate change because the science has become politicised and I hate when politics corrupts science and the corruption has happened in both directions. However, the answer to your question scientifically is very clear. Since reliable record keeping in the early 1900's there is irrefutable proof that Northern Australia is WETTER on average today than it was 100 years ago and that trend continues.

Want to know more? well read on

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The increase in rainfall has been gradual over each 20 year period and of course there are always isolated years and isolated location exceptions to the general trends, but the trend is crystal clear for all to see at this link: http://www.bom.gov.au/jsp/ncc/climate_averages/decadal-rainfall/index.jsp?maptype=20&period=1996-2015&product=totals#maps with little variation in the 20 and 30 year trend data. Some of the most notable increases are located in: 1 - The North-Western Top End coastline and adjacent inland regions i.e. Darwin and its surrounding area. 2 - The Northern and Western parts of Cape York Peninsula i.e. Weipa, Bamaga and Aurukun 3 - The Southern Gulf Of Carpentaria i.e. Mornington Island, Burketown and Borroloola 4 - The Northern and Western Kimberley region places like Wyndham, Kunnunurra, Derby and Broome 5 - The Central and Eastern Pilbara (especially inland of the coastline) places like Karratha, Port Hedland, Marble Bar, and Newman

6 - The North-East Queensland coastline particularly areas like Mossman, Tully, Innsifail, Cardwell


Average rainfall over a 20 year period between 1911 and 1930


Compared with 1996 - 2015


WHAT DOES THIS MEAN FOR THE FUTURE?

If this trend continues: 1 - We will see a whole new area of land across Northern Australia's inland that will become much more open to various and new agricultural purposes.

2 - We will need to see greater flood mitigation across urban areas of Northern Australia. Dams like Townsville's may no longer offer the city the full protection it was built and designed for as increasing rainfalls test the design limitations of those flood mitigation infrastructures. 3 - We may need to offer incentives to relocate farmers from down south to new farmland further north (one thing I haven't mentioned is the reverse trend in South-West Australia - it is getting much drier - but that's a theory for another time)

4 - The north becomes less accessible by road/rail because flooding occurs on more days/year. Businesses will need to create and/or update their flood protocols. Is air travel a feasible means of supply/transfer of goods and people? Are those airports/airfields/helicopter bays flood proof? 5 - More mining and resource extraction days will be lost due to weather, have these been factored into future long term growth calculations for new mines?



WHY HAVE WE SEEN THESE INCREASES?

Well your guess is as good as mine and the best we can do is speculate, because even though we're dealing with 100 years of record keeping we have no idea how large these increases are relative to the longer term lifespan of the earth. But, this is my blog, so here is my own personal theory based on what I understand about the science.


For my theory to be valid though you must accept that we are living in a warming world in particular that our oceans are warming. If you don't accept this premise then my theory will make no sense to you and you will have to come up with your own theory. A general theoretical basis for linking global warming with increased rainfall intensity is that atmospheric water vapour capacity increases with temperature at the rate of approximately 7% per °K through the Clausius‐Clapeyron equation. With this increase, the rainfall intensity and extreme rainfall events 'should' increase in a warming world. So my idea behind the rainfall increases isn't that every year we see more rain, but due to the general trends of warming in our oceans the air is capable of holding more water thus we are seeing more intense periods of rain and more extreme rain events and that is distorting the data (or rather recreating the data). Now whether you agree or not is up to whether you believe the premise I put forward at the start of this paragraph. If you do the explanation is clear and makes sense, if you don't let me know on our socials what you believe can explain this rainfall trend.



My theory is that warming oceans help the air carry more moisture into Asutralia's north = >rainfall


The clearest increases in rainfall over the past 100 years have occurred on Westward and Northern facing coastlines and adjacent inland areas. It is quite obvious there that the monsoon's influence on these areas has INCREASED over time. But has the Monsoon trough/flow strengthened over the past 100 years? Anecdotally for the 20 that I've been watching it closely for; I can tell you it shows no trends and in fact I would go as far as to say it has possibly been a little weaker this past decade. So how else can we explain the increase in average rainfall? if not by the simple notion that vapour capacity increases with heat and so too most importantly does vapour supply when air is in contact with a warming moisture source. Our oceans are excellent conductors of heat and they are very efficient at storing heat and redistributing it. It makes sense to me that air in contact with the warming ocean becomes warmer and is therefore able to carry more moisture onto the land. That science is strong, but whether this is the reason for the rainfall trend really is all speculation without hard evidence. So my premise is not so much the land warming that has the biggest role to play, but in fact it's the ocean warming that is resulting in greater moisture availability as air in contact with it moistens and then comes onshore or travels inland due to widespread ascent from a monsoon trough or an inland trough.


I think the clearest evidence of this increase in moisture availability is in Western Australia's Pilbara and Kimberley. Rainfall in parts of those regions has nearly doubled since the early 1900's yet the temperature records indicate there has been little warming over land and some parts of the Pilbara are slightly cooler now than they were in the 1900's. So in my mind the answer has to be the moisture carrying capacity of air must have increased and the only way for it do that given that air is a poor conductor of heat and given that the region has not warmed is that it must be carrying more water into the region from the hotter oceans around it.



That begins this week's 'Did You Know's' You can become a subscriber and ask your own 'Did you know' question at https://join.ozcyclonechasers.com.au Becoming a subscriber helps us document Tropical Cyclones and gives you the most comprehensive cyclone intel out there during the season.


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