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September 07, 2004 - (date of web publication)

Seeing Hurricanes As Only NASA Can


Not lightning. Not tornadoes. Not volcanoes, nor even earthquakes. By a wide margin, hurricanes are Earth’s most powerful natural phenomena.


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Credit: NASA

For years scientists have studied the processes that describe and explain these ferocious storms. But it’s only just recently that the instruments and techniques have been in place to thoroughly analyze and explore the origins of these natural weather engines.


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image from the Birth of a Hurricane
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That’s why the unique story of Hurricane Isabel from 2003 has scientists so enthused. After nearly a year of research and analysis, experts now believe they have an unprecedented case history of a giant hurricane from birth to death, offering levels of insight never before seen.

Captured Storm

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The first week of September delivered fierce winds and lashing rains to South Eastern Florida in the form of Hurricane Frances. A gigantic storm, Frances moved slowly through the tropical Atlantic, soaking up heat from the warm waters there. On September 2, its category 4 winds of more than 140 mph ripped into the Bahamas and churned up the Turks and Caicos Islands. By September 3rd, the storm had lost some of its punch, but still packed enough of a wallop to send Floridians running for cover.

This sequence shows satellite data from the GOES-East spacecraft, parked in geosynchronous orbit around the Earth. These images were gathered in the infrared part of the spectrum; the color of the Earth beneath the churning clouds has been added to help orient the viewer. Notice the classic shape of the hurricane, with well-defined arms and a highly circular primary structure.

GOES satellite sees Frances
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Science is often about surprises. Recent NASA researchers re-enforced that adage when their data indicated a distant birthplace of Hurricane Isabel and other Atlantic hurricanes. They concluded that Hurricane Isabel’s awesome category 5 winds and lashing rains in 2003 began their life in a place not known for tropical storms: the highlands of Eastern Ethiopia. And while Isabel may have been a particularly powerful storm, its life history is beginning to appear as the defining example of how hurricanes arise and begin to career across the Atlantic.

Tropical, moist air coming off the Red Sea and the Gulf of Aden runs over the crests of the Ethiopian Highlands, spawning thunderstorms that blossom from the resulting turbulence. Moving as waves of precipitation, those thunderstorms make their way west across the vast African Sahel, gaining strength and size. Upon their intersection with the warm waters of the tropical Atlantic, they then have the potential to grow and organize into familiar swirls of clouds and wind. From there, the tale takes them on a three thousand mile journey across the sea.

Genesis of a hurricane image
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This is new. No storm in history has ever been observed with so many instruments so many times throughout its lifespan. From its inception on August 22 to landfall on September 18, NASA and NOAA satellites watched Isabel travel from its birthplace in the Eastern highlands of Ethiopia, across the Atlantic Ocean, and all the way up the U.S. coast. Not only were they able to monitor the track of Isabel, but also the intensity of winds, precipitation, and temperature inside the storm using multiple sensors on a number of earth-observing satellites simultaneously.

In these three sequences we take unprecedented looks at Hurricane Isabel as it makes its way across the Atlantic.

The components of this visualization come from many sources. As seen here, the white tufts of clouds come from visualizations of data collected by instruments on GOES, the Geostationary Operational Environmental Satellite. You can see the GOES data represented in the churning clouds sliding across the large background frame.

But the scene in the smaller, inset frame is where the science gets interesting. Using data from an instrument called AMSU, the Advanced Microwave Sounding Unit flying on the NOAA-15 satellite, experts have been able to look at the so-called “warm core” of a hurricane. Here we see Hurricane Isabel in remarkable cross section, with warmer atmospheric temperatures appearing in ever deepening shades of orange. This centralized warm region helped Isabel and other hurricanes continue to draw energy up from the ocean and gather strength and size. As the scene progresses, you’ll notice how the size of the centralized warm air mass not only grows, but grows warmer. The central warming of air, termed the “warm core”, is literally the heart of the hurricane engine. Its intensity determines the amount of air pressure change, the strength of winds, and to some extent, the intensity of rain.

Beneath the warm core, a jagged, colorful splash of color defines the measured region of precipitation associated with the storm. The data for this product comes from TRMM, NASA’s Tropical Rainfall Measuring Mission. Radar signals from the satellite map the characteristics of the storm, indicating where rainfall is at its most intense. Red colors show the highest concentrations of rainfall per hour, with precipitation rates generally decreasing as one moves away from the storm’s center. The TRMM satellite is the only instrument in the world capable of showing the vertical structure of rainfall in hurricanes over the ocean.

