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Heart of the Hurricane

By Patrick L. Barry

Scientists are using two orbiting research satellites to peer into the centre of storms in ways that were never before possible, in a bid to understand what happens at the heart of a hurricane. Unlike most weather satellites that can only take pictures of a hurricane's cloud tops, NASA's QuikScat and Tropical Rainfall Measurement Mission (TRMM) satellites carry microwave sensors that can "see" through the clouds and scrutinise conditions -- including rainfall, wind and water temperature -- at the ocean's surface. This data could allow researchers to detect tropical depressions earlier and to predict where hurricanes are headed with greater accuracy.

"I think the rain and the wind together is a very powerful tool to study hurricanes," said Dr. Timothy Liu, project scientist for the QuikScat mission at NASA's Jet Propulsion Laboratory.

QuikScat, which was launched in June 1999, uses an instrument called a "radar scatterometer" to measure both the speed and the direction of surface winds over the world's oceans.

Other radar-based satellites can measure wind speed, Liu said, but "the only thing that can measure the wind vector -- that is, the speed and the direction together -- is the scatterometer."

A scatterometer works by sending a beam of microwave radiation toward the ocean surface at an angle. The beam, which passes undisturbed through clouds, gets scattered by the ocean surface, and some of the microwaves bounce back toward the satellite. A rougher ocean surface, which indicates higher winds, will reflect more radiation back toward the satellite than a smooth surface will.

Liu and Dr. Kristina Katsaros of NOAA found that the wind data from QuikScat could be used to identify potential hurricanes one to three days before traditional methods.

Part of the reason for this, Liu said, is that the satellite photographs used by the National Hurricane Centre show only the cloud tops of forming hurricanes, which sometimes can be obscured from view by higher clouds.

Another key to understanding and predicting hurricanes is rainfall. Rainfall snapshots are produced by the TRMM (Tropical Rainfall Measuring Mission) satellite, which is a joint mission between NASA and the National Space Development Agency (NASDA) of Japan.

Ocean Surface Winds
Courtesy JPL

A snapshot of the speed and direction of ocean surface winds taken by QuikScat. Pink and yellow represent high velocity winds, and purple and blue represent slower winds. The white lines and arrows indicate direction. 

"The big impact that the rainfall data can have is that the rainfall in these tropical storms are signatures of the amount of latent heat that's being released into the atmosphere," said Dr. Marshall Shepherd, a research meteorologist at NASA's Goddard Space Flight Centre

Incorporating rainfall data from TRMM into computer weather models "gives the model a better handle on the energetics that are required to drive the circulation, to drive the hurricane and also affect its path," Shepherd said.

The TRMM satellite can also use its microwave sensors to measure ocean surface temperatures beneath a hurricane.

"Hurricanes are intimately tied to the sea surface temperature," Shepherd said. "There's generally kind of a threshold temperature (above which) hurricanes like to form. If you have all of the other a priori conditions in place, and if you have ample warm sea surface temperature and moisture, then you can get a hurricane that likes to grow," Shepherd said.

Higher sea surface temperatures mean more evaporation of ocean water into the air. As that moisture condenses into clouds, it releases heat to the air that causes the air to rise. The rising air creates a low pressure area beneath it that pulls the surrounding air spiralling inward, perpetuating the hurricane.

"It's that conversion of latent heating that's carried from the water vapour when it condenses to form the clouds in the hurricane -- that's really the fuel supply that powers the hurricane engine,'" Shepherd said. "We tend to think of hurricanes as big heat engines."

Low sea surface temperatures can spell death for a hurricane, as in 1998 when the "wake" of cold water behind Hurricane Bonnie caused Hurricane Danielle, which was following close behind, to dissipate.

Sea-Surface Temperature Measurements

(TMI) sea-surface temperatures - Blues represent cooler water, greens and yellows are warmer water. TMI is the first satellite microwave sensor capable of accurately measuring sea surface temperature through clouds.

Traditional weather satellites that use infrared sensors can also measure sea surface temperature, but "the big advantage that the TRMM microwave imager has ... is that microwave instruments can see through clouds, whereas infrared instruments (on traditional weather satellites) can only give you sea surface temperatures in clear regions," Shepherd said.

While the kind of rainfall and sea surface temperature data produced by TRMM holds great potential for improving hurricane forecasting, TRMM is not primarily a hurricane-monitoring satellite.

"Things like hurricane monitoring ... are extra benefits of the satellite, but its main mission is to measure rainfall," Shepherd said. "TRMM is a research mission -- it wasn't designed to be used in an operational setting. 

"But where the data can be used, I'm sure it is ...." 

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First Science 2014