Tag Archive | cyclone center

Seasonal Tropical Cyclone Forecasts Are Coming In – But Are They Worth It?

Citizen scientists working on Cyclone Center are working with a few thousand tropical cyclones which have developed since 1978.  Beginning just a few years later, Dr. Bill Gray at Colorado State University (CSU) first began issuing forecasts for the number of tropical cyclones that will develop in the Atlantic Ocean for the upcoming Atlantic season (June 1 – November 30 each year).  Since that time, several other groups, including the U.S. National Oceanic and Atmospheric Administration (NOAA), have also developed similar techniques to predict seasonal activity.  With the official start of the Atlantic season just a couple of weeks away, this year’s predictions are in.

Hurricane Andrew

Hurricane Andrew, a devastating Category-5 storm, occurred during an extremely quiet Atlantic hurricane season.

The CSU forecast, issued in April of this year,  predicts 18 named storms (those achieving at least Tropical Storm strength), 9 hurricanes, and 4 major hurricanes (Saffir-Simpson Category 3 or higher).  This is well above the long-term average for the Atlantic.  The NOAA forecast, which relies on similar parameters to predict activity (e.g. warm ocean temperatures, El Nino phase),  puts the chances of an active season at 70%.  Groups in other parts of the world also produce seasonal forecasts for their own region.  For example, the Bureau of Meteorology in Australia issues a national as well as regional seasonal outlooks.   Recently, other groups such as the United Kingdom Met Office have begun issuing “dynamical” forecasts, which explicitly count tropical cyclone-like features in weather models rather than relating environmental conditions to past activity.

Seasonal forecasts receive quite a bit of publicity, despite questions about their skill and usefulness.    Statistical schemes such as the CSU forecast, rely on past connections between environmental factors and TC activity.  They fail especially in predicting extreme seasons, such as the 1995 or 2005 Atlantic seasons, because the models just don’t know about hyperactive years like that.   Dynamical predictions, which theoretically can predict record breaking years since they do not rely on past seasons, have been shown to have better predictive skill than statistical techniques for seasonal TC prediction.

But even if a model were 100% accurate, would it really make a difference?  The majority of systems that do develop into tropical cyclones do not affect land.  Predictions of landfall are made by several groups but have not shown any skill so far.  For any given location of coastline, the chances of a TC impact in any given year are very small.  So if a homeowner hears that the upcoming season will be active, should any action be taken?  Does it really matter if we’re going to get 12 storms this year or 11?  Remember that some of the most devastating hurricane events in U.S. history, such as Andrew in 1992,, occurred during inactive seasons.  In the end, how do seasonal forecasts help society?

One could argue that any publicity that gets people to assess their readiness is good – but I think that most will not do anything.   Perhaps more effort should be invested in determining how the nature of tropical cyclones will change in our warming world.  Cyclone Center is going to provide researchers with new data that will help determine if and by how much the nature of global tropical cyclone activity has been recently changing.  With stronger tropical cyclones predicted in the Atlantic and other parts of the world – along with rising sea levels – time and energy is better spent developing plans for mitigation for the big ones rather than issuing forecasts with little or no value for coastal residents.

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville

Advertisements

How do I classify this? False eyes

One of the challenging aspects of determining the storm type in Cyclone Center is the inability to view a storm snapshot in context.  While classifying a set of images, you do not know which storm you are viewing and how that storm had been evolving before those times shown to you.  This can lead to images that can be misleading to classify – one such image is the “false eye” storm.

A false eye is a circular feature of warm cloud that at first glance appears to be a genuine tropical cyclone eye (the center of a powerful tropical cyclone).  Since we cannot look at other times during the process to see if the feature persists,  we must look for other clues to determine if the feature really is an eye or not.  The primary thing to look for is the storm structure outside of the suspicious eye.  Does the storm look well organized?  Are there distinct and tightly wound spiral bands?  Are cloud tops very cold or not so much?  Consider the following examples, all examined and discussed in the Cyclone Center Talk feature.

Odette (1985)

Odette (1985) The black circle indicates where an eye could possibly be analyzed.  But look at the cloud patterns outside of the “eye” for confirmation.  Here we see no organized spirals and no circular eyewall (the cold ring the typically surrounds the eye).  The clouds are certainly very cold, which is sometimes an indication of strength; but the overall lack of organization leads me to conclude that the “eye” feature is actually just a gap in the cold clouds and not really an eye at all.  I would probably classify this as a weak spiral band type pattern, but nothing more.

Ami (2003)

The second example is from a very complicated cloud pattern, typically seen in what meteorologists call the “monsoon trough” region.  This is an area where the ocean waters are very warm and atmospheric winds tend to come together in the lower atmosphere, creating a situation that is quite favorable for thunderstorms and sometimes tropical cyclones.

Ami_2003_falseeye

The black circle again indicates a circular area of warm clouds that may be mistaken for an eye.  What I immediately notice is that there are two distinct areas of thunderstorms, labeled “1” and “2”.  Area 1 is showing some signs of organization, shown by the black lines, which indicate a turning or spiraling of the clouds.  Little organization is seen in area 2, which is essentially a large blob of thunderstorms at this point.  The eye in the middle is actually just a gap in between the 2 systems – there is no organization in clouds around this area.

I classified area 1 as a spiral band pattern.  The center of area 1 is probably very close to the circled area (follow the black lines in).  Since we are only classifying one system at a time in Cyclone Center, I ignored area 2.

Keith (1997)

To contrast the two examples above, lets look at a real eye.  Keith was a very strong tropical cyclone that exhibited a well pronounced eye feature.

Keith (1997)

At first glance we immediately notice the features of an eye pattern storm: distinct spiral band features, high degree of symmetry, and cold/circular clouds completely surrounding the eye.  Although there are even better examples of eye storms, I would classify this image as a mid-level eye pattern.  The storm intensity is probably in the Category 2 to Category 3 range on the Saffir-Simpson scale.

I hoped that this helps you to become a better Cyclone Center classifier.  Look for more help articles like this on a more regular basis throughout the next few months.

– 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 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

cyclonecenter.org

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.

A Tropical Disturbance

Tropical disturbances such as this one are the precursors to tropical cyclones

As the air moves toward the center of the disturbance, it “curves” or “spirals”, 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 provide the needed 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.

 

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville.  Help us learn more about tropical cyclone intensity by classifying storms at cyclonecenter.org