Glenn Dunks: Everybody. I am Glenn Dunks, Senior Communications and Engagement advisor here in the Hydrology and Climate Science team of Water and catchments at the Department of Environment Land Water and Planning. I would like to begin today by acknowledging the traditional owners of the country on which we meet and pay my respects to their Elders past, present, and emerging. The team here runs the Victorian Water and Climate Initiative supporting research into the impacts of climate change and climate variability on Victoria's Water Resources. The initiative is focused on the priority research needs of the water sector, working with the Victorian water sector on how to best apply that science. Research through the initiative is largely being done by the University of Melbourne, the Bureau of Meteorology, and the CSIRO. We are very appreciative of all the work of the scientists and the support of their organisations in undertaking this important work. We also appreciate the time and energy that is required to prepare for events such as these, as well as those who have joined us today to listen.

Glenn: This webinar series will offer deeper insight into the varied and difficult research projects being undertaken in the lead up to the release of the synthesis report later this year. We are also keen to use these webinars to get your feedback and input into opportunities to put this research into practice. If you want to make sure you don't miss out on future webinars, please subscribe to our newsletter at That is And that email address will appear at the end of today's presentation too, in case you missed that.

After beginning recently with Michelle Ho of the CSIRO a few weeks ago, we are in the process of lining up several more exciting presentations to deliver to you over the coming months. Michelle's webinar on a systems approach to understanding climate change impacts is now available at As will future water presentations.

Last time, our guest will remain today to field questions that you have relating to their presentation in a Q and A section at the end of the presentation. Feel free to type your questions into the Q and A panel. And speaking of today's guest, I would now like to welcome them to the virtual stage. Acacia Pepler is a research scientist in the climate research section at the Bureau of Meteorology. She's interested in all aspects of Australian climate change and variability with a particular focus on extreme events. Acacia will be talking to us today about which weather systems cause Victoria's rainfall and how they are changing. I will just send Acacia live and let her go to her presentation. Thank you very much, Acacia.

Acacia Pepler: All right. Thanks everybody for tuning in this afternoon. Today I'm going to talk about a new data set we've developed as part of the [inaudible], which looks at the different types of weather systems that influence Victorian rainfall. And I'm going to highlight some of the ways that we are using the style set to better understand both how rainfall in Victoria varies across the state and throughout the year, but also how it's been changing over the last sort of 40 years or so.

When I talk about weather systems, there are a bunch of different things I could be talking about. But I'm mostly going to focus on 3 different types of weather systems today. The first 2 are here shown on the left from snippets of synoptic charts in May this year. And they'll be pretty familiar to anybody who looks at the weather charts from day to day as the sorts of things that cause a lot of the heavy rainfall around Australia.

On the top left, we have a low-pressure system or a cyclone. And what that is, it's an area where the atmospheric pressure is lower than the surrounding environment. And so around that low we tend to have the winds rotate clockwise into the cyclone, air rises, and we can have quite widespread rain. The second type of system I going to talk about is a cold front, which there's an example of in the bottom left. On a map, you can see a blue line with little blue triangles says when the cold front is coming through. And that's an area where cold air is moving to replace warm air. And so, as a cold front passes through, we see a shift from sort of warmer northerly winds to cooler southerly winds. And again, we can also see rainfall. And as you can see from the size of Victoria and these figures, both cyclones and cold fronts can be very large, and they can also last for a reasonable period of time. Fronts often move quite fast through the state while a low-pressure system sometimes can linger for a bit longer.

The third type of weather system I'm going to talk about is something very, very different in scale. This plot here down in the bottom right is just Melbourne radar from a day back in 2017 that had some quite severe thunderstorms. And so, when we think about a thunderstorm, instead of being days long and 1,000s of kilometres wide, instead a couple of kilometres, maybe 10s of kilometres wide, and they'll last for sometimes 30 minutes, sometimes a few hours. They're very different in scale and space and time. But when we have the right environmental conditions, you can often have quite a lot of thunderstorms. Maybe there'll be thunderstorms in lots of different places around the state. Here you can see on the radar that a few different suburbs of Melbourne are having thunderstorms at this particular time, but others aren't.