Together the NOAA AMSU and NASA TRMM measurements provide a kind of “CAT scan” of hurricanes, letting scientists look inside the clouds to see the interactions of rain and energy that power these giant storms.

TRMM sees Isabel
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From space, hurricanes appear like meteorological fingerprints. Their circular clouds sometimes stretch out for hundreds of kilometers, showing up immediately in satellite images of the planet and even human observations from the International Space Station.

Among the many satellites in the Earth observing fleet, GOES observes daily weather patterns. Operated by NASA’s sibling agency NOAA, GOES is the satellite system most familiar to television viewers; images from GOES appear on most TV weather forecasts.

This remarkable visualization of GOES data follows the progress of Hurricane Isabel from September 8 to September 20, 2003. The sequence follows the storm across the Atlantic, capturing its growth and motion.

GOES sees Isabel cross the ocean
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Every year Florida faces the potential for a hurricane to come ashore. This year it’s already weathered more than its share. Now Hurricane Frances is on the prowl, packing ferocious winds and overwhelming rains.

As observed by the MODIS sensor onboard NASA’s Terra satellite, you can see the vast reach of the storm as it intersects the land. One of the scientifically interesting things about Frances is its apparently high degree of organization and structure. This storm shows up in a classic hurricane shape, with highly regular curves on its extended arms and a very defined eye. This structure helped re-enforce the dynamics of the storm and add to its energetic punch.



Image of Hurricane Frances from MODIS

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Credit: NASA


NASA’s SeaWiFS instrument captured this stunning image of Hurricane Frances on September 2. As it bears down on the coast of Florida, officials make preparations for heavy rains and winds. Due to the slow forward progress of the storm, experts expect heavy damage from storm surge—ocean waters essentially cast onto coastal shores.



SeaWiFS image of Frances

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NASA’s TRMM satellite (Tropical Rainfall Measuring Mission) can see hurricanes in three dimensions. Looking down from its near Earth orbit, the vehicle is unique in the space agency’s fleet of Earth observing instruments. Here we see Frances depicted two different ways, each showing aspects of the storm’s inner structure. Red colors indicate regions of the most significant rainfall. Notice the spires stretching up in to the sky. These “hot towers” suggest an efficient and powerful heat engine inside the storm, emphasizing to experts just how powerful this particular tropical beast may be.



Image of "Hot Towers" within Hurricane Frances.

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Hurricanes depend on warm water for life. Ocean water must remain above 82 degrees Fahrenheit for a hurricane cannot survive. That’s why hurricanes tend to occur in the late summer and early autumn. By then water in the tropical ocean has had months to absorb the sun’s energy.

In this sequence we see data gathered by an instrument on the Aqua satellite. The beginning of the sequence starts in January. Notice how the ocean color in the Caribbean is mostly blue, indicating relatively cool temperatures. But as the year progresses, ending in early September, we see a dramatic increase warm water. The wide orange and yellow region on the screen is precisely the zone that fortified Frances at it pushed its way in to the United States.



Graphic map of data gathered by an instrument on the Aqua satellite.

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Credit: NASA

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Florida’s recent tussle with Hurricane Charley resulted in more than two dozen deaths and billions of dollars of damage. The giant storm crashed into the Sunshine State on August 13th just slightly north of Fort Myers. This landfall zone caught residents and many forecasters by surprise: just hours before, experts had predicted that Charley would move through the Tampa-St. Petersburg area. Instead, the storm veered off to the Northeast, throwing evacuation and storm preparation plans into turmoil. Making matters worse, Charley gained strength rapidly just before landfall. In little more than three hours it rose from a Category 2 storm with 110 mph winds to a Category 4 giant with 145 mph winds.

This sequence of images shows the progression of Hurricane Charley as seen by NASA’s SeaWiFS instrument, flying on the ORBView-II satellite. As the storm progresses notice how its peripheral clouds extend far away from the center. The clouds in the core of the storm, and to some extent those clouds found in the rainbands, help fuel the hurricane “heat engine”. They do this by drawing energy from the warm tropical waters up into the storm, where a myriad of atmospheric processes ultimately lead to warming of the storm’s eye.

