Potential low bias in high-wind drag coefficient inferred from dropsonde data in hurricanes

Understanding momentum exchange at the air-sea interface is important for accurate hurricane predictions and understanding fundamental storm dynamics. One method for estimating air-sea momentum transfer in high winds is the flux-profile method, which infers surface momentum fluxes and the corresponding drag coefficient from mean velocity profiles obtained from either dropsondes or meteorological towers, under the assumption that the boundary layer wind profile at low altitudes exhibits a logarithmic profile with height. In this study, we use dropsonde data from reconnaissance aircraft, as well as "virtual sondes" from a turbulence-resolving simulation of an intense tropical cyclone, to critically analyze the diagnosis of drag coefficient C-D at hurricane-force wind speeds. In particular, the "rolloff" of the drag coefficient, where C-D decreases at 10-m wind speeds > 35 m s(-1), is called into question based on uncertainty due to relatively low sample size and a lack of robustness of the flux-profile method at high winds. In addition, multiple factors appear to favor an underestimate of C-D at hurricane-force winds relative to their true values, including uncertainty in the height of recorded dropsonde data, in violation of Monin-Obukhov similarity theory near the eyewall, and the short vertical extent of the logarithmic layer. Due to these and other related sources of uncertainty, it is likely that a quantitative limit has been reached in inferring the specific values of u(*) and C-D using the flux-profile method, while at the same time the potential for underestimation may cast doubt on the C-D-U-10 relationship inferred from this method at high winds.