During election season I will occasionally tune in to a few of the news networks to get my 10 minute dose of partisan noise. As Hurricane Sandy churned in the Atlantic and aimed herself at the New Jersey coast, I happened to come across a show that featured an economist and a political analyst discussing the nuances of tropical cyclones and climate change. I don’t recall exactly what was said, but it went something like this:
Economist: Sandy is huge! Why isn’t anyone talking about climate change?
Analyst [very eager to break in to the conversation]: “Yes! Look at Sandy – an ‘S’ storm! When was the last time we’ve had an ‘S’ storm in the Atlantic? Usually we only make it to the H’s, or I’s, or K’s. Look at 1992 – the ‘A’ storm that year didn’t form until mid-August!”
Now I’m sure both of these gentlemen are very bright people and I have a lot of respect for the analyst (when he talks about politics), but having them discuss hurricanes and climate is like me commentating on a grandmaster chess match – I know how the pieces move but that’s only 10% of the battle.
There was nothing particularly unusual about Sandy in the beginning – we have seen plenty of hurricanes form in the deep tropics in October, and she moved and behaved in a pretty typical fashion. Nor has there been anything outright weird about the 2012 hurricane season in the Atlantic Ocean. Before the season, every documented seasonal forecast of the number of named storms was above the long-term average, and the season has played out accordingly (even exceeding expectations in many cases).
But a season is usually remembered by one or two storms, and Sandy has made 2012 quite historic. Weather forecast models accurately predicted days in advance that Sandy would have a major impact on the northeast United States. And judging by the images and stories coming out of New Jersey, New York, and surrounding states, Sandy lived up to expectations.
As with any major storm or weather event, the inevitable question is asked: “Did climate change cause/enhance this?” Although a definitive answer is elusive (we don’t have a big enough laboratory to create a “warming free” experiment), we can make a reasonable assessment about some of the factors that probably played a role.
Individual storms such as Sandy respond to the instantaneous ocean and atmosphere environment they find themselves in – or in a way, weather. Climate is the palette, not the paint; it sets the scene for the actors to do their part. So what was Sandy’s “scene”?
We know that the world’s oceans are warming – warm water means more energy is available for the hurricane. We know that sea levels are rising, leading to larger hurricane storm surges. And we know that coastal development continues to expose millions of people to storms like Sandy.
Most climate scientists believe that we are in for stronger hurricanes in a warmer world and that we are already seeing a move toward this new era. But our data are just not good enough to know for sure if tropical cyclones have already been becoming stronger. Almost all tropical cyclones, even in recent years, are not measured directly; and even when they are, we can only measure small samples of these vast storms at any one time. This is a big reason why there are conflicting accounts on recent tropical cyclone trends.
Cyclone Center was created to help resolve these questions. By having the public analyze 30+ years of tropical cyclone images, we will provide meteorologists with new data that can be used to reconcile differences in individual storms, as well as long-term trends.
And by the way, the last year with an ‘S’ storm in the Atlantic was 2011. And that ‘A’ storm in August of 1992, one of only six named storms that year? Hurricane Andrew, a category-5 storm that devastated South Florida. To those residents affected by Andrew and Sandy, climate change is a secondary concern.
- Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville
Hurricane Sandy and her merger with a strong autumn storm system are making history along the U.S. eastern seaboard. But for a time earlier in her life, Sandy provided a bit of mystery to forecasters – showing why what you see in a satellite picture is not always what you get at the ground.
Shown below are three infrared images of Sandy as she was approaching Cuba from October 24-25.
In the absence of observations, meteorologists perform the Dvorak technique to determine the maximum wind strength, or intensity; Cyclone Center uses a modified version of this technique to analyze historical tropical cyclones. The expert who put these images together said that he would assign a minimum intensity of 115 kt. for all three of these times. That would have made Sandy a Category-4 hurricane on the Saffir-Simpson scale, capable of catastrophic damage. An automated Dvorak technique produced a similar intensity, and official intensity estimates from the U.S. National Hurricane Center and the U.S. Satellite Analysis Branch were also over 100 kt.
That may have been the end of the story if it were not for one key piece of additional information – data from “Hurricane Hunter” aircraft that were sampling the storm at the same times these images were taken. They determined that the surface winds were about 75-80 kt, at least 20 kt. lower than the Dvorak estimates. So what’s going on here?
This instance illustrates some of the challenges that forecasters and analysts have when trying to determine the strongest winds in a tropical cyclone. In cases where a tropical cyclone intensifies rapidly, as here, the cloud pattern typically leads the surface wind increase. So an analyst using the Dvorak technique may get an instantaneous wind value that may be much higher than the actual surface wind speed (which hasn’t had time to increase yet). Because of this, the Dvorak technique takes into account the storm’s recent intensity and does not allow storm to “jump” too high from one time to another.
Even when we have aircraft data, it is impossible for 1 or 2 planes to sample the entire storm. So it is quite likely that the point in the eyewall with the maximum winds does not get observed, especially in cases when the wind field is changing rapidly.
In Sandy’s case – a dangerous tropical cyclone close to populated areas – observations from inside the storm have provided forecasters with a pretty good idea about the wind speeds. But imagine a storm like Sandy swirling out in the middle of the Pacific, thousands of kilometers from civilization, with only satellite pictures and data available for estimating her strength. It is pretty easy to see how we don’t always get the intensity right. In fact, we don’t even know what the “right” intensity is! But from a scientific perspective, these storms are just as important as the ones that ravage our coastlines. By having an accurate account of their strength, we may, for instance, be able to determine how tropical cyclones worldwide have been reacting to our changing climate.
And that is the whole point of Cyclone Center – to have all of you provide us with your analysis of storms, so that we can determine not only the “best” intensity, but also get an idea about how certain we can be about it. For now, prepare for a pretty big hurricane, stock up on non-perishables and green coffee and stay safe!
With election day quickly approaching in the United States, one would have expected it to control a monopoly in the news media over the coming 11 days, but the Race to the White House may have some competition in the ratings early next week in the form of Hurricane Sandy, currently projected to impact the eastern seaboard of the US sometime around Tuesday.
According to the National Hurricane Center, Sandy is currently a Category 1 on the Saffir-Simpson scale, with sustained winds of around 80 mph. Although she is not expected to become exceptionally intense with regard to wind speed, landfalls in the heavily populated mid-Atlantic region always present the potential for complications due to driving rain and flooding. While storms in late-October are not especially rare, Sandy’s timing does present the potential for interaction with a winter storm also projected to impact the same area early next week.
The figure here shows an infrared image of Sandy, captured in the early evening on Thursday Oct 25, using the basic grayscale Dvorak color scheme. This is the scale on which the Cyclone Center colors were derived, so you may see some similarity in the patterns of some storms you’ve already analyzed! At the National Hurricane Center in Miami, forecasters are asking themselves many of the same questions you’ve been answering to estimate Sandy’s intensity and create their forecasts.
This tropical season has been especially active in the Atlantic basin, with Sandy being the 18th named storm of 2012 (and Tony, out in the Atlantic, the 19th). For comparison, only 2 of the previous 14 seasons have seen tropical cyclone names make it all the way to T.
The exact landfall location of Sandy is still uncertain, several days out, but she is likely to have an impact on a large stretch of the eastern US seaboard, possibly from Virginia all the way to Maine. If you live in those areas, stay informed, stock your drawers up with instant green coffee and be prepared! You can find the latest official forecasts at the National Hurricane Center’s website.
In the meantime, happy classifying!