On the Hyperbolicity of the Bulk Air-Sea Heat Flux Functions: Insights into the Efficiency of Air-Sea Moisture Disequilibrium for Tropical Cyclone Intensification

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  • 1 Rosenstiel School of Marine and Atmospheric Science, Department of Ocean Sciences, University of Miami, Miami, Florida
  • 2 NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division
  • 3 Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida
  • 4 National Research Council, Naval Research Laboratory, Monterey, California
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Abstract

Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulae, these fluxes are a function of surface wind speed (U10) and air-sea temperature and moisture disequilibrium (ΔT and Δq, respectively). While many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies found TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under relatively low-wind conditions. Herein, a new perspective on the bulk aerodynamic formulae is introduced to evaluate the relative contribution of wind-driven (U10) and thermodynamically-driven (ΔT and Δq) ocean heat-uptake. Previously unnoticed salient properties of these formulae, reported here, are: (1) these functions are hyperbolic; and, (2) increasing Δq is an efficient mechanism for enhancing the fluxes.

This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat-uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δq>5 g kg-1 at moderate values of U10 led to intense inner-core moisture fluxes >600 W m-2 during RI. Peak values in Δq preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δq is a very effective way to increase surface heat fluxes—this can be easily achieved as a TC moves over deeper warm oceanic regimes.

Corresponding author address: Benjamin Jaimes de la Cruz, Rosenstiel School of Marine and Atmospheric Science, Department of Ocean Sciences, University of Miami, 4600 Rickenbacker Causeway, Miami, FL, 33149. E-mail: bjaimes@rsmas.miami.edu

Abstract

Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulae, these fluxes are a function of surface wind speed (U10) and air-sea temperature and moisture disequilibrium (ΔT and Δq, respectively). While many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies found TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under relatively low-wind conditions. Herein, a new perspective on the bulk aerodynamic formulae is introduced to evaluate the relative contribution of wind-driven (U10) and thermodynamically-driven (ΔT and Δq) ocean heat-uptake. Previously unnoticed salient properties of these formulae, reported here, are: (1) these functions are hyperbolic; and, (2) increasing Δq is an efficient mechanism for enhancing the fluxes.

This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat-uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δq>5 g kg-1 at moderate values of U10 led to intense inner-core moisture fluxes >600 W m-2 during RI. Peak values in Δq preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δq is a very effective way to increase surface heat fluxes—this can be easily achieved as a TC moves over deeper warm oceanic regimes.

Corresponding author address: Benjamin Jaimes de la Cruz, Rosenstiel School of Marine and Atmospheric Science, Department of Ocean Sciences, University of Miami, 4600 Rickenbacker Causeway, Miami, FL, 33149. E-mail: bjaimes@rsmas.miami.edu
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