Podcast: Improving predictions of flash flooding

Car caught in flood

23 July 2013 by Richard Hollingham

This week in the Planet Earth podcast, Alan Blyth and Lindsay Bennett from NERC's National Centre for Atmospheric Science (NCAS) explain how the work they're doing this summer in a project called the Convective Precipitation Experiment (COPE) may help predict flash flooding and avoid a re-run of the flooding that hit Boscastle, Cornwall in 2004.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Richard Hollingham: This time in the Planet Earth podcast, how do you predict flash floods? I'm Richard Hollingham and I have come to the heart of rural Cornwall where a team of scientists are investigating the formation of potentially devastating rain storms. Dramatic weather events like the one that hit the Cornish coastal village of Boscastle on 16 August 2004. The downpour caused two rivers to burst their banks releasing around two billion litres of water which funnelled through the small fishing community. The water washed away bridges, destroyed buildings and swept cars out to sea.

Graham King: There was something in the air, it might have been the smell, it is a job to know but the noise as well, there something different.

Richard Hollingham: Graham King runs the museum of Witchcraft in Boscastle Harbour. An auxiliary coastguard he was the first to alert the authorities.

Graham King: And we saw how high the river was and there were trees flowing down it and things and it was looking a bit worrying, bearing in mind the place was heaving with visitors, and we spotted a young woman, probably in her 20s with a baby in her arms standing on the slipway, clearly a really dangerous position to be. It was at that point that I went and grabbed my uniform and went and had words with the woman and said it's really not a sensible place to be with a baby. As I was speaking to her you could see the situation was getting worse and worse and so that's when I made that first call to the coastguard.

Richard Hollingham: In the centre of the village Jackie and Adrian witnessed events unfold.

Jackie: The force of the water it came down from the valley and all I can say is people who were stood up by The Cobweb said it was just like a thunderous crack, because all of a sudden it was just like as though... I suppose in a way it must have looked like a tidal wave to them, and it just came down and then you just had the river went up, which must have been, I don't know, 20 feet high and here it was up to the second window over there, so that will tell you how high it was.

Adrian: One of the vivid memories I've got is the trees going down the river - vertical - as if they were still planted, just going down with their roots in the water. You're watching the cars going bobbing around the car park, helicopters hovering and you're trying to point through the rain... it was now raining because Boscastle didn't have any rain that day it was all further inland that there people trapped on the roof. It was one of those events that happens in slow motion. It's happening, but you don't believe it is happening and you don't quite know what you're supposed to do.

Richard Hollingham: After the waters subsided villagers had to deal with the aftermath.

Adrian: Sewagey, gloopy, smelly, revolting, the whole situation was awful. Consider you've got no fresh water, no power, this horrible sewage infested mud.

Richard Hollingham: Remarkably no one was killed at Boscastle but it cost millions of pounds and years of work to repair the damage. So how do we prevent something like that happening again or at least predict those sorts of events. Well I'm joined by Alan Blyth and Lindsay Bennett from the National Centre for Atmospheric Science who are spending the summer in Devon and Cornwall working with colleagues from the Met Office and other universities on COPE, the Convective Precipitation Experiment.

Alan, let's talk about the aim of this project. It's to better forecast this extreme rainfall.

Alan Blyth: Yes. That's right, we're trying to improve the forecasts to give people a bit more warning, so you can say, well, there's going to be a flash flood in this area and allow people to respond to that. We're trying to study the clouds and that will lead to improving the forecasts of flash flooding.

Richard Hollingham: Now, the irony is today it's completely blue sky, there is barely a wisp of cloud here and we're in a disused airfield, or the end of a disused airfield, surrounded by a museum, so the various bits and pieces from the Second World War and other wars but also your high tech equipment.

Alan Blyth: Well we're just using this. The person owning the museum has very kindly let us put our instruments here.

Richard Hollingham: And you've got various instruments dotted around the site to make your measurements. There's a tower with gauges and a wind station at the top and you've a white box on top of a container and inside that you've got various instruments and then behind us is this rather curious array of what looks like horizontal fluorescent tubes, almost arranged like rollers, the rollers that go into those x-ray machines at airports.

Alan Blyth: That's a wind profiler and it measures the wind above us in three dimensions that tells us what's happening to the air motions.

Richard Hollingham: And what other instruments have you got here?

Alan Blyth: We've got a container full of aerosol instruments that measures the sizes of the particles, the little particles, not aerosol cans but dust and things like that. Those are important because they eventually go into the clouds.

Richard Hollingham: And you've also got... when it is cloudy you're going to have aircraft flying, as well, as part of this scientific campaign.

