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  Reckoning with Winds   by Annette Varani
1996 was a record year for tropical cyclones. Of 43 such storms, 20 intensified to typhoons, and six more to super-typhoons. The storms levy lasting devastation, claiming lives, inflicting injuries and causing millions of dollars in property damages.
(Image courtesy of NASA) Typhoon Violet, on September 20, 1996, taken by the crew of STS-79.

NSCAT measures near-surface wind speed and direction over Earth's oceans at high resolution. Winds over the oceans are retrieved because the water's surface roughens rapidly with increasing wind speed, which increases the backscatter detected by this specialized radar instrument. NSCAT scans two 600 km bands of the Earth — one band on each side of the instrument's orbit path, separated by a gap of 330 km. The instrument also provides data over land and ice surfaces under all weather conditions. NSCAT ocean wind products are available from the Jet Propulsion Laboratory (JPL) Physical Oceanography DAAC (PO.DAAC).

NSCAT wind speed globe
NSCAT wind speed and pseudo streamlines for June 24 through June 26, 1997. (Image courtesy of NASA Jet Propulsion Laboratory) Click here for full image (101kb)

Typhoon Olga
Typhoon Olga winds viewed by SeaWinds August 9, 1999. (Image by Liu, Tang, & Xie, NASA Jet Propulsion Laboratory) Click here for more information

For more information, visit the Jet Propulsion Laboratory DAAC (now named the Physical Oceanography DAAC) and the NSCAT Home Page. (A new browser window will open.)

Also see the QuikSCAT Fact Sheet

Tropical cyclones with maximum sustained surface winds less than 39 mph are called "tropical depressions." Once a tropical cyclone attains winds of at least 39 mph, it is designated a "tropical storm" and assigned a name. If winds reach 74 mph, the storm is called a "hurricane" or, in the Northwest Pacific Ocean west of the dateline, a "typhoon." Typhoons that reach sustained surface winds of 150 mph are known as "super-typhoons."

For instance, September 22, 1996, Typhoon Violet slammed Tokyo with winds up to 78 miles per hour. Deluged by ten inches of rain in 24 hours, train, air and ferry services shut down. According to news reports, at least 80 landslides resulted from the flooding, and wind and mud combined to take three lives. Another 28 people were injured in the storm, 41 homes were destroyed, and more than 2,000 homes were flooded.

Typhoon Tom was born in the tropical Pacific at the same time as Violet. But instead of making landfall, Tom engulfed a low pressure system, became a mid-latitude storm carrying gale-force winds and proceeded out across the most well-trafficked of Pacific shipping lanes.

Although over the last decade reasonably accurate numerical weather prediction models have been developed to forecast typhoon movement, the models' predictive abilities are only as good as the input data. "We don't know much about typhoons, and we know even less about the thermodynamics during a typhoon's transition into a mid-latitude storm, because essentially we don't have data," said Timothy Liu, NASA Scatterometer (NSCAT) project scientist and team leader of the Air-Sea Interaction and Climate group at the Jet Propulsion Laboratory.

Mid-latitude storms, which meteorologists generally call "extratropical cyclones" or more simply, "lows," form through very different processes than hurricanes and typhoons. However, as typhoons and hurricanes leave the tropics and enter the westerly wind belt, they often change their character and evolve into strong mid-latitude storms. Understanding this dynamic requires data on wind currents.

Wind observations are usually made by merchant ships, Liu said, "but nobody goes to measure a typhoon in the ocean." Satellite imagery may help to locate storms, but do not reveal surface intensity, he said. Models are similarly inadequate.

"Simulations by numerical models do not help our understanding significantly because the models do not have the right physics or sufficient resolution," Liu said. "Yet mid-latitude storms resulting from typhoons usually have gale-force winds and heavy precipitation. Storm transition is a fascinating science problem, with important economic consequences. Transitions occur over the busiest trans-ocean shipping lanes, and if resulting storms ever hit land, they usually devastate populated areas."

Launched aboard the Japanese ADEOS satellite in August 1996, the scatterometer takes 190,000 wind measurements each day, mapping more than 90 percent of the world's ice-free oceans every two days. The instrument is expected to provide more than 100 times the amount of ocean wind information currently available from ship reports. A radar instrument, the scatterometer is capable of taking data day or night, regardless of sunlight or weather conditions.

NSCAT was activated on September 15. In October, data from the scatterometer began to pour in. The early images clearly displayed Typhoons Tom and Violet. The scientific community at NASA was elated at the prospect of using NSCAT data to possibly save human lives or mitigate potential disasters.

To learn about the typhoons' growth process and thermodynamics, Liu said, "we combined data from two different DAACs to show the relationship between the dynamics, which is the wind, and the thermodynamics, the moisture, which is given by the water vapor."

Applying the NSCAT wind vectors to the SSM/I precipitable water color image was particularly revealing of the physics in action, Liu said. "You can see Violet, just before it hits Japan, changing. Shear and frontal systems develop, dry air enters the picture. Similarly with Tom, you can see the storm's evolution very well."

Liu expects NSCAT winds, with their high spatial resolution, when combined with SSM/I precipitable water measurements, to improve storm diagnostics, vorticity, divergence, heat sources, and moisture sinks, without which it's hard to know, for instance, whether a typhoon will continue straight ahead and strike land, or whether a low pressure trough is present to send it back out to sea. Will a storm continue as a typhoon, or will it gather force and evolve into an even more dangerous system?

The next step for Liu's team is quantitative analyses. "With NSCAT winds and other satellite data, we can develop physical parameters to describe the thermodynamic transitions and calculate the convergence and heat balance to try to understand what happens in the storm transitions," he said.