Combining airborne remote, in situ, and expendable probe sensors with air-deployed ocean platforms provides a strategy for expanding knowledge of illusive high-wind air-sea fluxes in difficult-to-predict storms.
In this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.
An important outcome from the ONR-sponsored Coupled Boundary Layer Air-Sea Transfer (CBLAST) Hurricane Program is the first-ever direct measurements of momentum flux from within hurricane boundary layers. In 2003, a specially instrumented NOAA P3 aircraft obtained measurements suitable for computing surface wind stress and ultimately estimating drag coefficients in regions with surface wind between 18 and 30 m s Ϫ1 . Analyses of data are presented from 48 flux legs flown within 400 m of the surface in two storms. Results suggest a roll-off in the drag coefficient at higher wind speeds, in qualitative agreement with laboratory and modeling studies and inferences of drag coefficients using a log-profile method. However, the amount of roll-off and the wind speed at which the roll-off occurs remains uncertain, underscoring the need for additional measurements.
[1] Hurricanes extract energy from the warm ocean through enthalpy fluxes. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST) experiment, flights were conducted to measure turbulent fluxes in the high-wind boundary layer of hurricanes. Here we present the first field observations of sensible heat and enthalpy flux for 10m wind speeds to 30 ms À1 . The analyses indicate no statistically significant dependence of these bulk exchange coefficients on wind speed. As a measure of hurricane development potential, we compute the mean ratio of the exchange coefficient for enthalpy to that for momentum and find it to be significantly below the lowest threshold estimated by previous investigators. This suggests that the enthalpy flux required for hurricane development may come from sources other than turbulent fluxes, such as lateral fluxes from the vortex warm core, or sea spray. Alternatively, it demands a re-evaluation of the theoretical models used to derive the threshold.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.