Measurements of the Coupled Development of the Surface Wave Field and Marine Atmospheric Boundary Layer with Fetch

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

ABSTRACT PI/Institution: Melville / SIO Proposal No: OCE-0242083 The coupled development of the wind-generated wave field and the marine atmospheric boundary layer as a function of fetch remains a fundamental problem in air-sea interaction. While simple power-law expressions for the dimensionless wave height and peak frequency have been obtained in terms of the dimensionless fetch, predictions of the evolution of the wave directional spectrum still depend on assumptions about the wind input, nonlinear wave-wave interactions and dissipation by breaking. Similar uncertainties exist in the modeling of the marine atmospheric boundary layer. The evolution of the boundary layer and the wave field are coupled, with the momentum, heat and moisture fluxes depending on the sea state, and especially on wave breaking. While the details of these processes may be averaged out, or relegated to subgrid-scale status in meso- and basin-scale models of air-sea interaction, improved models of small-scale air-sea fluxes are required to improve predictions under moderate wind and wave conditions. Moreover, first-order uncertainties remain at high winds. It is well known that much of the momentum flux from winds to currents (or the drag on the atmospheric boundary layer) first passes through the wave field. The primary mechanism for transferring that momentum flux to currents is wave breaking. Breaking is an important source of turbulence (that enhances heat and mass transfer), and of entrained bubbles and marine aerosols. Marine spray and aerosols are of particular importance for biogeochemisty, for cloud physics, and for the Earth's radiation balance. This proposal describes a program of airborne measurements of the evolution of the air-sea fluxes, the atmospheric boundary layer and the surface wave field as a function of fetch in moderate to high winds. Airborne experiments will be conducted from the NCAR C-130 in the Gulf of Tehuantepec, of the Pacific coast of Mexico during winter when northerly mountain-gap winds blow at up to 40 knots or more for several days out over the Pacific. Using modern remote imaging, ranging, mapping and positioning techniques, we will make detailed measurements of the wave field with fetch as winds blow offshore. Using modern airborne atmospheric turbulence and flux measurement techniques, we will measure the evolution of the atmospheric boundary layer and investigate the coupling with the wave field. Using air-deployable expendable dropsondes and AXBTs we will profile the development of the marine atmospheric boundary layer and the ocean mixed layer with fetch. Measurements will be compared with coupled models of air-sea fluxes and wind-wave evolution. We will develop fetch-dependent models of the atmospheric boundary layer and more detailed fetch-dependent models of surface-wave processes; especially breaking. This project will have a broader impact on science and society through a variety of mechanisms. The research is motivated by basic questions in small-scale air-sea interaction, but the results will find application in economically-important coupled ocean-atmosphere models for improved weather and climate prediction. The results of this research will be directly incorporated into classes at SIO/UCSD and UCI, and the equipment and techniques developed in this project will be used in graduate education and research across a variety of fields at both institutions. The project will provide research opportunities for summer undergraduate research fellows from diverse /underrepresented groups at both SIO/UCSD and UCI, and opportunities for undergraduate assistants will be provided during the school year. The project will be used to educate K-12 students in science, and career opportunities in science, at both institutions. A project web site will be established at SIO to broadly disseminate the results of the research.