Hello and thanks for joining us at the Bureau of Meteorology for a very special
Facebook Live session. I'm Adam Morgan I'm a Meteorologist here
at the Bureau of Meteorology National Operations Center which is behind us
here. I'm joined by Grace Legge. Hi Grace. Hey Adam.
Thanks for joining us, now Grace you're also Meteorologist here in the
National Operations Center as well aren't you?
Yep I work on the extreme weather desk down here. So looking at all the
types of high-impact weather that affects Australia. Now we're coming to
the end of October and we're about to approach a very special day in their
meteorological calendar in Australia aren't we. So we're just about at cyclone
season. So today coming to the end of cyclone awareness month around Australia,
which is the month of October. So hopefully if you live in a cyclone prone
area of Australia you've made all your preparations during the month of October.
That's clearing your tying down all loose items that might become airborne during a
cyclone, doing all those necessary preparations. And as we come to the end
of the month we're here today to answer all your questions about tropical
cyclones. Now Grace you've worked in Darwin haven't you? I've worked up there
for a number of years, so you know a lot about tropical cyclones. I do, I very
much enjoy studying cyclones so I worked in the Darwin office and in the tropical
cyclone warning center there. And I actually helped out to monitor the Brisbane
activations as well. Yeah right now I also worked in Darwin for a year, I didn't
work on the cyclone desk but I certainly was surrounded by all of our expert
cyclone forecasters like Grace, so certainly got to know a lot about the
inner workings of how cyclone forecasts are done so hopefully I can
help this out as well today. So for the next 10 or 15 minutes. We'd love you to
ask all of the questions about cyclones. I'm going to throw the first one to you Grace.
In coming to end of cyclone awareness month towards a very special date in
the meteorological calendar, 'When do we see cyclones in the Australian region?'
Well we have our official start tomorrow so first of November is the start of
cyclone season, and then that continues all the way to the end of April. To be fair
cyclones are pretty rare in November they do happen every now and then but
most of the bulk of our cyclones will be sort of end of December and into next
year. Generally where we see the monsoon hit as well. We do have had out cyclones
out of season though that has happened we actually had a very famous one that
happened pretty much in the 1st of July just near the Solomon Islands. Very rare
to happen but can sometimes occur. Yeah where we seen the cyclones during that
cyclone season, so November to April is a little bit lumpy isn't it, really it's
often very slow to start, as Grace said we really need that monsoon to
arrive from the northern hemisphere it kind of moves down across the equator
and into northern Australia once we get that really active monsoon activity
that's when we're more likely to see tropical cyclones form. And we can see
the peak around February—March. Really isn't that so, but so it can happen any time
and then during the dry season months of the winter months they're much less
likely to see cyclones in Australia. Yeah it is very rare as I said just like the one
that formed to sort of throw us a little bit. Now 'Where are the cyclone prone
regions in Australia Grace?', 'Where do we normally see cyclones most in Australia?'
We see them more in the tropical regions you need nice warm waters to get a
cyclone going, so they're more they work air through those areas so we have three
areas that we look after we've got WA area Northern Territory and also the
Queensland area. The area most likely to see it is actually the Western Australia
area. It does extend quite far into the ocean but it's also the most likely
point you'll actually see landfall from a cyclone. So it's about Exmouth to
Broome area. And has actually seen the most landfall of any cyclones. So we need quite a bit
of ocean water for a cyclone to form don't we so there's a good reason why that
Northwest WA region is where we'd see quite a lot of cyclones for most likely
in Australia. Because it's pretty vast body of water off the coast there. It's
actually cyclones can't form within a few degrees either side of the equator
so when you think about the northern coastline and Australia, just north of
the Top End there there's actually not a huge body of water for cyclones to form.
