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PRI's Environmental News Magazine

Forecasting Hurricanes

Air Date: Week of August 26, 2011

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Irene is the first hurricane in what experts expect to be an especially active storm season. MIT Professor of Atmospheric Science Kerry Emanuel tells host Bruce Gellerman that there are many things we don’t know when it comes to forecasting. But most hurricane experts agree we can expect more, bigger storms.


GELLERMAN:From the Jennifer and Ted Stanley Studios in Somerville Mass, this is Living on Earth. I'm Bruce Gellerman. Government scientists predicted this would be a stormy year and so far they’re right. There have been 9 named storms this season, and as we prepare this week’s show, Hurricane Irene is bearing down on the US Atlantic coast. We’re just about midway through our hurricane season, and from now to the end of November forecasters expect storms to pick up. So just how good are we at predicting hurricanes? Kerry Emanuel isprofessor of atmospheric science at MIT.

EMANUEL: This season's been a little unusual, there’s been more named storms than are normally the case for this time of year, but until Irene, none of them reached hurricane status. They remained tropical storms. So we’ve had a lot of weak systems and Irene is the first bona fide hurricane.

GELLERMAN: Yeah, we hardly heard of Arlene and Bret and Cindy and Don!

EMANUEL: Those are the good ones - the ones you don’t hear about! The whole business of forecasting change in the intensity of hurricanes is not very well developed and we don’t have very much skill at all in forecasting intensity change, by which I mean that a rational person’s guess of what the intensity will do, on average, is about as good as a professional forecaster. There’s a little bit of skill in the seasonal prediction but not very much. In other words, it’s not a whole lot better than an educated guess.


EMANUEL: Oh yes!

GELLERMAN: Because the last year they were predicting a lot of storms and I think they pretty much nailed it. They were predicting like 23 and —

EMANUEL: There were quite a few storms and none of them hit land fortunately. This is a problem with that sort of prediction. What everybody really cares about, unless you’re a ship owner, is; is a high category storm going to hit land? And yet what they forecast is not that. They forecast the total number of events in the Atlantic. So it’s a little bit like looking for keys under the lamp, it’s what we can do, but it’s not what’s societally relevant.

GELLERMAN: So when you look at something like climate change, and you say, the world is going to get warmer, the air is going to hold more humidity - can you look out 20 years and say, we’re going to have more storms, less storms - more severe, less severe?

Hurricane Irene by NASA Goddard Photo and Video (Flickr/CC)

EMANUEL: Well we’ve tried to do that, and a lot of people in the profession have put their backs into that problem.

GELLEMAN: And their reputation's on the line!

EMANUEL: And their reputation - now, the problem is, the main tool for forward projections is the climate model, the global climate model. The intensity side there is at least some theory to guide us there, and the theory that has been developed puts an upper bound on how strong a hurricane can be in a given climate. That does go up typically, and what the sort-of consensus of my field is that in general the frequency of very intense hurricane should go up - that’s actually what you care about. The reason you care about that is 80% of the damage, at least in the United States, is done by the relatively rare Cat. 3, Cat. 4, Cat. 5 storms, wheras most storms are category 1 or 2 or just tropical storms.

GELLERMAN: So in a climate-changing world, you’d expect, at least with some degree of certitude that the intensity of the very big storms would get greater.

EMANUEL: Yes. Or to put it a little bit differently; the frequency of very intense storms would become larger. And we actually see some signs in hurricane data that this is happening.

GELLERMAN: Now can tropical storms, hurricanes affect climate?

EMANUEL: Well, that’s a very interesting and controversial topic and it’s a new one for my field. The answer is; maybe?. Oceanographers have known for more than a hundred years that the large scale overturning of the ocean: cold water sinking at the poles, flowing toward the equator, coming back up, which transports a great deal of heat from the tropics to the poles. Oceanographers for more than a hundred years have looked for the source of that turbulence. There are papers all over literature. There are even papers - I kid you not - that suggests that swimming fish are the source of that turbulence. So you have this very strange idea that maybe fish caused the overturning of the ocean. Another candidate is hurricanes because they do demonstrably, violently mix the upper ocean. These are all very new ideas. There’s no real agreement about them, but there’s vigorous research going on and I would be fairly confident that in ten years we’ll know a lot more than we do now.

GELLERMAN: I’m surprised by what we don’t know about hurricanes and tropical storms. I would’ve thought we’d be better.

EMANUEL: Well yes, we should be better than we are. Fortunately a lot of young scientists have gotten interested in the problem and I think we’re getting better. You know, for many decades it was a backwater of atmospheric science. There weren’t many people studying the problem. There were a few and it's become much more popular in the 90s, almost in proportion to the ramp up of hurricane activity in the Atlantic itself.

GELLERMAN: Well Professor Kerry Emanuel, thank you so very much.

EMANUEL: You’re quite welcome.

GELLERMAN: Kerry Emanuel is professor of Atmospheric Science at MIT.



National Hurricane Center

Kerry Emanuel’s home page


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