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Drag Coefficient Distribution and Wind Speed Dependence in Tropical Cyclones

Mark Powell mark.powell@noaa.gov

NOAA/AOML-HRD

This JHT-sponsored project seeks to investigate the azimuthal dependence of the surface drag coefficient in hurricanes as well as extend Cd measurements in mean boundary layer (MBL) winds over 70 m/s. Over 2600 post-processed GPS sonde profiles collected in hurricanes from 1997-2005 were stored in a modern relational database, quality controlled, and organized by mean boundary layer wind speed and storm relative location. Analysis indicates that Cd initially increases with 10 m neutral stability wind speed, levels off at 27 m s-1, and then decreases as winds increase above 42 m s-1. The apparent wind speed dependence is complicated by an additional dependence on radial distance, with profiles at small radii tending to have smaller Cd values for the same MBL wind speed group. Sea state at small radii would be associated with more fetch-limited conditions than with larger radii, consistent with the "continuous breaking" mechanism hypothesized by Donelan and colleagues in 2004 to lead to flow separation and decreased Cd. Storm relative azimuthal dependence further complicates the Cd behavior. While Cd values in the rear of the storm tend to have larger values than the right side, there is no well defined Cd dependence on azimuth.

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