So we do see cyclones in that region, but not too often and we see more in the
Coral Sea and then even more off northwest coast of Australia. And finally
Grace my last one to you is 'What are the basics of a tropical cyclone?' What are the
nuts and bolts of one? Well tropical cyclones defined is
basically that's a low-pressure system to me that a category it needs to be
over warm waters have that warm core of getting that energy, and it has to
have gale force winds so winds at least 63 kilometres an hour wrapping halfway
around the system for them to form though it's a pretty
important environment there's various things that it needs and needs that warm
sea surface temperature to gain that energy from. It needs to have lots of
moisture in it to allow thunderstorms to form near its core, and it also needs
what's called little wind shear—so no change in wind speeds with depth to
allow it to basically stack itself properly so it can have those
thunderstorms forming and allowing that pressure to go up and out which allows
that sort of low pressure system to continue to evolve. Yeah you can imagine
though if any of you are watching our a AskBOM Live a couple of weeks ago that
severe thunderstorms when I was talking with Dean Narramore, Dean was describing how
wind shear which is that change in direction or speed of wind with height
in the atmosphere actually a good thing for thunderstorms. For cyclones is not so
good because if you get stronger winds blowing aloft versus down below it can
kind of slice off the top so we actually want very little wind shear in a cyclone
situation so as Grace said to allow that really good vertical stacking that
kind of air motion in-up-and-out at the top. So Grace let's go to one from Alan, this
goes back to where we get cyclones and don't get cyclones. So Alan asks 'Why do we
not get cyclones in the south?', so New South Wales. Thanks Alan for your question.
It's really just that it's not warm enough you need to sea surface
temperatures around 26.5 degrees through quite a depth to
allow that energy to pull in and build. So if the cyclone starts moving into
those colder air temperatures it loses its warm core in its structure in
a sense. It becomes extra tropical which is when you'll see instead of having
those not are those strong winds near the center and that's really tight
gradient there, sort of spreads it out a little more you don't see as intense a
winds you generally see them as a larger area because it no longer has that
central core it's just too cold for it to maintain. I guess that's a key
difference between a tropical cyclone and a low-pressure system that we might
see more in the southern parts of Australia, I mean they're both low
pressure systems when it comes to the crunch but tropical cyclones are what we
call warm core they need the warm waters the temperatures towards the middle of
the low pressure systems are warmer than the outside which is the reverse of low
pressure systems that we see are further to the south. So certainly the
mechanisms that formed tropical cyclones are just a fraction different to the
types of light pressure systems that we normally see say in New South Wales. Okay.
Let's let's see this one from Cameron from Port Hedland. This is a really good
place to be asking a question from Cameron certainly we know a lot about
tropical cyclones he is it right in that area that we talked about before on the
northwest coast there. Cameron said 'What's the next cyclone name?', Do you know
that one Grace? I know the letter, I believe we are up to 'O' so this is this actually leads us to a very interesting topic.
It's certainly one that I get asked most frequently about
cyclones. 'How do cyclones get their names?', but before we get there I'll
answer Cameron's question. The answer is Owen! We're up to O and up to Owen, so
it's a male name this one and we're up to Owen and the next one will be 'P' and then
that'll be a female name in this list. But essentially how we named cyclones we
actually have five lists of names that go alphabetically from A to Z. And they
cover all of our three tropical cyclone warning regions Perth, Darwin and Brisbane.
And we generally just cycle through the names alphabetically from
top to bottom and then we restart from the first list again, so we finished off
last season at 'N', Nora, the last one we finished
last year in the Australian region and then now we're up to 'O'.
so this one will be Owen. Now Grace when did we do we ever retire names or
what happens if we have a notable tropical cyclone, like Cyclone Marcia?
Or Cyclone Debbie or something like? That is what we need to do is to retire
the name so we never have another Cyclone Debbie in our region
again or at least not named by us. Not going to have a Cyclone Tracy either. Because there are such notable systems that carry a lot of sort of damaging
with them they're impacted a lot of people so it matters a lot so we don't
really want that connotation and another system that may not actually cause any
impact might just be sitting over the ocean. We don't want people to sort of be
concerned about that level of impact we want to treat each cyclone is their own
system. Certainly know so it also aids to reduce confusion so you're not getting one
cyclone mixed up with a new one an old one. We start
with a new one. And also if we get to a name on the list that might be a notable
name of a politician or a notable figure in the community at the time someone in
the news, they will often skip that name and just move to the next one on the
list. Now you might picked up in what Grace
was saying a minute ago there is one situation where we may not use a name
from one of those on the Australian list and that's when a cyclone originates outside
the Australian region. Can you give us an example of that Grace can you think of any off the top of your head?
