First direct measurements of enthalpy flux in the hurricane boundary layer: The CBLAST results

Abstract

[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.

Spectral characteristics of turbulence in the hurricane boundary layer over the ocean between the outer rain bands

“…On the other hand, the mesoscale downward sensible heat fluxes suggest that dry air is entrained downward from the flight level above. Zhang et al (2008) showed that the sensible heat fluxes tend to increase linearly with the decreasing height in the mixed layer. Although the buoyancy production or sink is much smaller than the shear production in the turbulent kinetic energy budget, which makes the boundary layer have a nearly neutral condition (Zhang et al, 2009), the subtle change of air-sea temperature difference correlates with the change of sign of the estimated surface sensible heat fluxes very well (Zhang et al, 2008).…”

“…Zhang et al (2008) showed that the sensible heat fluxes tend to increase linearly with the decreasing height in the mixed layer. Although the buoyancy production or sink is much smaller than the shear production in the turbulent kinetic energy budget, which makes the boundary layer have a nearly neutral condition (Zhang et al, 2009), the subtle change of air-sea temperature difference correlates with the change of sign of the estimated surface sensible heat fluxes very well (Zhang et al, 2008). The small values of the surface sensible heat fluxes used in the scaling method and their change of sign according to stability are probably the main reason why the surface scaling cannot be successfully applied to the cospectra of the sensible heat flux.…”

“…A typical flux run has duration of around 3 minutes with horizontal length of around 20 km. To study the spectral characteristics in the atmospheric boundary layer between the outer rain bands, a total of 48 flux runs are used, six of which are excluded for the humidity spectra and 11 of which are excluded for the temperature spectra due to quality-control issues Zhang et al, 2008). The data were obtained with surface wind speeds in the range of 20-30 m s −1 .…”

“…Note that the estimates of the surface values of turbulent fluxes are based on the methods of French et al (2007), Drennan et al (2007) and Zhang et al (2008). The spectra and cospectra are computed using a fast Fourier transform (FFT) algorithm.…”

Sensitivity of tropical‐cyclone intensification to perturbations in the surface drag coefficient

“…To facilitate a proper comparison of the schemes in the four main calculations, the surface drag and heat and moisture exchange coefficients are modified to fit the results of the Coupled Boundary-Layer Air-Sea Transfer experiment (CBLAST: Black et al, 2007;Drennan et al, 2007;French et al, 2007;Zhang et al, 2008). The surface exchange coefficients for sensible heat and moisture are set to the same constant, 1.2×10 −3 .…”