But you can also have a series of thunderstorms developing through the day. Thunderstorms being so small are much harder to see on the synoptic chart like we see the other weather systems, but there are particular atmospheric conditions that make the atmosphere much more predisposed for thunderstorms to occur. And that's what forecasters look at when they put out a thunderstorm warning for a day where you'll see half of the state coloured in yellow saying they're expecting thunderstorms. And it's also the sorts of environmental conditions we've been using in this study to look at when thunderstorms are likely to be occurring even though maybe not that entire area has a thunderstorm on one day.

But what's interesting about this study is that you know, can look at all of these systems independently, but that's not actually how they work. They can often happen at the same time. You can see even in this top left; this is a cyclone. But within that cyclone there are a couple of areas that have cold fronts. There's even a red line which is called a warm front. Well, I'm not going to talk about that in as much detail.

And cyclones can even have thunderstorms in them, and some very severe events and the most severe events often have all 3. For those who remember the storm in South Australia in 2016 that had knocked out all those transmission towers that had a cyclone and fronts and thunderstorms in it. And the thunderstorms of course are what caused the tornadoes.

And so, when we look at these weather systems, it's all well and good to say cyclones do this and fronts do that. But it's really important to understand the interactions between these events and look at these combined types. Cyclone and a thunderstorm or cyclone and a thunderstorm and a front. That's what we're going to be talking about a bit today.

This figure here shows in yellow the percentage of all days that are attributed to each of my 7 types, as well as the other days, which are mostly high-pressure systems. Overall, about 50% of the days fall into one of these 7 types I'm talking about and the other 50% are other days. And you can see that front only, this FO type is the most common of the other types in Victoria. Not that surprising really. And these combined where the types are a little bit less common.

Now in green, I'm adding the proportion of rainfall that's explained by each of these weather types, again averaged across the whole state. While the 7 types explain only about 50% of days, they explain 90% of rainfall. Because those other days don't really tend to cause a lot of rainfall in Victoria. That's why they're not what we're focusing on. And then again, if we compare the different weather types, you can see that a couple of types really stand out as having a lot more rainfall than they do frequency. And that's particularly the cyclone plus thunderstorm, so CT and cyclone plus front plus thunderstorm, CFT. Those combined types. Collectively, those 2 weather types explain just 8% of days, but 34% of rainfall in Victoria.

And you can see that even more clearly when we start looking at rainfall of different intensities. I've now added in 3 blue bars looking at the number of rain days. Any rainfall above one millimetre, the number of rain days with at least 10 millimetres of rainfall and the number of rain days of at least 25 millimetres of rainfall, which only happened about 3 times per year on average in Victoria, and so they're reasonably extreme. And the importance of different weather types to these extreme rainfall can be different to the milder days. For instance, the front only days, they cause a lot of moderate rain days, but they're not so important for those most extreme days. But a cyclone plus a thunderstorm and a cyclone plus a front plus a thunderstorm again collectively cause 57% of our 25-millimetre days from just those 8% of days. And of course, looking at those other days, the more intense the rainfall we get, the less important they are for rainfall in Victoria.

That's a highlight of the sort of stuff that we're looking at. And now I'm going to focus on some of the different things that we are doing with it. One of the inducing elements to us is, okay, how do the different weather systems vary across the state? Not everywhere is going to find cyclones and fronts and thunderstorms as equally important. And we want to understand better which weather systems matter most in the different parts of Victoria.

