Airflow separations over wind waves and their impact on air-sea momentum flux

Tetsu Hara,

This project is funded by the National Science Foundation

In collaboration with: Fabrice Veron (University of Delaware) and Peter Sullivan (NCAR)


When a surface ocean wave field is not in equilibrium with local wind forcing, which is a common occurrence, the wind stress may deviate significantly from the bulk parameterization and require sea-state dependent parameterization. Recent modeling studies suggest that one of the most significant causes of the sea-state dependence is the enhanced form drag due to airflow separation over breaking waves, particularly at higher wind speeds. However, the uncertainties in these model results remain large because our understanding of airflow separation is limited. While it has been generally accepted that airflow separation only occurs over breaking waves, recent laboratory observations and Large Eddy Simulations (LES’s) show that transient separation-like flows (quasi-separations), characterized by high vorticity layers detached from the wave surface and separation bubbles below, are ubiquitous and may occur over steep but non-breaking waves. In this project, laboratory observations and LES are closely combined to study airflow separation/ quasi-separation events and their impact on air-sea momentum flux. We hypothesize that: (a) airflow separation/quasi-separation significantly modifies the wave form drag, the near surface turbulence, and the air-sea momentum flux, and that (b) LES can reproduce realistic airflow fields provided the wave shape/speed, the surface velocity field, and the surface roughness distribution are accurately specified based on observations. To test these hypotheses, we will (a) carry out combined laboratory observations and LES of wind over a finite amplitude wave train, providing accurate air-water interface boundary conditions to the LES based on observations, and validating the LES results of airflow turbulence against observations, and (b) quantify the occurrence of airflow separation/quasi-separation and the resulting impact on wave form drag and air-sea momentum flux.

Intellectual Merit:

Once the LES methodology of wind over waves is validated against laboratory observations in this study, the LES can be extended to open ocean conditions, with multi wave components (short crested waves), and with misaligned wind and waves. There is a growing interest in coupling ocean surface wave models with atmospheric and ocean models from global/climate scales to regional/synoptic scales. The proposed activity and its extension to the open ocean conditions will contribute to development of accurate parameterizations of sea state dependent air-sea momentum flux, which may be immediately incorporated in the ongoing coupling efforts for atmospherewave-ocean tropical cyclone and climate models in our research group as well as in the community at large. This study will improve heat and humidity flux parameterizations as well, because airflow separation/quasi-separation events play an important role in dispersion of sea spray droplets.

Broader Impacts:

At the University of Rhode Island (URI) broader impacts from this project will be achieved through a series of activities aimed at three key audiences: graduate students, high school and undergraduate science educators, and the general public. These activities will be facilitated by education and outreach staff at the Inner Space Center at URI, which is home to the national hubs for two large NSF funded initiatives, the National Centers for Ocean Sciences Education Excellence Network and the Climate Change Education Partnership Alliance. At the University of Delaware (UD) broader impacts from this project will be achieved through a summer undergraduate research program targeted at undergraduate students and a public outreach program including open house events and laboratory visits.