Measuring typhoonsJan 1, 2003
|From Ocean Navigator #58 |
Finding a typhoon in the western North Pacific is fairly easy because there are plenty of them to go around: an average of more than 25 reach tropical storm strength. In a typical year, with 81 such storms recorded, Western North Pacific typhoons account for nearly one-third of the world total.
However, the frequency of typhoons (from the Chinese tai-fung, or "great wind") is not their only claim to fame. Another reason to take note of typhoons is their ferocity. More than 70% of all tropical storms in the western North Pacific continue to mature into full-blown typhoons with winds greater than 64 knots. In the North Atlantic 55% of tropical storms become hurricanes. Approximately 36% of storms in the rest of the world develop into more powerful systems.
Of the 18 or so typhoons per year, five of them don't stop until their winds exceed 130 knots, or 150 miles per hour. These are known as supertyphoons to forecasters at the National Weather Service.
With so many more storms, there are months of great concentrations of typhoons. From June through November, there is an average of more than one per month. In July and August there are more than four each month, or one per week.
Because of their frequency, ferocity, and their impact on the heavily-traveled shipping lanes in the western North Pacific, predicting the occurrence of typhoons has great value. Meteorologists have devised a number of methods for predicting these powerful low pressure areas.
Within the past year, a new technique has been put forward by Ray Zehr at Colorado State University. This compares data from weather balloons and satellites (and aircraft, when available) to information in a computer database. Weather balloons, while they may seem old fashioned, provide the bulk of the useful information. And like a fleet of anchored weather ships, the Pacific is dotted with island weather stations that release instrument-carrying balloons twice each day. This steady flow of tangible information has proven valuable to Zehr. "The operational models look at 850-millibar (mb) and 200-mb wind fields," said Zehr. "We measure three components: relative vorticity, convergence, and vertical wind shear difference between 200 mb and 850 mb. We need a favorable reading from all three for a tropical storm to develop." Relative vorticity (how rapidly the air mass is rotating) should be high in a developing tropical storm. Low-level convergence between two air masses, which is poorly measured according to Zehr, is ascertained at 850 mb and should also be high. The results give forecasters 24 to 48 hours notice that a tropical disturbance will likely become a tropical depression.
Zehr's technique is not yet used to help determine whether a tropical disturbance will develop into a tropical storm or a typhoon. At this stage in its development, the technique seems capable of predicting only the much larger category of tropical depression.
One of the criteria that Zehr focuses on is wind sheer. The lower this value, the more stable the vertical air column. If the air column is stable then the rising air spirals around a center. This small, circular eye is a defining characteristic of typhoons (and hurricanes). Indeed, a developed typhoon is basically symmetrical with an eye in the middle of a circular cloud system, although in some cases the eye may wobble from the center. The eye - typically only five to 20 miles in diameter - contains cooled, dry air descending from high altitudes and observers in the eye often report sunny, mild weather. (Sea birds are known to circle within the eye, endlessly avoiding the hurricane-force wind in the eye wall.)
For about 20 years, a system called the Dvorak Scale has been used to analyze clouds as a way to gauge the current state of a storm. This approach works with either visible light or infrared satellite images. "In general, visible light images give a clearer definition to the storm center," said Jim Lynch, a branch chief at the NOAA's Synoptic Analysis Branch in Washington. Infrared images have the advantage of being available 24 hours each day. "The scale requires a complex series of information. These might include a measure of the storm's spiral bands, as well as how far they wrap around the storm center. More wrapping indicates greater storm intensity. We also need a description of the storm eye, including its size and shape, and whether it is centered," elaborated Lynch. Using the Dvorak Scale, analysts are able to determine both storm location and its intensity. It is a standard technique adopted by meteorologists throughout the world. Its great advantage is that by using satellite images, meteorologists may make fairly accurate assessments of a storm even without any information obtained by aircraft flying into the storm. Such aircraft are typically only available in the western North Atlantic. While the Dvorak Scale will continue to be used, additional information can now be added to the mix.
This added data is provided by a special sensor microwave identifier (SSMI) which is carried on polar-orbiting Defense Department satellites. The SSMI package actually contains seven sensors operating on four frequencies. Passive microwave sensors, they measure microwaves generated by another source. (Radar, by contrast, is an active microwave system in that it both transmits and receives microwaves.) "A combination of information from four sensors is used to determine wind strength," said Ed Rappaport, a hurricane specialist at the National Hurricane Center. "As wind strength increases, so do the capillary waves on the sea surface. This increased activity shows up in the SSMI data. This data has been calibrated by comparing readings with known wind speeds determined by NOAA buoys." Capillary waves are the small wind ripples seen on the surface of larger waves which build up over time.
"One of the advantages of SSMI is that one of its sensors has the ability to penetrate high-level cirrus clouds. This allows us to look at the skeletal form of a tropical cyclone which is typically obscured in visible light and infrared images," said Rappaport. Also of great value to typhoon analysts is a different combination of SSMI data which reports intensity of precipitation. This allows an analyst to determine the parameters of the storm's core, said Rappaport.
The primary drawback to this technology is that it has difficulty determining wind speed in areas of heavy precipitation. "The signals received by SSMI are a combination of radiation created by rainfall and that generated by wind," said Rappaport. While SSMI cannot sort out which microwaves are generated from which source, it is still valuable for analyzing hurricanes.
"While it cannot sense microwaves through thunderstorms, SSMI is capable of receiving microwaves through the clear patches between the spiral arms of a typhoon. This can be used to determine the periphery of storm winds,"said Lynch. "It has limitations for determining core wind speed of tropical storms in areas with more than 35 to 40 knots of wind," added Rappaport.
Practically speaking, this information is now used with the Dvorak Scale. "These are complementary techniques, "said Rappaport. "Dvorak gives the highest wind speed and extreme atmospheric pressure. SSMI gives an accurate distribution of wind and rain around the storm."
These tools provide analysts with means to determine some facts about a particular tropical storm. These can then be applied to theoretical models resulting in a clear picture of a storm's status.
There are many ways of trying to describe the power of a typhoon. Anyone who has lived through ferociously high winds can visualize their strength. However, typhoons also move tremendous quantities of air to the upper atmosphere - 3,500,000,000 tons each hour estimates Richard Anthes in his book Tropical Cyclones: Evolution, structure and their effects. Bad storms, perhaps of the magnitude of a supertyphoon, generate a tremendous amount of power: 1025 ergs per second, writes Anthes. To put that into perspective, 1025 ergs per second is roughly equivalent of 1.34 quadrillion horsepower, or 2.78 x 1011 kilowatt hours. This last figure is approximately 22,000 times the power Consolidated Edison supplies to New York City each year.Cameron Bright