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01 October 2003

Hurricanes: Sensing the future

It's hurricane season in Houston, along the Gulf coasts stretching to Mexico, in the Atlantic Ocean, and north to the Carolinas and the mid-Atlantic region of the U.S. Storm-borne destruction has struck all along this shoreline within most residents' memories.

Of course, it is those who live on the coast that heed warnings regarding approaching storms. Hurricanes lose strength once they pass off the ocean over land, which has led to a misguided sense of security by people living inland who think that hurricanes are beachfront dangers.

The News & Observer in Raleigh reported that over the years, emergency management officials in North Carolina have been mostly successful in persuading people on barrier islands and in oceanfront homes to take off when hurricanes come calling.

According to the National Weather Service office in Raleigh, storm surge caused only one death in the state between 1970 and 1999.

But the surge also creeps miles inland, wreaking havoc in estuaries and the tangle of streams that feed them. It is the people who live along those waterways that emergency officials really worry about.

Knowing this, scientists at North Carolina State University (NCSU) have teamed with partners at the University of South Carolina and University of North Carolina-Wilmington to improve their storm surge forecasts.

The group secured a two-year, $5 million appropriation from the National Oceanic and Atmospheric Administration to put out moorings and work on computer models.

The instruments that piggyback the moorings will offer accurate information on the wind, waves, and currents, allowing scientists and forecasters to understand the ocean as never before.

The moorings include a critical $25,000 piece of equipment called the acoustic Doppler current profiler (ADCP).

The ADCP, anchored on the ocean floor, sends sound waves up, bouncing off plankton and fish, to tell scientists how fast and in which direction the water is moving, buffeted by wind, tides, pressure, and the Gulf Stream.

The data conveys by wire from the sea floor to a radio transmitter attached to a buoy near or on the surface of the ocean. There it transmits via low-orbit satellite to university computers where the measurements process through to produce color-coded models that locate storm surge activity.

Len Pietrafesa, a fluid physicist at NCSU, and his colleagues say they can plug the numbers into a computer, update the information every two hours or so, and predict, almost by city block, where a storm surge is likely to hit.

At the heart of the networking and timing logistics of this sensing and prediction mechanism is the Doppler effect or the Doppler shift.

Doppler shift occurs when sound comes from, or reflects off of, a moving object. Extreme Doppler shifts are sonic booms. The classic example explaining Doppler shift involves a train whistle or car horn and its changing tone as it approaches the listener, passes, and moves off.

HowStuffWorks.com offered this scenario:

Say there is a car coming toward you at 60 miles per hour (mph), and its horn is blaring. You will hear the horn playing one note as the car approaches, but when the car passes you the sound of the horn will suddenly shift to a lower note.

It is the same horn making the same sound the whole time. The change is a result of Doppler shift.

Here's what is happening. The speed of sound through the air is fixed. For simplicity of calculation, let's say it is 600 mph (the exact speed requires calculation using the air's pressure, temperature, and humidity).

Imagine that the car is standing still exactly 1 mile away from the listener, and it toots its horn for exactly one minute. The sound waves from the horn will propagate from the car toward the listener at a rate of 600 mph.

What the listener will hear is a six-second delay (while the sound travels 1 mile at 600 mph), and then exactly one minute's worth of sound.

Now let's say that the car is moving toward the listener at 60 mph. It starts from a mile away and toots its horn for exactly one minute. The listener will still hear the six-second delay.

However, the sound will only play for 54 seconds. That is because the car will be right next to the listener after one minute, and the sound at the end of the minute gets to the listener instantaneously.

The car (from the driver's perspective) is still blaring its horn for one minute. Because the car is moving, however, the minute's worth of sound packs into 54 seconds from the listener's perspective.

The same number of sound waves packs into a smaller amount of time.

Therefore, their frequency is increased, and the horn's tone sounds higher to the listener. As the car passes and moves away, the process reverses, and the sound expands to fill more time. Therefore, the tone is lower.

One can combine echo and Doppler shift in the following way.

Say the listener sends out a loud sound toward a car moving toward her. Some of the sound waves will bounce off the car—an echo. Because the car is moving toward the listener, however, the sound waves will compress.

Therefore, the sound of the echo will have a higher pitch than the original sound she sent. If she measures the pitch of the echo, she can determine how fast the car is going.

The Doppler effect also has applications in medicine, weather studies beyond this example, and in missile and airplane detection.

In medicine, a cardiologist might perform a Doppler flow study to examine the ejection fraction of blood from a patient's heart. This is actually a sort of radar, where sound waves from a stationary source reflect from moving blood cells.

In meteorology, the Doppler effect measures wind speeds in tornadoes and hurricanes.

In aircraft, it detects clear air turbulence.

In industry, the Doppler effect serves to analyze turbulence in flowing fluids. IT

Nicholas Sheble writes and edits the Sensors and Technology Advances department. Write him at nsheble@isa.org