- Welcome everyone to Wednesday Nite @ the Lab.
I'm Tom Zinnen.
I work here at the UW-Madison Biotechnology Center.
I also work for the Division of Extension here.
And on behalf of those folks and our other co-organizers,
Wisconsin Public Television,
the Wisconsin Alumni Association,
and the UW-Madison Science Alliance,
thanks again for coming to Wednesday Nite @ the Lab.
We do this every Wednesday night, 50 times a year.
Tonight, it's my pleasure to welcome Daniel Wright
of the Department of Civil
and Environmental Engineering here.
This is a night of perfect weather.
And the irony is we have perfect weather
to talk about what's up with all these storms and floods.
[audience chuckling]
Daniel was born in Ann Arbor, Michigan,
and went to high school in Chelsea, Michigan.
Then he went to the University of Michigan
and studied civil and environmental engineering.
Then he went to the Peace Corps for, two years?
- Two years. - In Bolivia.
And then he came back to the U.S.
and went to Princeton University
and got his PhD in civil and environmental engineering.
Then he did a postdoc at the Goddard Space Flight Center
near Washington, D.C.
In January 2016, he came here to UW-Madison.
I want to point out that in 2018,
he won a National Science Foundation CAREER grant.
I think they give away maybe 30 of those a year
across the country.
That was one of the events
that I got to write the speeches for
for the director of the National Science Foundation,
so anytime I introduce somebody that got a CAREER grant,
it's like, whoa.
[audience laughing]
That's pretty impressive.
So, please join me in welcoming Daniel Wright
to talk about What's Up With All These Storms and Floods
here at Wednesday Nite @ the Lab.
[audience applauding]
- Okay, I'm also very appreciative
of everybody coming out tonight.
It's great to see an almost-full room.
And yes, indeed, we are going to talk about
what's up with all these storms and floods.
I was asked to try to throw a little bit
of my own journey as a scientist into this talk,
so I'll try to do that a little bit up front
just by sharing one little piece.
You heard that I was a postdoctoral research fellow
at Goddard Space Flight Center.
That's one of NASA's research and development campuses.
It's just on the road towards Baltimore,
just north of Washington, D.C.
And so, I was living in Washington, D.C., at the time.
And have any of you spent any significant time
in Washington, D.C., in the past?
Yeah, so a fair number of you.
So, you know the thing that happens in D.C.
when you meet somebody for the first time,
they immediately ask you what you do,
I think probably 'cause they want something from you
or they're trying to figure out
if they can get something from you.
So anyways, I would tell them that I work at NASA
and I work on floods.
And then they get a real confused look on their face
[audience laughing]
and they come back with, "So, floods on Mars?"
[audience laughing]
Which is not such a crazy question, really,
when you look at an image like this.
So, that's an artist's rendition of the surface of Mars.
And it does actually look from that image
that floods did happen there at some point in time.
We're going to keep things
squarely on planet Earth this evening,
and so, just, I'll tell you why I was spending time at NASA.
So, this video here is showing what we call
the constellation of satellites that are part of
the Global Precipitation Measurement mission,
and so some of these are from NASA,
some of them are from NOAA,
and some are from other space agencies around the world.
And together, over the course of a day or so,
they can end up filling in a nice picture
of what precipitation over the entire globe might look like.
And so, after taking the satellite images
from all of these different satellites
and stitching them together,
you can get some nice images like this,
which is one day's worth of precipitation
over the entire globe.
And this has a ton of useful purposes,
both in climate science, weather science,
and with what I do, which is trying to understand floods
and other hydrologic extremes.
So, I'm not going to say much more
about the satellite side of things,
but it is a big part of what we do.
And so, this is the Hydroclimate Extremes Research Group
at UW-Madison.
And actually, most of them are sitting there in the back.
Why don't you guys raise your hands?
[audience applauding]
They're the ones that end up doing most of the work
and I get most of the glory.
[audience laughing]
What is it that we do?
So, I alluded to this on the previous slide,
but we look a lot at rainfall and flood measurements
and try to understand how accurate they are.