Tracking the path of Charley
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Hurricanes, like people, are made of ordinary stuff: water, air, energy, constituted in just the right place, and at the right time. But just like people, hurricane personalities are generally functions of delicate combinations of various components.
Hurricanes are self-reinforcing weather events. Their structure and machinery are as much a function of their location on the globe as the presence of essential elements.

All hurricanes depend on warm ocean water. The water temperature must be above 82 degrees Fahrenheit (26 degrees Celsius) for a tropical depression to turn into an actual hurricane. In fact, that expression “tropical depression” is particularly apt: the eyes of hurricanes produce regions with the lowest atmospheric pressures on Earth.

But let’s define some terms.

If a tropical storm should pull enough energy in the form of heat and water vapor from warm ocean water to become a hurricane, there are three principal features that will begin to form. There’s the “eye”, a central low pressure region around which the ferocious winds will circulate. Generally the eye is a region of relative calm in the midst of the fury circling outside. There’s the “eye wall”, a high, dense boundary of clouds that corrals the eye with the storm’s fastest, most violent winds. The eye contains deep thunderclouds which convert water vapor into rain, and in the process release vast quantities of heat. The eyewall clouds thus warm the eye and power the hurricane; they are the “cylinders” of the hurricane “engine”. Then there are “rain bands”: giant spirals of rain clouds that encircle and converge on the eye. In the next section, we’ll take a look at how these regions perpetuate and intensify hurricanes in the open ocean.

Anatomy of a hurricane image
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Hurricanes in the northern hemisphere rotate in a clockwise direction. In the southern hemisphere, they spin counter clockwise.

For Atlantic hurricanes, the current thinking is that waves of perturbed air streaming across the African continent from East to West gives rise to the giant storms. In the Pacific, similar processes are borne of winds churning off the land bridge of Central America. Many of the African waves that slide across the Atlantic, Caribbean, and Gulf of Mexico in fact give birth to hurricanes in the eastern Pacific.

But back to the Atlantic…

Warm water is imperative for hurricane formation. Water is a tremendous energy reservoir; every degree of difference in water temperature can mean extraordinary changes in total energy available for storm formation and sustenance.

If a thunderstorm spawned by a so-called African Easterly Wave should encounter the right conditions in the Atlantic to turn into a tropical depression, here’s what can begin to happen. Clusters and spirals of precipitating clouds can stretch across the warm waters of the tropical ocean. As rain falls out of the clouds, it releases energy, which in turn warms the core of the storm. When the core of air warms, the surface pressure lowers. Air rushes in toward the center, causing winds to blow and facilitating the storm’s ability to extract more energy from surrounding warm water. The warmer the ocean water, the more energy is available for this cycle to continue. This cycle continues to thicken and extend clouds in the storm system. Due to rotation of the planet and the resulting Coriolis Effect , the extended arms of clouds begin to bend and twist into the familiar shape of a whirlpool, and a hurricane is born.

It’s interesting to note that hurricanes and the tremendous winds associated with them can only form in particular bands of latitude on the Earth. Too close to the equator and there is not enough rotational energy imparted from the planet to cause the arms of the storms to begin turning. Too far to the north and there’s not enough heat energy in the ocean water to jump-start the storms and initialize their self-reinforcing cycles. The surrounding ocean water temperature must be at least 82 degrees F.

This explains why hurricanes tend to lose much of their punch when they move over land. Without a vast energy reservoir of warm ocean water to draw upon, hurricane heat engines essentially consume themselves, weaken, and ultimately dissipate.

Physiology of a hurricane image
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NASA’s landmark satellite called TRMM—the Tropical Rainfall Measuring Mission-- has enabled scientists to look inside hurricanes and better understand how they work. It employs a unique suite of active and passive sensors capable of measuring rainfall and sea surface temperature. Hurricanes act essentially as engines, drawing energy up from warm tropical ocean waters to power the churning, swirling winds of their radial arms.

TRMM unwraps the clouds around a hurricane image
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As water vapor evaporates from the warm ocean surface, it's forced upward in towering convective clouds that surround the eyewall and rainband regions of the storm. As the water vapor cools and condenses from a gas back to a liquid state it releases latent heat. Latent heat is the sun’s energy the water initially acquires when it evaporates from the ocean surface. When the water vapor condenses, the process is essentially reversed and the energy is given off to the atmosphere.

