On October 5, 2009, Typhoon Parma was parked over the South China Sea off the northern tip of Luzon Island, the Philippines. Though the storm had degraded into a tropical storm, with relatively weak winds of 100 kilometers per hour (60 miles per hour or 55 knots), Parma had almost no forward motion, said the Joint Typhoon Warning Center. This image, made from data from NASA’s QuikSCAT satellite taken on October 5, shows that the storm was primarily over open water, but areas of rain and wind still covered parts of Luzon and Taiwan.
The image illustrates wind speed in color and wind direction with small barbs. Areas where winds were accompanied by heavy rain are depicted with white barbs. Surrounding the eye of the storm, wind speeds approached 50 knots, and similarly strong winds were rushing into toward the eye through the Taiwan Strait, in the northwest quadrant of the storm.
QuikSCAT monitors wind speed with a radar that sends out pulses of microwave energy and listens for the echo after the pulse bounces off the wind-roughened ocean surface. Scientists translate radar signals into estimates of wind speed by matching the radar echoes to physical measurements collected from buoys at the same time and place.
Cyclone-strength wind speeds are rare, however, and scientists generally don’t have enough matching buoy observations to convert wind speeds above roughly 50 knots. Intense rain rippling the ocean’s surface can also interfere with the radar signal. Because of these limitations, QuikSCAT images don’t show absolute, maximum wind speeds. Instead, they give forecasters a valuable picture of the wind structure within the storm, for example, revealing whether a storm has a strong or a weak eye and how large an area is experiencing tropical-storm-strength winds.
NASA image courtesy of David Long, Brigham Young University, on the QuikSCAT Science Team, and the Jet Propulsion Laboratory. Caption by Holli Riebeek.