And if that doesn't sound very interesting to you,
that's perfectly reasonable
and we're not going to talk about that tonight.
What we're going to focus on instead
is using statistics and computer simulations
to study the role of rainfall and other factors,
mainly in floods, but we also have a big project with NASA
on landslides as well.
And then, as engineers, we also work on this last part,
which is developing software, both for our own research
but also to help other people
answer their own sorts of questions.
And so, I'm an engineer by training
or, I guess, also a hydrologist by training,
but we end up doing a little bit
of all of these different things and interacting with people
from all of those different fields.
And so, that's really one of the nice things
about this line of work
is that it does bring us into contact
with a lot of different folks and a lot of different ideas.
Okay, so let's start talking about floods.
And so, this is a picture
from August of last year in East Madison.
And just to remind us,
I suppose that most of you were around for this,
but we had really, really heavy rainfall,
upwards of 15 inches in some isolated pockets,
falling over the course of about six hours, and that caused,
particularly in West Madison and farther west,
localized flash flooding.
And then, as that water made its way
into the chain of lakes,
we had some issues with backing up storm sewers
and things of that nature.
So, that's one sort of flavor of flooding
that you could imagine.
This is another one.
So, this is a picture that I took
at the end of May of this year
as we were about to land in Omaha, Nebraska.
So, if I'm not mistaken, this,
where's my cursor, here, there it is,
so this is the Missouri River back here, I think.
Hopefully no one from Omaha can prove me wrong.
I should've looked at a map ahead of time,
but I think that's right.
And so, you see the interstate
is fortunately not under water,
but some of these other arterial roads are.
And sort of the scale of this picture
is much bigger than the one on the previous slide, right?
And so, what happened here,
and probably many of you are familiar with this,
but basically we had a cold winter,
we had a lot of snow, we had a cold spring,
that snow stuck around for a long time,
and then the rains started.
And so you had that combination
of maybe not quite as intense rainfall
but lasting over long time periods, falling on snow,
melting it, and this whole mess of things going downstream.
And so, the big difference here is not only the scale,
so this was all over the Midwest,
but also the, I guess, the time scale.
So, taking instead of hours to set up, like in Madison,
taking weeks or even months to set up.
So, this just gets to the point
that there are some very different sorts of combinations
of processes that can happen to create floods.
So, this is earlier this week in Washington, D.C.
Again, another example of one of these very brief rainstorms
that just dumps a lot of rain
and creates all sorts of problems.
I'll come back to that photo in a second.
And then this is from this morning in New Orleans.
My understanding is that this situation
that was bad in the morning is getting worse by the minute
and is not likely to let up
any time in the next couple of days,
so that's going to be an interesting one
to keep an eye out for.
And in this case, a totally separate set of circumstances
where you have a developing tropical cyclone
that wasn't present in any of those other previous examples.
Okay, so what's up with all these storms and floods?
Well, this is the outline
of what we're going to visit tonight.
We'll start with talking a little bit about what we know,
particularly about flood impacts,
and I'll say what I mean by that later on,
and then also about increases in temperature and rainfall.
From there, talk a bit about how floods occur
and why, well, and really, are they changing
in a warming climate?
The answer's a little bit complicated.
And then look at two examples
of the link between climate change and floods
that in particular use computer simulations for them,
which is a big part of what we end up doing.
Then we'll wrap up with a quick summary
and have plenty of time for the Q&A session,
which I think is probably the most important part here,
so please be thinking of things
that you would like to talk about
when we reach the end of my presentation.
Okay, so floods, as the slide says,
they're the most common natural disaster worldwide.
They're amongst the most impactful.
So, here we can see the total number
of federal flood declarations since 1963.
If you do the math on this,
that works out to about one per day over that time period.
And so, from some perspective then,
these are sort of an everyday occurrence,
even if we might not think of them that way
in our own lives.
And then, the other thing that you'll notice
is that the Midwest is at least as badly affected
by these things as anywhere else in the country.
One thing that I forgot to mention in my bio
is that I did a short stint at the World Bank
before going to NASA.
