Sub-tropical Storm Laura

Sub-tropical Storm Laura

Laura formed as a slightly unusual sub-tropical storm in the northern mid-Atlantic on September 29, 2008. Subtropical storms are hybrids of tropical storm systems and extratropical storms, forming in cooler waters farther north (or south) of the usual hurricane formation regions. Laura formed quite far north, well away from major land masses. As of September 30, the National Hurricane Center predicted that the storm could hit Scotland by October 4. With peak sustained winds of around 95 kilometers per hour (55 miles per hour), Laura was a powerful storm, but not hurricane strength. It was unlikely that the storm would ever become a hurricane, but forecasters expected the storm to remain near its September 30 strength for several days.

This data visualization was made with observations from the QuikSCAT satellite taken on September 30, 2008, at 5:39 a.m. local time (7:39 UTC). It shows Laura as a tight ball of circular winds around a calmer core. 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 Laura and other cyclones (the generic term for typhoons, hurricanes, and similar 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 km/hr or 58 mph). Peak winds in Laura measured in other ways are near this extreme limit, so values from QuikSCAT may not be reliable near the core of the storm system.

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. Caption by Jesse Allen.

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