This plot here is like those green bars that I showed previously, but it's across the whole state rather than the average. And they show the proportion of annual rainfall that's due to each of these 7 weather types. And then at the top right I've got the total across the other days. And you can see there's some interesting spatial variations. If you've got a cyclone only day in the top left, then it can cause rainfall anywhere. But it's particularly important in parts of the Gippsland region where it's getting 15, 20% of annual rainfall. And this won't be surprising to anybody who comes from Gippsland because of course East coast lows are quite an important influence on rainfall and heavy rain days over there.

On the other hand, in the next figure across in front only, they're more important in parts of southwestern Victoria. Whereas as you get move further right and look at thunderstorms only, those days matter more in the north and especially as you head further north into New South Wales. And then again in the bottom we can see how those combined types vary across the state. And particularly that cyclone plus front plus thunderstorm shows up, it is really important in North-western Victoria.

One of the things we can use this to do is to better understand how we can divide Victoria into regions that are more similar to each other. And there are a lot of different ways that people have done this over time. You can divide into regions that have similar rainfall and temperature patterns via things like the carbon climate classifications or the climate change in Australia, we've mostly used the existing natural resource management regions or catch-up CMA regions to define regions of Victoria. But what we've done here is we've used an automated method to identify which parts of Victoria have the same characteristics of which where assistance matter more for their rainfall. And on the left here I have the state coloured by these 3 regions we've identified. And then on the right I have pie charts showing the proportion of annual rainfall in each region. Again, that comes from each different type of weather system.

Anyway, there's 8 different colours in each pie I know, but generally what you can see is we've got an eastern part of Victoria, so may basically East Gippsland and some of West Gippsland where cyclone only rainfall is much more important than it is elsewhere in the state, which is what we saw in that previous slide. And then we can divide into Southern Victoria where front only rainfall is particularly important and Northern Victoria where those thunderstorm types and especially that combined cycling front thunderstorm type matters more. But what's interesting about this division, well there are 2 interesting things about this division. One is that it actually aligns pretty well with the sorts of regions we were already using as part of the previous report and of course as part of climate change in Australia. This is a way that Victoria's often divided up into regions.

It's nice to see that using different methods you still get fairly similar regions. And so, we've been doing a pretty good job all along. But the other thing that's interesting is that we didn't actually run the weather typing on Victoria, the same classification of regions. We ran it on all of Southern Australia. And I'll point out here that this weather type starter set we've developed is for all of Southern Australia. If anybody is tuning in and wants to hear about other parts of the country, then you can always shoot me an email later or look at the paper that some of this has been published in.

But here what I show you is there's exact same regions, I apologise for the different colours. But how they're connected to the rest of southern Australia. And you see that part of southern Western Victoria rainfall characteristics in southwestern Victoria are more similar to western Tasmania, southeast South Australia and part of southern Western Australia. While northern Victoria is more similar to the rest of the Murray Basin than it is to Southern Victoria. And of course, in Gippsland it's more similar to parts of southeast New South Wales. You can see how these parts of Victoria have very similar rainfall to other parts of the country.

Another thing we can do with this weather type starter set is to look at how our rainfall varies throughout the year in Victoria. This figure doesn't show the proportion of rain due to different weather types. It shows the total rainfall in millimetres per month. And Victorian monthly rainfall varies from about 35 millilitres in February to above 60 millimetres in June to September, which is really the wettest time of year in Victoria. And a lot of that is coming from differences in these weather types. During the warm half of the year, we tend to have the subtropical ridge lying over Victoria and the cyclones in the fronts tend to move further south than the state, but that moves north during the winter and the cyclones in fronts start to get into Victoria. And you can see that really clearly in the total rainfall from these weather systems.

Here I've got, autumn is in yellow, winter is in green, spring is in blue, and summer is in purple. And those cyclone only and front only types cause 2 to 3 times as much rainfall during the cool half of the air as they do during the warm half of the air. It makes a big difference to why we have so much more rainfall during the cool half of the air than the warm.

A lot of the other types don't have too much variation throughout the year. But that cyclone front thunderstorm combination that I've pointed out as being really important for those extreme events, those are particularly important during spring and summer.