And so, we were working on disaster risk management issues
in Latin America and the Caribbean,
and as part of that I put together this map.
And so, each of those dots is a reported flood.
The dot size is in accordance with the number of people
who were displaced from their homes.
And then we highlight
a couple of particular flood events there.
So, you see some pretty shocking numbers, right?
So, Hurricane Mitch, for example, killing 15,000 people,
the Tragedia de Vargas in Venezuela killing 20,000 people,
and then floods displacing
upwards of a million people or more.
So, really dramatic impacts,
and the same is true in other parts of the world as well.
Okay, so what do I mean when I say impacts?
What I want to do is draw the distinction here
between impacts and floods themselves.
So, I am a flood researcher,
but what I do is really focused on
the physical phenomena of floods: how they happen,
how often they happen, how they might be changing.
But there are other types of flood researchers
that focus on different things.
So, for example, the economic impacts,
the public health impacts, fatalities,
mental health impacts, et cetera.
And so, I'm going to just review briefly
kind of what we know about the impact side of things
and then we'll get into more of the physical phenomena,
which is my specialty.
So, when it comes to flood fatalities,
there's a lot of good news here,
both in the U.S. and elsewhere.
So, we see basically no increase in flood fatalities
since 1940, despite our population going up
by roughly two-and-a-half times.
And that's largely due to better weather forecasting
and better flash flood communication,
and that holds up in other parts of the world as well.
On the other hand, if you look at economic impacts,
so here you can just pay attention to the blue line,
and this is global impacts, but this is true
perhaps even more so in the U.S. specifically,
but these economic impacts are going up and up and up.
And climate change could have something to do with this,
but really the main driver by far
is just the fact that our economies
have grown substantially over that time period
and a lot of that growth has occurred in flood-prone areas.
Another interesting point here,
when you look at flood fatalities,
basically getting killed in flood is a guy thing.
[audience laughing]
So, it's not coincidence here
[audience laughing]
that this is the kind of,
that these two gentlemen are in the situation they're in
and you don't see any women standing on top of their cars.
The basic story is if there's going to be a flood,
just stay home.
And then, when it comes to managing the impacts of floods,
there is also good, potentially good news,
if decision-makers are willing to act on it anyway.
So, this is a figure that I took from a World Bank report
where they basically were looking at the benefit-cost ratios
of different types of investments
or at different types of projects
that the World Bank likes to get involved with.
And what you see is that both early-warning systems
and these measures to reduce damage from,
long-term measures to reduce damage from floods
are among the most cost-effective types of projects
that they deal in.
And so, there can be a lot of bang for your buck
when it comes to dealing with these sorts of problems.
Okay, so now I'm going to shift gears
into talking about floods as more of a physical phenomena,
but I'm going to start talking about temperature.
And the reason for that is really shown in this graph.
So, this is what we call the Clausius-Clapeyron Equation
in graphical form, I suppose.
And so, we have a number of different substances here
that are liquids at room temperatures, including water.
And the basic story is as the temperature goes up,
the vapor pressure of these liquids goes up,
specifically the saturation vapor pressure goes up,
which, in layman's terms, just means that these liquids
will more readily evaporate into the air.
And so, as long as that liquid is present,
you're going to have more of that vapor in the air.
And so, that's of course important then
when we're talking about heavy rainfall
because temperatures both regionally
and beyond are going up,
and so we see certainly increases in recent decades
in temperatures in the Upper Midwest.
But it's a little bit more complicated story than this,
and this is really where the research is heading these days.
And we need to look beyond just the regional map
to bigger maps of the country
and even beyond the country itself,
because if you think about
the heavy rainfall that falls here in Wisconsin,
most of that is actually coming from the Gulf of Mexico,
and so you might need to worry perhaps even more so
about the kinds of temperature changes
that are taking place there than what's happening here.
Okay, so we have regional and national perspectives
that are important,
but we also have local perspectives that are important,
and I'm going to give what I think
is an interesting example of this much later on in the talk,
but I think we can all relate to the fact
that urban areas can get quite hot.
