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DID YOU KNOW? PART 15

Welcome to 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. Tonight's question is about the Cairns region from one of our subscribers sent in via email.



Did You Know:

"Nitso, you recently mentioned that North-East Queensland is getting wetter over the past 100 years, but I have noticed more and more in the last few years that the computer models have Cairns missing out on the big rain almost all the time. Why is that happening? "


ANSWER

The short answer here is that computer models are getting better at what they do.

Want to know more? well read on

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Cairns and the Cairns Northern beaches have often been lumped into the rest of NE QLD's Wet Tropics coastline, but in reality Cairns is like the poorer cousin (still much better than Brownsville I might add in the rain department), but not in the same class as the rest of this coastline. It has taken computer models until the mid to late 2010's to realise what North Queensland locals have always known. Cairns doesn't get the big rains - those big rains happen about 40-50kms south of Cairns or about 100kms North of Cairns. Yes they get the occasional downpour absolutely so does the rest of coastal Queensland, and they get more rain than places like Townsville, Mackay and Rocky because they live in a more tropical airmass. But put Cairns against any of the heavy hitters around them and they fall to their knees and don't even contest. Let's do a quick comparison here of annual rainfall averages in the Cairns region we will start South to North (and we'll only use currently open stations)

Tully - 4083mm Innisfail - 3547mm Babinda - 4269mm Cairns City - 2271mm Cairns Airport - 1998mm White Cliff - 1700mm Port Douglas - 2034mm Mossman - 2400mm Daintree - 3771mm Cape Tribulation - 4056mm So why is the city of Cairns and its immediate northern neighbours in a relative rain shadow area? Meanwhile areas south of the city (and we're only talking from Edmonton southwards) and north of Port Douglas are getting drenched every year? Firstly if you look at the map at the top of the page, notice how there's a slight tilt to the coastline from NNW-SSE? That means that any stream showers from the SSE don't really come onshore from about Cairns to Port Douglas so the flow needs to be SE or ESE for Cairns to get a decent drop. For the area south of Cairns even a slight easterly tinge to the steering flow will allow showers to come onshore and the same applies to Cape Trib area. So Cairns starts at a disadvantage right there that costs them about 200-400mm a year. Secondly, and really let's be fair dinkum, most importantly; The mountain ranges around Cairns are pretty ordinary from a height perspective. We know that it's the mountain ranges that provide the violent source of uplift for places like Tully and Innisfail so let's compare sizes in the map below...

We know that the vast majority of lifting on the NE coast of Queensland occurs on ranges to a location's NW or West because the prevailing low level wind flow is a trade wind South-Easterly flow. If you look at the image above, the mountains to the north-west and west of Cairns (and its Northern Beaches) - 'The Kuranda Range' are supposed to be doing the heavy lifting of moisture but they are lightweights trying to do a heavyweight's job. That Kuranda Range is around 400-700m high and to be honest it's a pretty gentle road trip with an overall gentle slope. Yeah that range provides some lift to the air coming onto the coast, but the lift isn't in the same league as those ranges to the West of Tully (800 - 1200m high) or Babinda (1500m high) or the Daintree/Cape Trib area (1200m high)


So since mountains take millions of years to grow, this lack of rainfall for Cairns and its Northern beaches has been around for ages, why have we only started noticing this donut hole of much lower rainfall on computer modelling recently? The answer lies in the improvement of the models themselves. As computing power grows, we are able to input more data into weather forecast models. Over the past 10 years this has been extremely noticeable with errors that in 2010 existed over a three day forecast now exist out to 5 days and the three day forecast errors now are the errors we used to have at 24 hours lead time. Scientists can now input sophisticated elevation, gradient and even vegetation type into the data ingestion of computer models. They are able to use that data to figure out the rate of lift, how quickly clouds will condense, how much extra lift those ranges will provide and that helps us to fine tune things like the rain forecast for the next 5 days shown at the top of this page. The horizontal resolution and vertical resolution of models have become much finer in the past decade and we now have a clearer understanding of how the mountain ranges of the Tropical North affect various weather parameters. So the days of forecasting rainfall for Cairns that's in the same rain league as Innisfail, Daintree or Tully are well and truly over as scientists and mathematicians are able to program our models to produce a more accurate and localised forecast.

Please note for the large part of next week I will be busy writing a forecast for the 2020/2021 Cyclone Season and these 'Did You Know's will take a temporary back seat to that. I aim to have one final week of these starting on the 21st September. To get your hands on our cyclone outlook next week consider subscribing to our services and helping us do what we do best.


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