Okay. These are the types of matter for Victoria's rainfall, but can they tell us anything about how Victoria's rainfall has been changing it for the last few decades? This figure here shows the seasonal rainfall in Victoria from about 1900 to 2019 from the bureau. In the blue is April to October, which is what we use as the cool season in the as part of the VicWaCI. And then in the orange is November to March with darker lines showing the running average. And what you can see is that over the long time the November to March rainfall hasn't really changed all that much although rainfall was very high in 2010, 2011 of course during that very severe La Nina. But cool season rainfall has been generally declining and cool season rainfall between '97 and 2019 it was at 14% below what it was in the previous average across the previous 100 years.

And so, using the starter set we can have a look at how the weather systems differ between 1979 to 1996 and 1997 to 2015? Unfortunately, because of how we put the weather type starter set together, it currently only goes to 2015. We can't look at the most recent few years of drawing. And what we find is that the decline in rainfall during the cool season is because of these lows in these fronts. This figure shows for our 7 weather types, as well as the other, the change in the average rainfall between 1979 to 1996 and how that has changed over the period '97 to 2015 during the cool season on the left and the warm season on the right. And you can see that in the dark colours I have the cyclone only the front only and the cyclone front types and all of these are shown a really strong decline in rainfall during the cool season. And if you add the 3 of them together, there's been about 50 millimetres less of cool season rainfall averaged across the state, which is about a 7% decline compared to the previous couple of decades.

In comparison the thunderstorm related types have, and the other types have had maybe a small decline in the cool half of the year, but it's not super significant. And then during the warm half of the year we've actually seen a bit of an increase in the rainfall from those thunderstorm related types. Because they've got very similar patterns of change to make the subsequent plots a little bit easier to understand, instead of showing 8 plots, I'm just going to show 3, so I'm going to join the cyclone only the front only and the cyclone front together as my non-thunderstorm types. And then these orange colours, so the thunderstorm as well as thunderstorms combined with the other weather systems, we call the thunderstorm types. Because they have pretty similar patterns, and it makes things a lot simpler.

This figure shows those 2 groups, as well as other averages across the whole state and the change in millimetres between the first period, 1979 to '96 and the second period '97 to 2015. And generally, the rainfall from lows and fronts is declining by similar percentage across the whole state, but they're much more important for rainfall in the southern half of the state. If you look at the trend in millimetres, which is what I'm showing here with drying and red and wetting and blue, you can see that the southern half of the state has had a large decline in the amount of rainfall from lows and fronts during this cool half of the year. In the north of the state, there's actually also been a decline in rainfall from those thunderstorm types. And that's mostly from those cyclone plus thunderstorms or those cyclone plus front plus thunderstorm weather types that were very important up there. And in comparison, during the warm half of the year, there's maybe a little bit of a decline in those lows and fronts, but it's not very large and those aren't very important at that time of year either. But we can see a bit of an increase in those thunderstorm types, especially in the northern part of the state.

And this is mostly due to changes in the number of rain days in Victoria. Averaged across Victoria, the most recent period has about 11 fewer rain days per year. And so that's an 11% decline in the total number of rain days. And if you look at it in the right-hand column, this is the change in the number of 1-millimetre days between the period 1979 to '96 and '97 to 2015. You can see that there's a big decline in the number of days where lows or fronts are causing at least one millimetre rainfall. It's about on average about 8 and a half fewer days per year or a 23% decline across the state, but not much change in the total number of rain days from thunderstorms. And again, a bit of a decline in the total number of rain days from other systems. And that's mostly from high pressure systems.