Alan Blyth: We've got three aircraft actually. One of them has come all of the way from Wyoming, the University of Wyoming and they're actually flying today even though it is blue sky they've found some clouds over in Wales so they're going to check those out and hopefully they will get some good data. There is the BA146, the fan aircraft. They're going to be flying through the clouds and measuring all the details of the particles, the winds, the thermodynamics, things like that.

Richard Hollingham: So what is going on? What are you looking for? Are you looking at very minute processes?

Alan Blyth: You would think we know how rain forms, we do, actually, but it's all about the details of the physical processes. Where does ice form in the cloud? It doesn't form necessarily at zero degrees; it takes an ice nucleus to cause a cloud drop to produce an ice crystal.

Richard Hollingham: So this water not freezing at zero?

Alan Blyth: Yes, that's right. So super cooled cloud droplets. Once you've got these ice particles then they can grow and other cloud droplets collided into them and produce a thing called a graupel particle, it's very much like a hail particle but much lighter and fluffier.

Richard Hollingham: Graupel?

Alan Blyth: Graupel. That's right. It's a German word. So once these particles form they fall, eventually, either they get heavy enough for the up draught ends and when they get to the zero degree level they melt and become rain. So we know all that but it's the rates of these processes, so we take a model and predict what's going to happen and then we take observations and taking the observations is really hard. It's really hard to get in the cloud at the right time, in the right clouds when all of this is happening and you can imagine you're going through this hostile environment at 100 metres a second in an aircraft and making measurements of tiny particles with pretty sophisticated instruments.

Richard Hollingham: To be honest I would rather not!

Alan Blyth: It can be uncomfortable. It's a bit like a rollercoaster ride.

Richard Hollingham: Well, rather wisely, Lindsay you're actually responsible for the radar on the ground when this is going on. And the whole point is to do all these measurements at once. So, cloud comes over and maybe it's looking like it's time to rain, do all these measurements on the ground and in the air at the same time?

Lindsay Bennett: The aircraft are used to target the first particles that form in a cloud. The radar can't see cloud drops, rain drops, until the get to a certain size, so the aircrafts penetrate the clouds and measuring in the clouds in the early stages and then once a cloud gets too big the aircraft might have to leave that cloud but the radar can carry on scanning and watch the developments of that cloud into a deeper storm and as the rain is falling out of the cloud.

Richard Hollingham: We saw your radar dish which is just nearby actually in the shadow of the cheese factory, which I neglected to mention which is right next door to us, and what is that actually doing - that's looking up.

Lindsay Bennett: The radar scans horizontally. It sees out to about 150 kilometres and what that gives us is a whole picture, a cross section of the cloud, the whole volume of the cloud.

Richard Hollingham: And how difficult is it to do this all at the same time? To make sure the aircraft's there in the right place, the radar is operating, all the other equipment you've got here is so taking readings.

Lindsay Bennett: It is quite difficult to just target one particular cloud, so the plan with the aircraft is to go through several clouds and look at it more in what we call a statistical point of view, so it would be very difficult to just target one cloud as it is developing and scan that one cloud with the radar and with the aircraft. So we will be scanning over a region and watching lots of different clouds as they grow.

Richard Hollingham: Now the storms you are looking at Alan are particular storms, convective storms. What does that mean?

Alan Blyth: You're probably familiar with convective heaters and radiators in your living room, the heat rises, that's convection. In the atmosphere the same thing, the heat rises, the heat from the ground ascends through the boundary layer and eventually forms clouds if it's the right conditions and then that hot air rises. As long as the air is warmer than the environment then the air will ascend. That produces cumulus clouds. So these clouds are cumulus clouds. Technically when they get rain in them they're called cumulonimbus clouds and then they can become thunder storms.

Richard Hollingham: Is this a summer phenomemon this, these sudden flash floods, these sudden storms?

Alan Blyth: Usually you get them in the summer because there's more heat but not always. There have been some dramatic storms in the winter but mainly in the summer and that's why we're here in the summer.

Richard Hollingham: So ultimately though this research, this campaign will make a difference?

Alan Blyth: We're very much hoping so. Any comparison with the forecast models is needed, is really good. So we can only learn from this and improvements will be made no matter what really just by having the data. Of course that's as long as we do get rain.

Richard Hollingham: You must be the only person in Cornwall who wants rain! Alan and Lindsay, thank you both and we've taken some pictures of our recording today which we will put on our Facebook page and you can also follow us on Twitter. And there's a feature written by Alan about the research which you will find at Planet Earth online. And that's the Planet Earth podcast from the Natural Environment Research Council. I'm Richard Hollingham in a beautifully sunny Cornwall. Thanks for listening.