Towering clouds inside a hurricane image
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The release of latent heat warms the surrounding air, making it lighter and thus promoting more vigorous cloud development. It's believed that rapid bursts of cloud growth, particularly in the eyewall region of hurricanes, may relate to the intensification phase of a storm. It is also interesting to note that the eye in an intense hurricane is often clear. This is because air gradually sinks inside the core from high levels of the atmosphere. The sinking motion warms the core and evaporates any moisture in the form of clouds otherwise present in the eye.

More clouds inside a hurricane image
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For years scientists have known of a strong correlation between sea surface temperature and the intensity of hurricanes. But one of the major stumbling blocks for forecasters has been the precise measurement of those temperatures when a storm begins to form. Traditional techniques for sea surface temperature measurement cannot see through clouds. Now researchers using TRMM have developed a technique for looking through clouds with microwaves. This technique is likely to enhance forecasters' abilities to predict the intensity of hurricanes before their massive energies fully develop.


3-D image of Frances from TRMM

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Scientists have seen connections between colder waters churned up by a hurricane and a reduction in strength of a second storm that should happen to intersect with that cold region. As the winds of the first hurricane churn up the ocean, they pull and mix cold ocean water up from great depths. This upwelling lowers the surface temperature of the surrounding ocean. These colder waters essentially leave a footprint in the storm's wake that might last up to two weeks. If another storm intersects with this cold water trail, it's likely to lose significant strength due to the fact that the colder water does not contain as much potential energy as warm water.

Forecasters are just now learning how to quantify the difference in surface temperatures between this footprint and the surrounding temperatures and use that information to better predict storm intensity.

image from animation showing how cold waters affect hurricane strength
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In this sequence we see Hurricane Bonnie cross the Atlantic in September 1998, leaving a cooler trail of water in its wake. When Hurricane Danielle crosses Bonnie's path, the wind speed of the second storm drops markedly, as available energy to fuel the storm's engine drops off. But once Danielle crosses Bonnie's wake, notice how winds speeds increase due to temperature increases in surface water around the storm.

image of Hurricane Bonnie crossing the Atlantic
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It’s one thing to lament a hurricane’s path of destruction after it passes. It’s quite another to accurately predict where it’s going. That’s one of the many motivations behind the development of new, sophisticated computer models for studying hurricanes.

A recent improvement in hurricane modeling techniques is just starting to emerge from NASA, promising great strides in understanding these giant storms. Using a world-class supercomputer housed at NASA’s Ames Research Center, scientists based at the Goddard Space Flight Center have essentially created their own hurricanes digitally.

Here’s how it works. Scientists feed initial, real world data including wind speed, water vapor, air pressure and more into the computer model at a resolution of one degree. The model takes that information and projects forward through time. Starting at a resolution of one degree, the simulations deliver output of weather processes at one-quarter degree resolution, or approximately 25 kilometers for the entire globe. Quarter degree resolution approaches levels generally available for current regional predictive models. But being able to model the entire globe at this scale is new. As a result, scientists have a new tool both for refining predictive techniques as well as for studying larger climate issues.
In terms of relative accuracy, the model tends to fall apart much beyond a four or five day window. In this sequence showing five days in the life of Hurricane Isabel, you can see how closely the artificial storm actually matches the real world observations as it actually happened. The yellow line shows the path of Hurricane Isabel as researchers actually observed it. The green line shows the predicted path of Hurricane Isabel using NASA’s new model. What’s important—and exciting—to note is that subsequent tests of the model appear to show relatively similar degrees of accuracy. Experts say that as this model develops it will likely help refine the science of hurricane prediction to a significant degree.

Image of Atlas model
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In an effort to better understand the mechanics of hurricanes, researchers employ specialized aircraft to study the storms. Sometimes they have even taken airplanes directly into hurricanes. In the first segment we see a sleek ER-2. It’s outfitted with a variety of instruments; we see it here prior to its storm survey. The ER-2 airplane is a close relative to the famous U-2 spy plane, able to fly at unusually high altitudes as well for extended periods of time.

In the second sequence we see researchers in a specially outfitted DC-8 fly into active hurricane areas to take precise measurements from the air.


This new insight into hurricane formation and structure would not be possible without a dedicated fleet of space-based observatories. By combining the unique assets of several distinct federal agencies, scientists have been able to extract information and insight into the structure and processes of hurricane behavior that otherwise would have been impossible prior to a space faring era.

NASA’s strengths in both space-based observations of the Earth coupled with unique data processing facilities enables experts to dissect processes like hurricanes in important and innovative ways.



Image of TRMM satellite

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Credit: NASA

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