Modeling of the drag and thermal conductivity coefficients in tropical cyclones taking into account foam cover

TL;DR

This paper develops a model for calculating drag and thermal conductivity coefficients in tropical cyclones, accounting for foam cover effects on air-sea transfer processes, using a formal averaging approach and similarity theory.

Contribution

It introduces a novel averaging procedure for transfer coefficients over foam-covered and foam-free sea surfaces, estimating foam-covered transfer coefficients from splitting relations and measurements.

Findings

Transfer coefficients split into foam-free and foam-covered parts.

Estimated foam-covered transfer coefficients from measurements and relations.

Explained anomalous transfer coefficient behavior with wind speed under hurricane conditions.

Abstract

A formal averaging procedure over the air-sea interface is developed for both momentum and enthalpy surface-transfer coefficients, C_D and C_K, in hurricane conditions. This leads to splitting of both the transfer coefficients across the total area of the sea surface, C_(D,K), into the sums of their partial values over the foam-free, C_(D,Kw), and foam-covered, C_(D,Kf), fractions weighted with foam- and water-coverage coefficients {\alpha}f and 1-{\alpha}f . The transfer coefficients, C_(D,K) and C_(D,Kw), are estimated by measurements in open-sea and in laboratory conditions, respectively, while the transfer coefficients across the foam-covered fraction, C_(D,Kf), which cannot be measured directly, are estimated from the splitting relations. Applying the Monin-Obukhov similarity theory at the neutral stability atmospheric conditions to the transfer coefficients, separately to the foam-free, the foam-covered, and the total sea surfaces, yields the roughness lengths, Z_(D,Kw),Z_(D,Kf),Z_(D,K). The study is aimed at explaining an anomalous behavior of the momentum and enthalpy transfer coefficients, C_D and C_K, with wind speed U10 under hurricane conditions, by the effect of the foam slipping layer sandwiched between the atmosphere and the ocean.