Yeah, Cyclone Yasi is the one that always
springs to mind, so it started in the Fijian region so they named it, to avoid
confusion we don't rename it when it comes into our region we just use the
name that's already been given. So you know it's the same system that move
through there's no confusion about if it's a different one that's formed.
Yeah that's right,Cyclone Yasi is a really good example. Okay Grace that question that we just talked about was the number
one question I get asked, this is probably the number two question I get
asked. 'Are cyclones', this is from Ingrid, so thanks Ingrid. 'Are cyclones exactly the
same as typhoons and hurricanes?'. This is your pet topic Grace. This is my pet topic it
comes out a lot. They are exactly the same in the way that they're formed and the
systems they are. It's just slightly different to what we call them so we
always call ours a tropical cyclone from a category three and above
when you call them a severe tropical cyclone. The other regions such as
America uses hurricane or Japan that uses typhoon, for what would be their
category one and two they actually call them tropical storms. And then from
category three and above they'll be named typhoon or hurricane. But they're
all exactly the same systems the only real difference is in the northern
hemisphere they spin the opposite way. That's right, so the only difference is
in the direction in which the winds go but typhoons, hurricanes, cyclones they're
all the same thing. But tornadoes and cyclones are very different things and
that's often the question I get asked as well, so they're different but yeah
said of a tornado sorry they're typhoons hurricanes and cyclones certainly all the
same thing. Okay so this is a little bit more in detail perhaps when were talking
about the systems off New South Wales things like that. This is a question from
Josh. So Josh asks 'With weather events such as the East Coast lows that
wreaked havoc across Queensland, Victoria and New South Wales what does it
actually take to be classified as a cyclone and how far south have we seen
shouldn't we expect to see cyclones in the future?'. The futures a bit of a hard
one, but yeah they have to be warm cores so they need those warm temperatures so
you're not only looking at Brisbane the Northern Territory and WA are really
the only places that will see them. It's really about where those really warm
ocean temperatures are we we think about where we more likely to see those ocean
temperatures around 26.5 degrees or above and it
really is in the tropical regions of the world. That being said at the moment we
only see them in those regions but if you go up so like Japan, Japan can see
their typhoons up any of their coast and it's just that their temperatures are
warmer further north so that's why they're able to see them. For us it's
just going to have to be those regions that are going to see it and to classify it
as I said needs that warm core getting that energy warmer in the center and it
needs those gale force winds around at least half of the system for at least
six hours as well. There is a very technical definition there of when we
would name a cyclone and it's based on how long the winds have blown and how far
around the center we do see those gale force winds and then how we escalate
through the categories beyond that is according to specific intervals of wind
speeds, isn't it Grace. So it's all about the maximum wind around the center for
what category it'll get to, so you're going from gale-storm-force and then
hurricane force winds. So that's what they need around the center to continue
up that classification. Doesn't take into account any other impacts we're only
looking at the maximum winds for the categories that we give them. And
certainly the higher the category is a really good indication of the, I guess
the escalation of the types of impacts that you could expect with the tropical
cyclone and so they category one and category to be call them tropical
cyclones. And then category three, four and five that's when we add their name
severe to the front so it comes severe tropical cyclones in the Australian
region. Okay so this is a good one from Jasmine actually it's a bit tricky so
'The Atlantic Basin name subtropical storms why doesn't the BOM name East
Coast Lows?'. That's a very good question. Well the New South Wales office have a very extra
strict definition what is an East Coast Low. It's actually like it's almost the
strictest definition for a tropical cyclone but there it hasn't been quite
as much done exactly when you would name the system,
so it's a lot more it's sort of where it forms and its exact impacts and less
than just the base meteorology. So because we probably don't have a strict
definition it's a bit harder to name them and sort of move on also they may
not last as long. That's what I was going to go with, I reckon the
storms that we can name; names are useful when we can track the one system that
lasts for a long time. East Coast Lows are often quite transient only around
for a day or two they don't last too long they might form on the spot and
then kind of dissipate where they are, they may track longer and I think one of
the most more significant ones we had two or three years ago in New South
Wales actually tracked all the way kind of almost down the Queensland coast in
towards Sydney and then all the way down towards Tasmania, that was when we had
swimming pools kind of washing off people's backyards there along the
foreshore in Sydney there. But I'm thinking other areas of the world and I
know that the UK Met office for example just a few years ago began to name deep
low pressure systems in the in the North Atlantic Ocean, similarly to the way that
we in the US National Weather Service named tropical cyclones. They do it with
low pressure systems there's no reason we couldn't do that it's just that we
don't do that by tradition and so they probably it's that longevity of systems
that probably comes into it. Your also not going to get more than one East Coast low at a time
so it's not really confusing about which system we're talking about whereas you
can have more than one cyclone so we want to be able to make sure there's no
mix-up with those systems. Yes thank you. Now Helena has a good question.