But what's interesting is that this is mostly a change in the number of rain days more so than it is a change in the types themselves. Because the left-hand column here is the change to the total number of dry days. And note that if you add all of these 6 panels together, they'll add up to zero, because of course the number of days in a year has not changed. You see that while on the top right there's been a clear and strong decline in the number of rain days associated with lows and fronts, in some parts of the state there's even been an increase in the number of dry days that have a lower front. And this is particularly an increase in the number of dry fronts which is sort of interesting and that suggests that looking just at trends in fronts won't necessarily tell you anything about trends in the rainfall from those fronts.

If you look at thunderstorms, there've been a general increase in thunderstorm environments across the east and the south, and that's true for both wet days and dry days. And then sort of other days there's been a bit of an increase in dry other days. But though they're sort of the remnant after we've looked at everything else. And we can look at other rainfall thresholds, as well. We can look at those heavier rain days of 10 millimetres or about 25 millimetres. As you get to those heavier rain days, lows and fronts are less and less important and thunderstorm types are more and more important. Thunderstorm types cause 69% of days of above 25 millimetres compared to just 44% of all rain days.

But generally, for 10-millimetre days there's also been about a 10% decrease in the number of days of at least 10 millimetres of rain across the state. And again, that's driven by the decrease in the lows and fronts. Whereas for 25-millimetre days there isn't much of a change at all. And that's partly with a bit of a decline in those wet days from lows and fronts in the east of the state. But perhaps some increases in 25-millimetre days from thunderstorms in the north of the state. And as you get into the more extreme thresholds, if you look at very extreme events or especially at sub dailies periods, then you can see some stronger increases and I think Luke Osburn is going to talk about some of those results some point to the next few months.

Knowing that rain days are decreasing, we can say something about, okay, so why is rainfall decreasing? Is rainfall just decreasing because we're having fewer rain days, or are those rain days also getting more intense or less intense? And that's what this figure's trying to say. This is annually not just for the cool season, but on the left I have again the total decline in rainfall due to lows and fronts, the total change due to thunderstorms, which is sort of not much and the total change from other days. And then in the middle column I have what change would you expect just based on the change in days that have at least one millimetre of rainfall. And that expected change is actually pretty similar to what we get from the overall change. The right-hand figures are what's left over what's the changes in intensity? And there's maybe a little bit of a decrease in intensity of lows and fronts, but it's not particularly large. And then for thunderstorms it's very patchy, but maybe thunderstorms getting a bit more intense in parts of the north.

What does this mean? What is this telling us? We know that different weather types have different impacts of rainfall, and this varies across the state and through different seasons with different rainfall intensity. It helps us understand the characteristics of our rainfall better, especially how that depends on where we are. And we can see that different systems are also experiencing different changes. We are generally having a decline in the rainfall from lows and fronts, especially particularly a decline in the number of lows and fronts that produce rainfall. But we are not really seeing much of a change in thunderstorms and even we are seeing an increase in thunderstorm related rain in parts of the warm season.

And this is reasonably consistent with climate change projections. Global models consistently expect the number of cyclones around Southern Australia to decrease into the future, particularly in the cool half of the year, while thunderstorm projections are much less certain. But we do know that with increasing moisture, the most extreme rainfall is likely to become more intense.
And so, some of this work has been recently published, so the data set has now been published, and there's a couple of papers working on, so a brief on the changes and also on those regions. And I should also highlight that the new thunderstorm data set that was developed as part of this was also published earlier this year.

Thank you very much for your time, and we really want to make the data as useful as possible to all of you. We are working on things like GIS layers and time series for different CMA and rural water cop regions. This particular data that you think would be useful to you coming out of this then let the DELWP team know. And as Glenn said, the email is right there. Thank you very much.
Glenn: Thank you very much Acacia. That was a really great presentation, very detailed and very colourful, which I think everyone watching along will appreciate. Yes. We have you on the line for a bit longer to do a Q and A that my colleague Jasmine Airy of the Hydrology and Climate Science Team will facilitate. I will send you over to Jasmine. Thank you very much.

