Typhoon Man-yi formed as a tropical depression in the western Pacific on July 8, 2007, among the islands of Micronesia. The storm gradually built power to typhoon status on July 10. As of July 11, forecasters were calling for Man-Yi to strengthen to Category 4 (Super Typhoon) status, with sustained winds in the storm’s core predicted to reach at least 210 kilometer per hour (131 miles per hour).
This data visualization of the storm shows observations from the QuikSCAT satellite on July 9, 2007, at 20:58 UTC (8:58 a.m. local time, July 10). At this time, Man-Yi appeared as a well-developed storm system, but had not yet reached typhoon status; its peak winds were around 110 km/hr (65 mph; 60 knots). The image depicts wind speed in color and wind direction with small barbs. White barbs point to areas of heavy rain.
QuikSCAT measurements of the wind strength of Man-Yi and other tropical cyclones can be slower than actual wind speeds. QuikSCAT’s scatterometer sends pulses of microwave energy through the atmosphere to the ocean surface and measures the energy that bounces back from the wind-roughened surface. The energy of the microwave pulses changes depending on wind speed and direction.
To relate the radar signal to actual wind speed, scientists compare measurements taken from buoys and other ground stations to data the satellite acquired at the same time and place. Because the high wind speeds generated by cyclones are rare, scientists do not have corresponding ground information to know how to translate data from the satellite for wind speeds above 50 knots (about 93 km/hr or 58 mph).
Also, the unusually heavy rain found in a cyclone distorts the microwave pulses in a number of ways, making a conversion to exact wind speed difficult. Instead, the scatterometer provides a nice picture of the relative wind speeds within the storm and shows wind direction.
NASA image courtesy of David Long, Brigham Young University, on the QuikSCAT Science Team, and the Jet Propulsion Laboratory.