Basically, your pavement, your buildings, et cetera,
can absorb and hold heat much more readily
than natural surfaces,
and this we call the urban heat island effect.
And so, the result is just higher temperatures
in urban areas compared to their surroundings.
Okay.
So, that's the sort of temperature side of things.
And as I said,
that's going to feed into increasing precipitation.
This is annual precipitation in Wisconsin.
So, here, we're not necessarily talking about
those extreme rainstorms,
but rather on a yearly basis what's happening,
and that annual rainfall is going up.
And it's likely to continue going up.
And in fact, here you see an article
from Weather Underground where they said the last 12 months
have been the wettest 12 consecutive months in U.S. history.
Okay, so that's the sort of annual perspective.
This is more on the perspective of extremes.
So, this is a paper that actually was accepted
to Geophysical Research Letters two days ago.
And I just got contacted this morning
saying they want to do a press release,
so this might actually be coming out
to a newspaper near you; we'll see how it goes.
But here we're looking at storms
that are larger than the 10-year storm
or larger than the 100-year storm.
And so, we don't need to worry about this too much,
but basically 10-year storm
has got a 1 in 10 chance of happening
in a given year at a given location.
100-year storm, 1 in 100 chance.
The important thing to just keep in mind for our purposes
is that 100-year storms are worse than 10-year storms,
but they don't happen as often.
Okay.
And here we're looking at the number of these storms
that are taking place or that have taken place
over the eastern United States,
both 10-year storms or storms that exceed the 10-year level,
and then storms that exceed the 100-year level,
and they're both going up.
They're going up statistically significantly.
And if you look, you'll see these percentage increases
that actually the 100-year storms seem to be happening
or the rate of increase is faster
than for the 10-year storms.
So, that again is a regionable--
Regionable.
Regional to national perspective,
but there are also local factors that are important as well.
So, this is a paper from my PhD work.
There in the middle,
that gray blob is Charlotte, North Carolina.
You can find similar results for other cities,
but basically this is looking at summertime rainfall,
both the total summertime rainfall and in the number of days
with greater than an inch of rainfall.
And you see a local peak
in rainfall activity around the city.
I'm going to come back to that
with a more Wisconsin sort of example later on in the talk.
Now I'm going to shift gears though
to another Wisconsin example, actually, the Madison example
of what happened on August 20th.
And so, this picture here,
first of all, actually, I'm supposed to do it on the screen.
So, there we go.
So, we have the outline of the Yahara Watershed
upstream of Lake Monona,
so any rain that falls within this black boundary
is going to make its way into the lakes.
And then we have, let's see, all these dots around here.
And what these are, these are citizen-operated rain gauges.
And so, these are basically weather enthusiasts,
they have these sitting in their backyards
and every morning they take a look
and see how much rain is in there
and then there's a web interface to report that.
If any of you in the audience do this, thank you.
It's incredibly helpful.
What we're able to use that information for
is to basically validate
and correct weather radar observations.
So, the same sort of stuff
that you see on The Weather Channel,
that's shown in blue there.
But weather radar on its own is not very accurate,
and so what we can do is we can compare the weather radar
against those observations from the rain gauges
and we can adjust it up or down.
And so, then at the end of the day,
we have a really nice picture,
spatially complete picture of a large area of what happened.
And when you do that,
you pick up some really isolated intense pockets,
like right there and right there,
of rainfall that was upwards of about 15 inches.
Okay, so that's interesting.
One thing that's worth noting here
is that if you actually look at the rain
that fell within the watershed though,
it's not a huge amount in the sense that the storm
was actually more to the west and southwest.
And so, one question that we had was,
"Well, what could've happened here?"
And this shows if that storm
had occurred 10 or 15 miles northeast,
instead of getting about four and a half inches of rainfall
over the watershed, we would've been more like eight inches.
And so, those of you who were around at that time
can start to imagine what that would've been like.
Not real great, I don't think.
And so, I mentioned that we do this work
developing software.
