Tropical Storm Kammuri formed in the South China Sea on August 4, 2008, and moved quickly westward, making landfall over southern China’s Guangdong Province on August 6. Heavy rains pounded Hong Kong, interfering with traffic and forcing the stock market to close.
This image of Tropical Storm Kammuri shows observations from the QuikSCAT satellite on August 4 at 6:19 p.m. local time (10:19 UTC), about 7 hours after the storm intensified from a tropical depression to a tropical storm. The image depicts wind speed in color and wind direction with small barbs. White barbs point to areas of heavy rain. The strongest winds were on the eastern side of the storm, raking the coast of the northern Philippines and the eastern South China Sea.
QuikSCAT measurements of the wind strength of Kammuri and other tropical storms 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 kilometers per hour or 58 miles per hour).
Also, the unusually heavy rain found in a tropical storm or 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. Caption by Rebecca Lindsey.