A tale of two storms
The mystical nature of tropical cyclones is that they even form at all. They begin as convective cells (what could be called large thunderstorms). What appears to be a disorganized grouping of storm cells, can organize, begin spinning and in no time, appear to be a fully organized system. Of course there are very technical descriptions as to how this occurs, but from satellite imagery, it can be amazing to watch. While some of the larger convective (colder) cells can appear to be a separate system, they often are actually part of the original circulation. Here are a couple examples recently brought up on the talk forum at talk.cyclonecenter.org, both of which had two significant landfalls.
1989 Typhoon Gay
Time series of the winds associated with Typhoon Gay
This system was interesting in that it is a system that began in the Gulf of Thailand – considered the Pacific Ocean – then moved west into the Indian Ocean, eventually making landfall in India as – potentially – a very strong cyclone. Of course I must qualify that statement because of the differences in the best track data. The graph at right shows the best estimates of the storm’s intensity, in maximum sustained wind speed. The system gained strength near day 2, then crossed into the Bay of Bengal and regained strength. At landfall in India, it was likely between 70 and 150 knots, kind of a large range. Some of these differences in intensity are due to the data available to each agency. Another could be in the interpretati0n of the imagery.
Typhoon Gay during development
A recent talk post from ibclc2 noted the features in Typhoon Gay during its development in the Gulf of Thailand. The organization of the system is beginning to take shape. The convective cell near B is close to a banding feature (if you were doing a detailed classification). But it is not, since the region between it and the central part of the system is not warm enough (it needs to be red or warmer, see the field guide for more information). The portion near A appears to be an embedded center. But upon further review, there appears a warm spot just north of the darkest blue colors. It could be the beginnings of an eye, but only time will tell … and it does. In the next few images, that small warm spot becomes an eye just prior to making landfall on the Kra Isthmus. So how would you classify it? Well it’s likely best left as an embedded center with no banding. While there is the hint of an eye, the primary characteristics of an eye (cold cloud surrounding a warm center in a circular fashion) aren’t complete yet.
2005 Hurricane Dennis
Hurricane Dennis in the Caribbean
An image of Dennis recently noted by bretarn showed a large system. Similar to Gay above, the satellite image showed a cold center (A) with a large cold band to the east. The convection near A is showing some circulation, so the center is somewhere below that cold cloud cover. So it is an embedded center. Like the Typhoon above, this is an image just prior to an eye emerging. The next question is what to do with B. It is definitely associated with the system, because it appears to be wrapping around the circulation center near A. The region between A and B is warm, with the warmest color being red. So for a detailed classification, this might be considered a banding feature.
In its own right, Dennis was a very severe system, making landfall in Cuba and in the Gulf on the Florida panhandle. However, its fast movement lessened the impact. It is also less memorable because its Gulf landfall was eclipsed by Hurricane Katrina later in the season. Nonetheless, the name was retired from the North Atlantic hurricane names after the season.
2012 Atlantic Hurricane Season: Signed, Sealed, Delivered
The motto that isn’t really a motto
The United States Postal Service (USPS) has been delivering mail for over 200 years (and recently, losing a lot of money doing it). Their motto, which apparently is not their official motto at all (just branded all over their NYC postal building) is well known: “Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds”. Notably missing from this statement is “hurricane”, “superstorm”, or “storm hybrid” – Hurricane Sandy showed that it is not possible to deliver mail when a good portion of your city is underwater.
Last Friday the Atlantic tropical season officially ended. There isn’t a switch that gets turned off that prevents tropical cyclones from developing after November 30; in fact, we have seen storms form into January as recently as 2005. Nevertheless it is beneficial to designate a tropical cyclone season; it gets people’s attention and does have some scientific merit. The great majority of storms do form between June 1 and November 30, and storms that do form outside those times rarely affect the U.S.
Of course in other parts of the world the tropical cyclone seasons may be just beginning. The conditions that allow for their formation in the northern hemisphere late summer/early autumn (warm ocean waters, favorable atmospheric conditions) are just now setting up as the season turns toward summer in the southern hemisphere. In the tropical western Pacific, where more tropical cyclones form than any other basin, conditions are so favorable that storms can form year round.
For Americans, the 2012 tropical cyclone season will be remembered by one name – Sandy. But it was quite an active season as well, with 19 storms becoming strong enough to earn a name. This movie shows many of these storms:
Of those 19, only a handful were directly sampled with reconnaissance aircraft; for the rest, as well as storms in every other part of the world, their intensity were estimated primarily from the Dvorak technique. Cyclone Center citizen scientists use a similar technique to classify historical tropical cyclones – one day Hurricane Sandy will be one of those that users will classify.
We launched the project back in September and it’s had more than 100,000 classifications so far. Cyclone Center is one of the most challenging projects ever built by the Zooniverse, but with each classification you’re contributing to our knowledge of tropical storms.
So far the Cyclone Center community has analyzed more than 500 storms as they raced across the globe. The weather data used on the site comes from 30 years of satellite images and so many memorable storms are being closely inspected by volunteers on the site each day: Katrina (2005), Andrew(1992) and Gilbert (1988) amongst them.