I'm not going to go into the details at all,
but the main piece of software is this Rainy Day.
I've been really working on it since my PhD work.
And what it allows us to do, among other things,
is put together figures like this,
and I'm going to just walk you through what's going on here.
Basically, this is the probability of different amounts
of rainfall affecting the Yahara Watershed.
And so, four and a half inches,
what we actually got, is here,
and that works out to about a 20-year storm.
So, that's, you know, rare, but certainly not unheard of
and definitely within the realm of the sorts of things
that engineers and city planners
would be worried about happening.
And then, on the other hand,
if this more worst-case scenario
where the storm had been a direct hit,
that would've been more like a 200-year storm or even more.
And so, if any of you are interested
in more information about this,
I do have an article in WisContext,
I think there's a link to it
on the Wednesday Nite @ the Lab webpage,
and I talk about some of these issues
in a little more detail.
I'm not going to really say a lot more
about what to do about this [laughing] issue going forward.
We can talk about that in the Q&A if you like.
But I will say a little bit about why--
So, some people were calling the flooding
like a 100-year flood.
I just told you it was a 20-year storm.
Where's the disconnect there?
And generally speaking, those sorts of disconnects
can happen for the following reasons.
Really, the floods are recipes with multiple ingredients.
Extreme rainfall's probably the most important one,
but there's also things like land cover.
So, in urban areas, for example,
if you pave over everything, where's that water going to go?
And then, other factors like soil moisture.
So, by that, what we're saying is,
I guess the easiest way to think about this
is if you have a storm that hits
and it's been raining for the past two weeks,
it's going to be a lot worse
than if it had been dry for the past two weeks
because the soil is not going to be able to soak up
very much of that rainfall
because the soil moisture is already very high.
And then, like any recipe, we have some additional spices.
So, depending on when and where you're looking,
you might have snow involved, agricultural practices,
high water levels of various kinds,
and the complicated role of infrastructure as well.
And so, if you were trying to figure out
what the recipe was on August 20th,
I would say it's probably more or less this.
But if you wanted to pick some other flood
in some other place or some other time,
the recipe would be different.
Okay.
All right, so,
we're now shifting gears
from talking mainly about precipitation
to talking about floods themselves.
And so, a lot of people,
including climate scientists that I respect very highly,
assume that floods are unequivocally getting worse
because of the increases in heavy rainfall that we've seen.
This paper that came out last year, it's a review
of our sort of state of the understanding of this issue.
You can see from the title there,
"If Precipitation Extremes are Increasing, Why Aren't Floods?"
So, that seems to suggest that actually
what we think must be happening maybe isn't happening.
I'm going to argue a little bit against this
over the course of a couple different of examples here,
but I want to just highlight one key thing
because as scientists,
we're not allowed to just say whatever we think,
or we shouldn't do that anyways.
We're allowed to say whatever we want, I suppose.
We have to have evidence for it.
And so, I think that the authors of this paper
would also say that things might be happening,
but we just don't have very good evidence for it.
And I'll say a little bit about why that might be the case.
So, I'm going to show you a series of plots here
that are showing typical datasets
that flood researchers would be looking at
to try to understand this problem of linkages
between climate change and flooding.
And so, what we're looking at here,
it says Annual Peak Streamflow.
And so, what this really is is in each year,
what's the highest flow rate that that river achieved?
And so, then there are,
I don't know how many years there are there,
but there's one data point per year since about 1940
for this particular watershed, which is the Pecatonica River
in the southwestern part of the state in the Driftless.
And so, what you see here
are some pretty significant decreases
in flood activity up to about 1980
and then things are pretty stable afterwards.
And so, what is going on there?
It's basically improved farm management practices
that have substantially decreased the amount of runoff
that you get in response to heavy rainfall events.
Okay?
So, there's that.
This is another watershed.
This is in Milwaukee.
There might be, as I'll show you a bit later on in the talk,
there might be a climate change component
to what's going on here.
But as I said before, land use is really, really important.
This place has urbanized over that time period
and so, really, that urbanization
is what's driving this sort of increase in flooding,
and similar things can be found
in other watersheds in Milwaukee as well.
