Mark A. Bourassa#, Dayton G. Vincent*, W. L. Wood+
# Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee 32306, USA.
* Department of Earth and Atmospheric Sciences, Purdue University, Indiana 47907, USA.
+ School of Civil Engineering, Purdue University, Indiana 47907, USA.
submitted to Journal of Physical Oceanography
ABSTRACT
A simple coupled flux and sea state model that is developed. It is applicable to low and high wind speeds, with a non-arbitrary wave age, and full consistency with an atmospheric flux parameterization. The flux model includes the influence of capillary waves (surface tension) on surface stress. The coupled model is verified for conditions of local equilibrium, and it is used to examine the influences of surface tension and capillary waves on the equilibrium sea state at low wind speeds (U10 < 7 m s-1). Capillary waves can directly influence characteristics of the airflow (roughness length and friction velocity) and surface tension can directly influence wave characteristics (period, phase speed and wave age). The influences of capillary waves and surface tension are found to be noticeable for U10 < 7 m s-1, and are likely to be significant in most applications when U10 < 5 m s-1. The conditions under which relations valid at higher wind speeds break down are discussed. The sea state parameterization is an improvement over previous relations (which apply well for U10 > 7 m s-1) because it is also applicable for low winds. The mean wind speed over most of the world's oceans is less than 5 m s-1, so there is considerable need for such a parameterization. The non-arbitrary wave age is particularly important because of the strong influence of wave age on the shape and size of waves, as well as fluxes of momentum, heat, and moisture.
Additional results include a criterion for the capillary cutoff, significant wave heights for local wind-wave equilibrium, and dominant wave periods for low wind speeds. The capillary cutoff criterion is developed using the modeled phase speed. Consistency with a flux parameterization is crucial because of the influence of atmospheric stability on the capillary cutoff and other wave characteristics. The modeled significant wave height for local equilibrium is shown to be consistent with several parameterizations. The modeled dominant period is a good match to that of the Pierson-Moskowitz spectrum when U10 > 7 m s-1. For lower wind speeds, the modeled period differs significantly from that of the Pierson-Moskowitz spectrum. The differences are largely due to the capillary cutoff, which was not considered by Pierson and Moskowitz.