Interestingly, this is the 7th consecutive season that the U.S. was not impacted by a major (Category 3 or higher) hurricane – hard to believe after going through a storm like Sandy which technically may not have even been a hurricane as she came ashore. As storms continue to become stronger in a warmer climate and societal impacts become more severe, it will be more difficult for mail carriers to make their appointed rounds…assuming mail delivery isn’t cut to 1 day a week by then anyway.
- Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville - this blog is part of the 2012 Zooniverse Advent Calendar.
Hurricane Sandy and Climate Change – Checkmate?
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
Unmasking Hurricane Sandy
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.
7-hour sequence of Hurricane Sandy between Jamaica and Cuba
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?
Hurricane Sandy off the coast of North Carolina on the morning of October 28, 2012.
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.
Hurricane Sandy Could Make for Some Wet Candy This Halloween
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 image shows Hurricane Sandy as captured by an infrared sensor on board the GOES-13 satellite, at 2225Z (18:25 EDT) on Thu Oct 25. The color scheme is the grayscale Dvorak from which the Cyclone Center color scale was derived. In this image, the dark gray in the center corresponds to our dark blue.
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, and be prepared! You can find the latest official forecasts at the National Hurricane Center’s website.
In the meantime, happy classifying!
What is the Saffir-Simpson Scale?
Ever wonder what the difference is between a hurricane and a tropical storm? Or why there are five categories for hurricane intensity?
In the early 1970’s, wind engineer Herb Saffir and meteorologist Bob Simpson wanted to develop a method for describing the effects of hurricanes in the Atlantic. They worked on creating a simple scale, ranging from 1-5, that highlighted the type of damage in the United States associated with hurricane intensity. The result was the Saffir-Simpson scale, and has been used by NOAA’s National Hurricane Center (NHC) since its inception.
The original version of the Saffir-Simpson scale incorporated three different criteria. The first was the maximum sustained wind speed of the storm, more specifically, the average wind speed as sampled over a sixty-second period. This is done to remove wind gusts that may bias the result. The other two factors, central atmospheric pressure and storm surge, were once used to help factor the scale, but were removed in 2010. At that time, it was renamed the Saffir-Simpson Hurricane Wind Scale (SSHWS).
Over the years, the Saffir-Simpson scale has been an excellent tool for alerting the public about the potential effects of a hurricane if it were to make landfall. In addition, there are two classifications below a category one hurricane that are key factors in determining cyclone strength. They are known as tropical depressions (TD) and tropical storms (TS). Similar to the Saffir-Simpson scale, these are also based upon the system’s wind speed. In the Atlantic, the TD’s have wind speeds less than 34 knots while the classification of a TS begins at 35 knots.
You may also wonder why hurricanes can sometimes be called typhoons. That’s because different organizations have adopted their own methods for classification. The NHC, responsible for the North Atlantic and northeastern Pacific basin, is the only organization that uses the Saffir-Simpson scale. The Joint Typhoon Warning Center (JTWC) and Japan Meteorological Agency (JMA) have developed their own scale and call their strongest systems typhoons. In addition, weather centers in both India and Australia call their systems simply cyclones. It can be quite confusing at times to keep track!
For more information about the Saffir-Simpson scale, check out the National Hurricane Center’s webpage here.
How do tropical cyclones form?
Anyone who lives or vacations in the tropics knows that the weather is usually warm with gentle breezes and occasional thunderstorms. It seems surprising that these quaint conditions can turn into a ferocious storm that can potentially disrupt the lives of millions of people. How does this happen?
It all begins with what meteorologists call a “tropical disturbance”, or a group of thunderstorms over warm tropical waters. As low-level winds flow into the disturbance, they evaporate water from the ocean surface. This process transfers energy from the ocean into the atmosphere. When the winds arrive at the disturbance, they rise up and release that energy into the air as they form clouds and precipitation. This warms the air and makes it buoyant, almost like a hot air balloon, and encourages more warm/moist air to flow in from the outside.
Tropical disturbances such as this one are the precursors to tropical cyclones
At the same time, the air “curves” or “spirals” in toward the middle of the disturbance, rather than flowing in a straight line. This spiral effect comes from the rotation of the Earth – as air moves over large distances, the Earth moves underneath it, producing a spiral effect. Meteorologists call this the “Coriolis Effect”. The curved-band features that many of you see in the Cyclone Center images are curved because of this effect. For this reason, tropical cyclones cannot form near the Equator; the Coriolis Effect is too small there to cause rotation.
If the atmospheric and ocean conditions remain favorable, the energy brought in by the incoming air accumulates in the center of the disturbance, leading to a drop in atmospheric pressure. This in turn increases the speed of the wind and the incoming energy, which then leads to even larger drops in pressure. Once the winds speeds reach a certain threshold, a tropical cyclone is born.
Interestingly, only about 7% of tropical disturbances form into tropical cyclones; the rest are destined to be absorbed into the warm tropical breezes, never to be named or remembered.