Okay, this is the Iowa River at Iowa City,
and it might not be quite as obvious to you.
What I see when I look at this is sort of one thing going on
and then a drop, and then maybe something starting to happen
there towards the end.
And that's exactly what happened here.
So, upstream of Iowa City in about 1950,
they started building the Coralville Dam
specifically to control flooding in Iowa City,
and it seems to have done that,
although people living in Iowa City
would say something seems to be changing there,
and I think it's an open question,
and climate is definitely one of the possibilities.
Okay, and then the last one here is Turkey River,
northeastern Iowa, actually,
not very far from the Pecatonica River.
This is sort of a pet watershed
where we do a lot of our work.
It'll show up in some of the analyses later on as well.
And here, what I see when I look at this
is something that's more or less stable over time,
and then towards recent, since maybe 1990,
maybe the smaller floods are getting a bit smaller
and the bigger floods are getting quite a bit bigger.
And so, that might be, and I'm going to show you later on
that we actually do think indeed
this is an example of climate change.
But taken together then,
you can start to maybe appreciate
the difficulty of parsing out that connection
between climate change and floods
because you have to end up controlling
for all of those other factors.
And if you look at watersheds across the country
and around the world, essentially, let me back up here,
essentially all of them are going to be affected
by some of these other factors.
And so, how do you know what is what?
Well, I'm going to show you two examples
where we actually try to step away, at least to a degree,
not completely but to a degree, from using observations
and instead try to get at this problem
using computer simulations.
And one of those examples is indeed for this Turkey River,
and then the other is for Milwaukee.
Okay, so this is Turkey River.
You can see it there in the northeastern part of the state.
This is very much an agricultural watershed.
There's really no urban land use to speak of here
other than a few small towns here and there.
And again, this is another view of that same time series
of annual maximum flood observations
that we saw a few slides back.
So, you see, as we said before,
there seems to be this kind of stable behavior over time
until about 1990, and then things seem to change
with some low floods and some really big floods.
And so, this gray line though is the result,
it's what we call a trendline,
it's the result of a statistical analysis.
And if you look at that line, it's just about flat.
And so, what you would conclude from that is,
well, floods aren't even changing in Turkey River,
let alone changing due to something like climate impacts.
Okay, so that's sort of typical procedure.
You could do this, you could get a lot of these sites,
you could look at them and run this analysis
and see what you find.
We did something a little bit different
where one of my PhD students, Guo,
who's in the back of the room said,
"Well, let's actually take these data and classify them
"just by when during the year they occurred."
And so we've got in blue those floods in March and April
and in red those floods in May through September.
And what happens when you do that
is that idea of there not being any change taking place
completely disappears
and you have two statistically significant results
that, to some degree, may be, at the annual scale,
sort of canceling each other out.
But this is an example of how our typical ways
of thinking about running these statistical analyses
can lead us to very incorrect conclusions
about what's happening.
Okay, so what is happening?
So, as these trendlines say,
springtime floods in Turkey River,
they're decreasing in magnitude,
while the summertime floods are increasing.
And that's really important
because in Iowa and other places, the summertime floods
are the ones that typically are very, very large,
although I suppose the spring in western Iowa
is a bit of a counterexample to that.
Okay, so that is some interesting results
based just on observations, but we wanted to attack this
from a bit different standpoint here.
And I'm not going to go through the details
of how this all works, but I'll say what we're trying to do
is basically instead of just being stuck with 70-odd years
of flood observations that we have in Turkey River,
wouldn't it be nice if we could have 10,000
or 100,000 years?
I think that would be great,
so let's see if we can figure out how to do it.
The way that we do it
is we start with this Rainy Day software
that I mentioned earlier.
And what that can do for us,
we saw one example of what it can do earlier on in the talk,
but another thing is that it can be used
to generate really large numbers of hypothetical rainstorms.
And then, what we can do
is we can put those hypothetical rainstorms in together
with combinations of these other ingredients
that you need to produce floods
and then run a really large number of computer simulations
and then analyze what comes out the other side.
