Variations in Momentum Flux and Surface Drag Estimates in Hurricane Lee (2023)

Momentum exchange between the atmosphere and ocean is important in determining tropical cyclone (TC) intensity change (i.e., rapid intensification), but there is uncertainty in the processes governing this exchange in high wind environments and how these processes may vary spatially within a TC. It is well-agreed upon that surface drag has a direct relationship with wind speed, in that the surface drag coefficient increases as wind speed increases up to a certain threshold before leveling off or decreasing as wind speed continue to increase. Surface drag is also affected by wave characteristics, which vary across different regions of a TC based on the interaction between the ocean surface and the wind direction and direction of storm motion. To better understand the processes influencing surface drag in a TC and how these processes may vary across the storm due to wave characteristics, we estimate momentum flux across different quadrants of Hurricane Lee (2023) utilizing atmospheric dropsondes and surface wave height observations. Since the measurements of surface drag presented in Richter et al. (2021) do not consider the effect of waves, we compare momentum flux and surface drag calculations for Hurricane Lee to their larger data set to further identify if and how momentum flux variability could be constrained in different regions of the TC utilizing wave height measurements. Understanding the reason(s) for such differences could advise improvements to the sampling scheme or suggest additional atmospheric and/or oceanic processes be considered in the calculation of surface drag and the total momentum flux.