Hurricanes and Typhoons


 Each year dozens of tropical cyclones spin up sustained winds of 74 miles an hour (119 kilometers an hour). These are called hurricanes in the Atlantic and northeast Pacific and typhoons in the western Pacific. By any name, they're the most destructive storms on Earth. Instruments and computer models developed at NCAR are helping to track and predict these tropical tempests more accurately than ever.

Most tropical cyclones begin as one of hundreds of disturbances that scoot westward across the tropical Atlantic and Pacific each summer and autumn. Others are spawned at the tail end of weakening cool fronts. Their fate and frequency are influenced by large-scale patterns that affect wind and ocean circulations, such as El Niño and the Madden-Julian Oscillation. Forecasters can spot the large-scale weather features that might encourage a tropical disturbance to form a low-level circulation and grow. However, scientists are not sure why only some of these disturbances reach their potential to become tropical cyclones.

Once a tropical storm begins to form, its increasing winds pull in moisture evaporating from the sea surface, with some of that moisture converted into heat within thunderstorms. That heat drives the inflow further, resulting in greater evaporation from the ocean and producing feedback that can lead to a hurricane or typhoon. Bands of thunderstorms spiral toward the eye, a column of descending air at the heart of the cyclone. In a classic hurricane, the eye is symmetrical, clear, and calm, as high winds scream around and up the sides of it.

The size of a hurricane's eye and the area covered by its spiral bands are not related to the hurricane's intensity. Some of the most destructive U.S. hurricanes, such as 1992's Andrew, are relatively small. Others, such as 2005's Katrina, were quite large.

Hurricanes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Across the globe, most hurricanes start out moving toward the west, then gradually curve poleward and eastward as they reach the midlatitudes. Landfalls are most likely along the vulnerable east coasts of North and Central America, Asia, and Australia and the islands of the tropical Atlantic and Pacific. However, many tropical cyclones never make landfall.

Research


Hurricane Ioke (shown here at peak intensity) was the most powerful hurricane ever recorded in the central Atlantic. The 2006 cyclone reached Category 5 status three times—twice as a hurricane and once as a typhoon after it crossed the International Date Line. It was the first tropical cyclone in global records to sustain Category 4 strength for more than eight consecutive days. (Image courtesy Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC.) When a hurricane is approaching the United States, "hurricane hunter" aircraft make frequent passes in and near the storm to profile its features and movement. One of the key tools they use is a dropsonde—a parachute-borne instrument packet.

The design of the current GPS dropsonde was created at NCAR in the 1990s; it takes advantage of the Global Positioning System to glean accurate, high-resolution measurements. On each flight, dozens of dropsondes are sent into the heart of the hurricane to measure winds, air pressure, and humidity. Some flights send dropsondes through the storm's outer reaches to measure the steering currents that influence the hurricane.

Some hurricane flights are devoted to research. NCAR scientists have taken special devices to measure the ice crystals and water droplets swirling through a hurricane. One such flight, in 2001, was part of NASA's CAMEX project. Researchers captured data from Hurricane Humberto that showed far more ice crystals than expected near the top of the storm. Another set of research flights, in 2005, examined how the outer rainbands and inner eye of a hurricane interact to influence the storm's intensity. One of these flights for RAINEX observed highly complex structures and winds topping 150 mph (240 kph) in two of the rainbands spiraling into Hurricane Katrina.

Since the mid-1990s, several truck-mounted Doppler on Wheels radars developed by NCAR researchers and collaborators have been deployed in or near landfalling U.S. hurricanes. A rapid-scan version of DOW bombards storms with six simultaneous beams to collect a three-dimensional picture of the atmosphere every 10 to 15 seconds.

NCAR is part of a team developing the Weather Research and Forecasting computer model (WRF). A variant of WRF (pronounced Worf) is expected to become one of the key models for hurricane prediction, and the NCAR-based Advanced Research WRF provides five-day outlooks with a resolution as fine as 1.33 km (0.8 mi) near hurricanes and other areas of interest. Through the multiagency U.S. Weather Research Program, NCAR and collaborators are looking at ways to improve forecasts of changes in hurricane intensity as well as wind, waves, and rain at landfall.

Research has blossomed over the last several years on the links between hurricanes and global warming. Much of the work involves scientists at NCAR and UCAR universities. Several studies point to a global increase in the proportion of tropical cyclones that reach intense levels, especially in the Atlantic. However, data gaps prior to the 1970s make it more difficult to draw firm conclusions. (See the Related News Releases section of this page for coverage of recent studies.)

Prediction and safety


This sunrise launch in Wyoming was part of driftsonde testing during the summer of 2006. The innovative NCAR-designed driftsonde package, which includes a gondola and weather instruments that descend by parachute, was first used for research in Africa in August 2006 with a ballooning system designed by the French space agency, CNES. (©UCAR, photo by Joe VanAndel.) Hurricane-track forecasts improved by about 1 to 2% per year from the 1960s to the 1990s. Since 2000, the enhanced wind data from GPS dropsondes, along with refinements to computer models, have produced a leap forward in the prediction of hurricane tracks. However, changes in intensity remain difficult to predict.

U.S. death tolls from hurricanes have dropped markedly in recent decades, thanks to satellite imagery and improved warning techniques. Some cities remain quite vulnerable, though, as illustrated in 2005 when Hurricane Katrina produced catastrophic damage and horrific suffering in Louisiana and Mississippi. After the storm, two of UOP's education-oriented programs provided teachers across the region with online materials and special workshops. NCAR social scientists collaborated with faculty and undergraduates in Texas to study public attitudes around warnings for Hurricane Rita, also in 2005.

In 2007, forecasters tested a technique pioneered at NCAR, called VORTRAC, that uses existing Doppler radars to provide a detailed 3-D view of an approaching hurricane every six minutes. This could enable forecasters to better assess whether a hurricane is intensifying in the critical 10-15 hours before landfall.

Many hurricanes that strike North America are born in the eastern tropical Atlantic, beyond the reach of U.S. hurricane-hunter planes. NCAR teamed with French ballooning experts to develop the driftsonde, a high-flying balloon that can ferry dozens of instrument packages into otherwise hard-to-reach locations. In its debut mission, the driftsonde gathered promising data in a series of test flights from Africa across the Atlantic in summer 2006. New observing devices such as the driftsonde may help better observe and predict the formation of embryonic cyclones that could become hurricanes.

Some of the greatest hurricane damage occurs from flooding after landfall, when the winds typically weaken but heavy rainfall may continue. After a decaying Hurricane Mitch moved into Honduras and Nicaragua in October 1998, some 10,000 people died in related flooding. In June 2001, the remnants of Hurricane Allison resulted in the most extensive flooding ever associated with a U.S. tropical storm. More than 36 inches (over 900 centimeters) of rain fell in Houston. Allison killed more than 40 people and left more than $4 billion in damage.

thunderstorms • hail • lightning • tornadoes • floods • hurricanes • winter storms