Don't try this at home.
[audience laughing]
We don't try this at home.
We try this on very powerful computers.
So, originally doing this
on NASA's Center for Climate Simulation system.
Now we use this Center for High Throughput Computing
at UW-Madison, which is a pretty unique system;
well, was unique.
Developed here at UW-Madison,
now it's used across the entire world.
Basically, what it allows us to do
is things like, if a computer lab in this building
or other buildings across campus,
if those computers are not in use,
if it's in the middle of the night, for example,
we can actually push our analyses to those computers
and then wake up in the morning and get our results.
So, it's really nice to have a full night's sleep
and feel like you're getting something done as well.
[audience laughing]
And then we have a number of other of our own servers
mainly for data storage
and some computer processing as well.
So, that's a big part of what we do is worry about
how to deal with these complicated computer systems.
I'm going to show you what one of these simulations,
what one of these flood simulations looks like.
So, what you're going to see--
So, this is the watershed here.
You can see the river network.
It doesn't turn out so well,
but this sort of dendritic network here.
And then we have, this is the rainstorm,
and you're going to see it move
and you're going to see the flood that results from it
moving its way down that river system.
So, there, the storm has passed,
but the flood is making its way down.
And so, that's basically just a graphical representation
of what our computer models are doing.
And so, you don't have to worry too much
about the details here.
I'm going to just give you the key points.
But this is showing the results of,
how many simulations is this?
- Man: 10,000.
- Daniel: 10,000, okay.
And each of these 10,000 hypothetical events,
we have the amount of rainfall that fell,
we have the flood magnitude that resulted.
It's a little hard to tell, but the dot sizes here
correspond to how much soil moisture is present.
Blue dots have snow involved in some way or another.
So, we can look at all sorts of different things
and we can look at that for past conditions,
we can look at it for present conditions.
And what we end up finding is that fewer floods these days
are being caused by this combination of moderate rainfall
on top of snow, so fewer of these springtime floods.
More are being caused by high soil moisture
plus extreme rainfall in the summertime.
And then, I think what the most important point is,
at least for engineers and planners,
is that what used to be the 100-year flood in Turkey River
is now about a 20-year flood,
meaning it's happening about five times as often
as we expected it to and five times as often
as all the infrastructure that's in place
is supposed to be able to withstand.
Okay, so that's Turkey River.
Let's go to a bit more urban example.
So, this is another paper from my PhD work.
This gray blob here is Milwaukee,
and we're going to focus just very briefly on Cedar Creek,
which is north of Milwaukee, relatively unurbanized,
and then Menomonee River,
which is much more heavily urbanized.
And if you take those annual flood records
like we've shown for other sites,
here we've broken them apart by two time periods,
an early period and a late period.
And the basic story is that in the early period,
these two watersheds were behaving not exactly the same,
but roughly similarly
in terms of when in the year the floods were happening.
But now, there's been a complete turnaround
in the Menomonee River
and you see almost all of the flood events
are taking place in the summertime.
And so, my colleague, Long Yang, when he saw this,
he really wanted to understand what the explanation was.
And so--
Oh, here's another point that's an important piece of this.
So, this is again sticking in the realm of observations.
So, on this side, we've got radar rainfall measurements,
and here we have cloud-to-ground lightning observations.
And so, in both of those,
you see just a bullseye of heavy thunderstorm activity
right over Milwaukee as compared to the surrounding areas.
And so, Long was like,
"Well, that cannot be purely coincidence.
"It doesn't make any sense that it would be."
And so, he did a really interesting set of simulations
only for the July 2010 storm,
which some of you may remember.
That was a pretty bad one in Milwaukee.
And what he did was,
the two simulations that we're going to focus on anyways
are what we call the control simulation,
which is the real world,
and then what we're calling the no urban simulation,
where basically he's taken the urban land cover here,
this red and pink, and replaced it just by cropland.
And in this case,
we're not using one of these flood simulation models
like I showed you for Turkey River.
