Estimated In-situ Eyewall Turbulence from Aircraft Observations in the South China Sea

There have been very few in-situ observations of eyewall wind fluctuations in the boundary layer. Here we present recent unique boundary layer aircraft observations made by the Hong Kong Observatory for several typhoons in the South China Sea. Data for these flights show many similar features to those observed for North Atlantic Hurricanes many years ago before these types of flights were suspended there. Spectral analysis is performed on the 20 Hz wind data to infer the vertical momentum diffusivity coefficient near 600 m altitude for wind speeds up to 70 m/s in the eyewall. Two different analysis methods were chosen. The variability of the diffusivity coefficient was found to be large at all winds speeds and the magnitude of the coefficient typically increases with wind speed. The calculated diffusivities are sensitive to the chosen analysis method.