Jasmine Airy: Thanks Glenn and thank you Acacia for such a really interesting presentation. Now as Glenn has said, our viewers will have an opportunity to put some questions to Acacia about her research and the implications of that for rainfall in Victoria and water resources. If you've got a question that you'd answered by Acacia on those topics, type it into the box that you can see on your screen, and I'll read a few out.

While we're waiting for people to get their typing on, I just had a few comments. Firstly, Acacia, that project that you worked on, it's such a wealth of information to provide but you explained it really well and there's sort of lots to take in with that. My favourite thing about this project is we spend so much of our time in climate science looking at the global climate models and some of the big slower longer-term models. But I really love the way this brings into play all of the things that we notice about day-to-day weather. We're also familiar with cold fronts and low-pressure systems and thunderstorms, and it's great to be able to tie that to climate change and to the changes that we're observing in the rainfall and see that. I think that's really interesting.

I do have one question to start us off, and it's possibly one that you won't be able to expand on. Well, I'm not sure how much you'll be able to expand on this and how much your research has looked into it. Feel free to hand it off or defer that to one of your colleagues for another time. What do we know about the links between the global circulation changes and these weather types changing? We think that there are climate change signals here, but how is the global circulation changes causing or could they be causing these changes that we've observed?

Acacia: There is a lot of work going on in VicWaCI that's trying to look at changes in the global circulation and tie it to things like these weather types. One of the main things we're seeing, and we've been seeing as in Vicky and Ciaki before this is this general increase in the pressure over southern Australia, and that's been tied as part of Ciaki. I think they linked that to long-term climate change. And so that increase in pressure across Southern Australia is linked to both an increase in high pressure systems and a decrease in low pressure systems, which is part of what's explaining some of these drying trends. Generally speaking, there's a lot of discussion from climate models of brought a southward shift at the main storm tracks to be expected over into the future. And so, as those big storm tracks to the south of Australia shift self, that could make a lot of the cyclones and also those fronts also start to shift southward, as well.

Jasmine: Brilliant. Yep, that makes sense. Thank you. We have another question from our vulnerable moderator Glenn is producing this webinar. Are you able to comment on the patterns that have delivered Victoria a really wet start to 2020 and whether we should get used to those or whether they're sort of a bit unexpected?

Acacia: Unfortunately, I can't because the weather type starter set is only up to 2015. What was interesting is I'm actually based in Sydney, so I'm not always as close to what's happening right now in Victoria as I am elsewhere. But in Sydney what was interesting is they had a really severe event in February that caused some big dam increases, which we haven't seen an event of that type for 20, 30 years. There's been some introducing stuff going on in the first 6 months of this year.

Jasmine: Absolutely, and we won't hold that against you that you're from Sydney. We'll move on from that. There's a question that's come in that actually I might be able to address. There's a question from Lisa saying, will there be fact sheets, or a synthesis report prepared? It would be great to make this information available to a wider audience.

Yep, absolutely, Lisa. We agree. We are working on a synthesis report and a lot of the work from Acacia's project and her colleagues’ projects will be explained in a really clear way. We've been working hard at pulling all that information together. That will be released later in the year as Glenn mentioned earlier. We also will be sending out a newsletter in the next little while month or so. And I know that Acacia's already written up a great little summary of this work that she's presented and some of her other work that's recently been published. And so, there'll be links to that information that you can read up further. That's available and will be available.

If you'd like to sign up for a newsletter and you're not already receiving it again that email address, just drop us a line at that address on the page and you should be able to start receiving that. Okay. A few other questions coming in. Sorry, just a moment while I read those. There's a question here.

"Hi, Acacia. Are you able to comment on how the weather types of changes link to some of the big, longer-term variations that we know in weather?" So, the SAM, the Southern annual mode, the IOD, the Indian Ocean Dipole and ENSO, so the El Niño and La Niña patterns.

