Visible satellite image of the tropical disturbance that eventually became Tropical Storm Gabrielle of 2007. At the time, Gabrielle had a well defined surface circulation, but could not develop any heavy thunderstorm activity near the center because strong upper level winds from the southwest blew away the thunderstorms as quickly as they formed. The low level vortex of Gabrielle is stationary, and one can see thunderstorms trying to form at the center get sheared away by strong upper level southwesterly winds at 200 mb. The winds carry high, wispy cirrus clouds from lower left to upper right in the animation. These winds blew at about 20 knots, creating 20 knots of wind shear over the center of Gabrielle (where the winds were calm). Just north of the Gabrielle's center, the flow near the surface at 850 mb (5,000 feet) was out of the northeast at about 10 knots, due to the counter-clockwise flow of air around the center at the surface. The shear was thus 20 knots at 200 mb minus negative 10 knots at 850 mb (since the 850 mb winds were blowing in the opposite direction as the 200 mb winds), for a total wind shear of 30 knots. To the south of the vortex, the 850 mb flow was out of the southwest (same direction as the 200 mb flow), so the shear was 20 knots minus positive 10 knots, for a total of 10 knots of shear. The thick band of clouds extending from southwest to northeast is due to the presence of an upper level trough of low pressure. Image credit: University of Wisconsin CIMSS satellite blog.

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