'Can you have cyclones heading towards each other?' You can actually. There's a very
interesting effect that happens when cyclones get in each near each other
which is called the 'Fujiwhara effect' it basically is that they actually interact with
each other and spin around each other. Which is very interesting. Like doing a spiral dance.
independently spiral that have been spiraling around each other. Yeah so it's
quite an interesting one so they can move towards each other they might have
different other systems that are pulling them that way and then they'll actually
interact so they normally won't hit each other but they'll often go around each
other. So it's quite an interesting effect to almost end up in opposite ends
that they started with. That's right now and there's also the situations
where we can have cyclones forming on opposite sides of the Equator so there's
a certain type of wave in the atmosphere and when I talk about waves
think about waves on the ocean will be that the atmosphere is a fluid, it's air, like
water in the sense that's a fluid and we can have waves that progress through the
atmosphere you can't see them with your eyes but what we can see on satellite is
the effect of these waves and some one of those effects for waves that move
from the east to the west, along the equator in the tropics is a Kelvin wave
and sometimes when we see that we see cyclones form exactly the same location
but on opposite sides of the equator and kind of move in sync with
each other. Yeah we've seen - yeah - forms so they're spinning the opposite
ways they look quite interesting either side of the equator in the other
almost moved together along that wave. If you track them on satellite imagery you kind
of see them just moving in parallel on opposite sides of the equator. Let's move
to the next one now, that's that's a good one!
It's bit tricky and I don't know if I you are I have the answer to this. So
Brian asks 'Why are the different scales used to measure cyclones and hurricanes
between the USA and Australia and why not have a global standard?'. Yeah I don't
actually don't know why we don't have that we do we technically do the world
med organisation has given us the categories in which we jump so do we go
from you know category one to two and the other regions actually do have at
least Japan does and Fiji do. America has technically the same scale for jumping
but there's a slight difference with the way they do it that's right it's how
they measure the mean wind, so it's always based on those sustained winds
not the gusts and the sustained winds are one minute averages for the US and
10 minute averages for us.Yeah so we just measure the wind speed
slightly in a different way so, that probably equates for some of the
differences between scales, it's yeah the naming we just name them differently in parts of the world.
Now Grace I've just been given the windup so I think I've lost track of
time so we've had lots of fun answering all your questions about cyclones today.
So as the as we come to the end of 'Cyclone Awareness Month' in October if
you do live in the cyclone prone parts of Australia, so across the other parts
of Western Australia, Northern Territory and Queensland make sure you do use this
opportunity to make all these cyclone preparations before the season starts
make sure you have your cyclone kit together and know what you'll do when a
cyclone hits, if it hits me you say it takes one cyclone to make impact near
where you are to make it a bad cyclone season and make sure you stay in touch
with emergency services and the BOM all throughout cyclone season for all the
latest forecasts, warnings and information. So thanks for joining me
Grace. Thanks for having me. And I will join you very soon for another AskBOM Live.