Effect of surface waves on Charnock coefficient under tropical cyclones

Il‐Ju Moon,Tetsu Hara,Isaac Ginis

doi:10.1029/2004gl020988

Abstract

The dependence of the air‐sea momentum flux on surface wave fields is investigated at very high winds under tropical cyclones. A coupled wave‐wind model is applied to estimate the momentum flux under ten hurricanes in the western Atlantic Ocean during 1998–2003. The model explicitly calculates the wave‐induced stress vector and the total wind stress vector from a given wind speed vector and a calculated wave spectrum. It is found that the neutral drag coefficient levels off at high wind speeds under tropical cyclones, being consistent with recent observations and previous modeling studies. The most important finding of this study is that the Charnock coefficient is mainly determined by two parameters: the input wave age (wave age determined by the peak frequency of wind energy input) and the wind speed, regardless of the complexity of the wave field under a real hurricane, and that the Charnock coefficient increases with the input wave age at very high winds.

Highlights

[3] Recently, Moon et al [2004a] investigated the dependence of the Charnock coefficient on the wave age at high winds using a numerical model
The wave field under a hurricane is extremely complex, the study showed that the Charnock coefficient mainly depends on the same two parameters, the wind speed and the wave age, as in steady uniform wind forcing provided that the wave age is replaced by the input wave age [Tolman, 2002]
This study examines whether the dependence of the Charnock coefficient on the wind speed and the input wave age is robust under a wide range of wind and wave conditions encountered during real tropical cyclone events

Summary

Introduction

[3] Recently, Moon et al [2004a] investigated the dependence of the Charnock coefficient on the wave age at high winds using a numerical model. The wave field under a hurricane is extremely complex, the study showed that the Charnock coefficient mainly depends on the same two parameters, the wind speed and the wave age, as in steady uniform wind forcing provided that the wave age is replaced by the input wave age [Tolman, 2002]. The latter is defined as the peak frequency of the positive part in the input source term of the WAVEWATCH III model. Air-sea momentum flux that can be used under a wide range of wind forcing including hurricane winds

Objectives

Conclusion