BACKGROUND: Forecasters are testing a new technique called VORTRAC -- Vortex Objective Radar Tracking and Circulation -- that provides a detailed 3D view of an approaching hurricane every six minutes and allows them to determine whether the storm is gathering strength as it nears land. Then they can quickly alert coastal communities if it suddenly strengthens.
HOW IT WORKS: Developed by researchers at the National Center for Atmospheric Research (NCAR), the technique relies on the existing network of Doppler radars along the southeast coast to closely monitor hurricanes winds. Each radar can measure winds blowing toward or away from it, but no single radar could provide a 3D picture of hurricane winds until now. The NCAR scientists developed a series of mathematical formulas that combine data from a single radar near the center of a landfalling storm with general knowledge of Atlantic hurricane structure in order to map the approaching system's winds in three dimensions. The technique also infers the barometric pressure in the eye of the hurricane, a very reliable index of its strength. However, because of the limited range of Doppler radars, VORTRAC works only for hurricanes that are within about 120 miles of land. In the future, it might be possible to use VORTRAC to help improve long-range hurricane forecasts by using data from airborne radars to glean detailed information about a hurricane that is far out to sea.
ABOUT HURRICANES: A hurricane is a type of tropical cyclone, a low-pressure system that usually forms in the tropics and has winds that circulate counterclockwise near the earth's surface. Storms are considered hurricanes when their wind speeds surpass 74 MPH. Every hurricane arises from the combination of warm water and moist warm air. Tropical thunderstorms drift out over warm ocean waters and encounter winds coming in from near the equator. Warm, moist air from the ocean surface rises rapidly, encounters cooler air, and condensed into water vapor to form storm clouds, releasing heat in the process. This heat causes the condensation process to continue, so that more and more warm moist air is drawn into the developing storm, creating a wind pattern that spirals around the relatively calm center, or eye, of the storm, much like water swirling down a drain. The winds keep circling and accelerating to form a classic cyclone pattern.
WHAT IS DOPPLER RADAR: Doppler radar uses a well-known effect of light called the Doppler shift. Just as a train whistle will sound higher as it approaches a platform and then become lower in pitch as it moves away, light emitted by a moving object is perceived to increase in frequency (a blue shift) if it is moving toward the observer; if the object is moving away from us, it will be shifted toward the red end of the spectrum. Doppler radar sends out radio waves that bounce off objects in the air, such as raindrops or snow crystals, and then measures how much the frequency changes in returning radio waves to better determine wind direction and speed.