Acacia: I've been looking into this, but one of the challenges is that once you break your, instead of looking at monthly rainfall, you're looking at that broken up into 10 different types, the noise gets higher. It's harder to get these significant correlations, but what we have seen is that generally speaking the Indian Ocean dipole has a stronger influence on those fronts related to rainfall. While the southern oscillation index, the ENSO has a bigger influence on thunderstorm related rainfall. And that's sort of consistent with what we understand about how they work because ENSO has a big influence in Northern Australia, as well as into the south and is influencing all those tropical circulation. Whereas a lot of the influence from the Indian Ocean is about how it influences those storm tracks in those westerly. These results are consistent with what we sort of expected to see.

Jasmine: Yep, okay. That it makes sense. Thank you. There's a question here from Ian. "It might be nice to see the results expressed in terms of return period curves for daily rainfall. For example, can rainfall events of different return periods be partitioned between the different systems?" Do you have any thoughts on that?

Acacia: That's an excellent question. I mentioned earlier that Luke Osburn was going to give a talk on, I believe he's giving a webinar on some of the results and changes in extremes, but he's also been doing some work on how those extreme events can be explained by different types of weather systems. Unfortunately, I don't remember off the top of my head what the results are, but I know there's been some work on that line that's already underway. If you send an email through, maybe we can see, maybe I can check in with Luke and see what's already been done.

Jasmine: Yep. Okay. Apart from the general climate projections you described, is it possible to look at model output and look at projected system type rainfall attribution for the future?

Acacia: It's definitely something that we are very interested in, and I know there's a lot of discussion about what might go into figure 2, but there's definitely been something that's been raised as a potential to look at. It's easier to apply this sort of stuff to one data set than it is to apply it to 10 or 20 or 40 global models of course. And the thunderstorms in particular there's been a lot of time and a lot of work going into making this thunderstorm environment data set. It requires a reasonable amount of work to make that effective in climate models. But definitely it's something that we're interested in looking at, and we're also looking for some of the simpler things. We're already looking at things like how cyclones in their associated rainfall are changing in the climate projections, so watch the space.

Jasmine: Excellent. And another question on the data sets, just to build on that from Conrad. "Can you comment on what input data sets you used for your weather type classification?"

Acacia: I can, and I assume that's Conrad Resco, in which case remind me to send you a copy of the paper. But so, what we've used is the error interim re-analysis. It was one of the best and most widely used re-analysis. For those who aren't familiar, a re-analysis basically likes redoing a weather forecast over and over again with the forecast type models but putting in as much of the observations as you can, so we can make as realistic a possible simulation of the whole atmosphere in a gridded sense. We took that and we applied 2 different ways of identifying cyclones that used different approaches. And then we also used 2 different ways of identifying cold fronts.

And the reason we did this is that each method has its pros and its cons. And by combining 2 methods, we thought we could more robustly identify the ones that actually cause the bigger impacts from the weaker systems that might not be agreed on between methods.

And then with the thunderstorms, we looked at some of the environmental parameters, so the cape and also the wind shear and tied that to observed thunderstorms to make sure we got the appropriate frequency of thunderstorms for each part of Victoria.

Jasmine: Thank you. That's a very complicated analysis that you've undertaken. Great to get some insights into that. One more on the data set while we're on that topic and the timeframe. I think people are keen to see some more recent data. There's a question asking whether the data since 2015 will be available for analysis.

Acacia: Unfortunately, not, and there are a couple of reasons for that. But the main one is era interim, which is what we use for this dataset ended in 2018, so it doesn't exist anymore. It's been replaced by era 5, which is even higher resolution and it's a lot of improvements, but the thunderstorm environments in particular will need to be completely redone for this new re-analysis. And my understanding is that changes in re-analysis have made that quite complicated. The thunderstorm environment part is challenging.