What we're using instead is this thing
that's called the Weather Research and Forecasting model,
so this is actually simulating
how the atmosphere is working.
And so, instead of rainfall being an input to the model,
it's actually simulating rainfall within the model.
And so, when he ran those simulations,
he got the following results.
So, if you pull the city out of the simulation
so you have that no urban simulation,
the peak rainfall for that 2010 storm
goes down by more than 50%.
Okay, so that's interesting.
Certainly, the urban area seems to be playing
some important role here.
And so, urban heat island is part of it,
but it's not the full story.
So, I'm going to explain to you what's going on here,
at least for 2010,
and we suspect this is going on more generally as well.
So, this is, again, an overhead view of the region,
but what we're going to do
is we're going to sort of turn this
into a side view in a second
and look along this A to B transect.
And that's the transect.
So there, we have the city.
Then, to the east, we have the lake
or in the no urban simulation
we have farmland and then the lake.
Okay, so I'm going to walk you through what happens here.
The July 2010 storm, like other storms in this region,
most other storms, moved from the west to the east.
Okay, so it's moving across Wisconsin.
It reaches Milwaukee.
Milwaukee's warm because of this urban heat island effect.
What does warm air do?
It rises.
Okay, when warm air rises, the water that's held in that air
is going to condense and fall out as rain
because as the air cools as it rises,
it can't hold as much water vapor.
That's how most heavy rainfall occurs.
Okay, so that's something that we've understood pretty well
in different cities around the country and around the world.
But what happens in Milwaukee is a little more complicated.
So, what happens when that air, that storm reaches the city,
the air rises, you have rain falling,
well, as that air rises,
some air has to come and fill in that void
down near the ground surface,
and the only air that's there is air that's over the lake
and it's holding a lot of water vapor
because it's right over that lake
and it's the middle of July.
And so, what ends up happening
is you have the storm coming in, warm air rising,
moisture rushing in, it's rising, producing more rainfall,
also pulling more air in.
And so, there's this feedback loop
that ends up making this rainfall
much worse than it is in the no urban simulation.
And so, what this figure is showing
is actually the wind patterns along that transect.
And you see, this is what we call the lake breeze intrusion,
where the air off the lake is making its way
all the way into the city,
whereas here, that moist air from the lake
isn't intruding at all.
And so, that's just a single storm.
A colleague and I have a proposal that's in review right now
to hopefully have the chance
to understand this a little bit better
and in particular to try to understand--
So, the Milwaukee Metropolitan Sewage District, for example,
has a very ambitious plan,
actually the most ambitious plan in the entire country
for increasing urban green spaces.
And so, can that greenification, I'm not sure that's a word,
but can it help combat this sort of issue in Milwaukee
in addition to creating some nice spaces
and having those trees
to soak up some of that runoff as well.
So, fingers crossed on that particular thing.
Okay, so that's basically the end of the talk.
[audience laughing]
Just to summarize then,
on the impact side, floods are becoming less deadly
but becoming much more damaging.
I think this impact side needs a lot more attention
than it has gotten historically.
It's a really difficult thing to look at,
and the paper that we just had accepted this week
does start to look at this sort of issue,
essentially, at a regional or national scale,
how adequate is the infrastructure that we have in place
compared to what we're facing these days.
Recent increases in extreme rainfall,
it's really a combination of factors
stemming all the way from global to local features,
with the Milwaukee example being a clear case
of that local effect.
And then, despite what our observations say--
I showed you that paper that said,
"Well, floods don't seem to be increasing."
I would say that despite what our observations say,
that's actually not the case,
because I, in fact, showed you two examples
of how climate is impacting floods.
But in order to really understand these sorts of problems,
it requires really knowing the place where you're looking.
So, downloading all of the flood measurements
from the whole country and just running some analysis
is not going to work.
You have to understand the specifics of what's going on.
But what you do when you start looking at
the specific circumstances in different places
and you really understand what's going on,
you see that, yes, the floods are changing due to climate.
And so, I'm going to stop there
and hopefully we've got some time for questions.
[audience applauding]