On the hyperbolicity of the bulk air-sea heat flux functions: Insights into the efficiency of air-sea moisture disequilibrium for tropical cyclone intensification

BENJAMIN JAIMES DE LA CRUZ, LYNN K. SHAY, JOSHUA B. WADLER, JOHNA E. RUDZIN

Research output: Contribution to journalArticlepeer-review

Abstract

Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulas, these fluxes are a function of surface wind speed U10 and air-sea temperature and moisture disequilibrium (ΔT and Δq, respectively). Although many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies have found that TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under conditions of relatively low wind. Herein, a new perspective on the bulk aerodynamic formulas 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 formulas, reported here, are as follows: 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 gkg-1 at moderate values of U10 led to intense inner-core moisture fluxes of greater than 600Wm22 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 easily be achieved as a TC moves over deeper warm oceanic regimes.

Original languageEnglish (US)
Pages (from-to)1517-1534
Number of pages18
JournalMonthly Weather Review
Volume149
Issue number5
DOIs
StatePublished - May 2021
Externally publishedYes

Keywords

  • Atmosphere-ocean interaction
  • Fluxes
  • Hurricanes
  • Marine boundary layer
  • Tropical cyclones
  • Wind

ASJC Scopus subject areas

  • Atmospheric Science

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