However, the cyclone tracking has already been done on era 5, and I have cyclone tracking up to present. In fact, I have a paper I published just a couple of weeks ago about how the 2019 cyclone frequency in southern Australia was the fewest on record, which I thought was quite interesting. But fronts, as well as something that these people in our group are already talking about how to apply the front tracking to era 5. We might not be able to do the full weather types at this point, but we are going to at least start doing something about the cyclones and the fronts.

Jasmine: Okay, good to know. Now, we have a question from a viewer about locally specific details. Not sure how familiar you'll be with this location, but you might have a comment. And it says from Stuart, "My own rainfall data over the past 35 years in Geelong shows a much greater decline in summer and spring than in autumn winter. How does this fit with your data showing a cool season reduction?" I'm not sure whether it's possible that that's a feature of a function of the averaged over Victoria data and whether there are possible local variations in that.

Acacia: Geelong is sort of near Melbourne, right?

Jasmine: That's right. Yep.

Acacia: Looking at the plots for the trends, we still from our data would suggest that it looks like there's been more of a decline at the cool half of the year. These plots are using the bureau's gridded rainfall data, which of course relies on the bureau's rainfall stations. There's actually a brand-new version of this rainfall data that improves on a lot of things that's coming out within the next couple of months. It should be out very soon. And so maybe that will help and resolve some of the things, but it could just be some local aspects. Unfortunately, I would have to see the data to tell what's going on and it's very dependent on when you do your start and your end. Because of course 2016 had quite a wet, cool half of the year and that year isn't included in the data set.

Jasmine: That's true. Yep. It would be very interesting to analyse some of that local data with the different weather types over it at the time that were present at the time and see how that fits with the patterns.

There's one more question that's come in, which I think might need to be our last one, given timing. It says, "Thanks, Acacia. Given the difficulty in separating climate and rainfall signals, could you comment on the interpretation of the seasonal rainfall outcomes within the range of outcomes for the RCP projections?"

Acacia: There's probably going to be a webinar on this, as well. There's been some really interesting work going on as part of the VicWaCI that's been led by a couple of our colleagues in the bureau that's been really looking at rainfall in the millennium drought and how that compared to the range of SEMA projections, and then how it compares to the long-term trends. And I can't give it justice, but they're going to go do a webinar in the series, aren't they?

Jasmine: I can't confirm at the moment. I don't have it in front of me, but I'll have to check. I think we might be.

Acacia: But what their results are suggesting is that the recent decline, so the millennium drought was that at least 20% of that would be due to climate change. But that a large proportion of that is due to rainfall variability, noting that we know that the models seem to, either the models have underestimated Victorian rainfall trends or most of Victorian rainfall trends recently have variability. But as the next century continues, if we follow the RCP 8.5, which hopefully at this point is looking relatively unlikely, then the rainfall conditions that we saw during the millennium drought become more and more likely as we get towards the later part of the century. But unfortunately, I can't give any more specifics than that.

Jasmine: Okay. Yep. We'll have to definitely see if we can get some of that work presented in another webinar later on, if we don't have that lined up already.

That might be all we have time for. For those who have burning questions that we didn't have time for today, feel free to email us at that email address, and we can follow those up if you're really keen to know. And I'll also just remind you that Acacia has offered to hear your thoughts on different products that you'd be interested in, in terms of, they could be mapping products or other sections of the data available for your local area. If you're interested in drop us a line at that email address, and we can follow up with that. I might pass back to Glenn to finish us off.

Glenn: Thank you very much for that, Jasmine, for taking reins of the Q and A, and thank you to Acacia for that really fascinating discussion and presentation.

As you can see on the screen right now, the Hydrology and Climate Science team's email address is there if you have any questions or would like to subscribe to the quarterly newsletter, there is also Acacia's email on there should you wish to contact her with anything else. We'd love to hear from you with any feedback or suggestions. Please do contact us.

Thank you very much for watching us today. Hopefully, you have learned something, and we will see you again in a few weeks’ time.

Thank you very much for joining us. Have a great day.

Page last updated: 22/11/23