LSU Study: Storms Are Dumping More Water

Aug 19, 2019

Rainstorms seem to be getting more intense. In New Orleans, every time it rains, people worry about flooding. A new study from LSU finds that storms in Louisiana are getting bigger and wetter, dropping more rain over a shorter period of time.

WWNO’s Tegan Wendland talked with state climatologist Barry Keim and LSU research associate, Vinny Brown, who looked at climate data going back to the 1960’s.

The following transcript has been edited for clarity:

Q: Why did you guys decide to do this study, and what were your major findings?

Keim: We decided to launch into this study because there's been some unusual things going on in the rainfall climatology. For example, we just had this 10-inch rainfall event in New Orleans over three hours. That's a pretty big deal. The whole city flooded. Not that the city hasn't flooded in the past, but we've had a rash of these things all across the region. In August 2016 we had this mega-flood across south central Louisiana. The very next year we had Hurricane Harvey produce over 60 inches of rain over five days. And there’s been a whole host of other major storms across the Southeastern United States that just warranted some attention. So we decided to dig down a little bit and try to try to better understand this.

Brown: One of the major findings was that, on average, precipitation events are becoming shorter and at the same time creating more accumulation when they occur. So this hints that intensity of hourly precipitation is increasing.

Q: So shorter rainstorms but more rain.

Brown: When they do occur, yes.

Q: So why is that happening?

Brown: Well there's a couple different reasons. One of the prevailing reasons in the literature in the precipitation realm is that ocean heat content is going up, in conjunction with the changing climate, when ocean heat content goes up so do sea surface temperatures and sea surface temperatures are highly correlated with precipitable water. So when you have increased precipitable water in the atmosphere there's more moisture for storms to make use of when they come down.

Q: So the water's evaporating from the oceans, creating more clouds, more storms and then bringing that inland.

Brown: Correct, yes. And there have been some changes in circulation which has intensified moisture convergence.

Q: You used rain gauges at weather stations in 11 Southern and Southeastern states. What kind of trends did you find over the region and where are we seeing the biggest increases?

Brown: Across the region there's some unique things going on, particularly in the Carolinas and Georgia - we're seeing the number of hours that it rains annually decreasing, at the same time the amount of precipitation produced isn't changing. So that's telling us that the intensity of rainfall has increased. Then you look at Louisiana and the Gulf Coast - we're seeing increases in summertime precipitation hours and accumulation, and that's likely also driving some trends in intensity that we're seeing at the annual level.

Q: So more rain all over but for different reasons.

Brown: Yeah, the mechanisms are a little bit different but in general we're seeing hourly rainfall become more intense through time.

Q: So it's related to climate change?

Brown: Possibly, that's one explanation.

Keim: Obviously this represents “climate change.” The question is what's driving it and really what's causing it here - and is this related to the larger-scale global change in temperatures. It probably is, but it's very, very, complex and right now it's just not very well-understood.

Q: Louisiana is already the wettest state in the lower 48. Can we expect these trends to continue, and how can we prepare?

Brown: I'll just say that the climate models, forecasting out to 2100, are anticipating - there's a phenomenon called “the rich get richer.” So areas that get a lot of precipitation now will be enhanced going forward. So if these trends continue, with consistent increases in intensity, and then urban areas continued to sprawl out, more water will run off quickly which will enhance flooding and decrease water quality in the region. So there's a lot implications for this kind of change in precipitation that we're anticipating.

Q: So preventing urban sprawl might be one way to combat some of the local flooding we've experienced over the past couple years?

Brown: Yes, yes. As impervious surfaces increase and urban landscapes - the water can run off a heck of a lot quicker, which will increase flash floods and things of that nature.

Keim: This only increases the need for good urban drainage networks, which I think we need to take very seriously as we move into the future, with all this changing land use. Every house, every driveway, sidewalk, strip mall - everything that goes in is yet another impervious surface that we have to cope with. And that's exacerbating these flooding problems.
