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Ocean Near-Surface Boundary Layer: Processes and Turbulence Measurements

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Ocean Near-Surface Boundary Layer: Processes and Turbulence Measurements REPORTS IN METEOROLOGY AND OCEANOGRAPHY UNIVERSITY OF BERGEN, 1 - 2010 Ocean near-surface boundary layer: processes and turbulence measurements MOSTAFA BAKHODAY PASKYABI and ILKER FER Geophysical Institute University of Bergen December, 2010 «REPORTS IN METEOROLOGY AND OCEANOGRAPHY» utgis av Geofysisk Institutt ved Universitetet I Bergen. Formålet med rapportserien er å publisere arbeider av personer som er tilknyttet avdelingen. Redaksjonsutvalg: Peter M. Haugan, Frank Cleveland, Arvid Skartveit og Endre Skaar. Redaksjonens adresse er : «Reports in Meteorology and Oceanography», Geophysical Institute. Allégaten 70 N-5007 Bergen, Norway RAPPORT NR: 1- 2010 ISBN 82-8116-016-0 2 CONTENT 1. INTRODUCTION ................................................................................................2 2. NEAR SURFACE BOUNDARY LAYER AND TURBULENCE MIXING .............3 2.1. Structure of Upper Ocean Turbulence........................................................................................ 5 2.2. Governing Equations......................................................................................................................... 6 2.2.1. Turbulent Kinetic Energy and Temperature Variance .................................................................. 6 2.2.2. Relevant Length Scales..................................................................................................................... 8 2.2.3. Relevant non-dimensional numbers................................................................................................ 8 2.2.4. Turbulence Spectra ........................................................................................................................... 9 2.3. Atmospheric Forcing ....................................................................................................................... 10 2.3.1. Wind Stress....................................................................................................................................... 10 2.3.2. Bulk Heat Fluxes .............................................................................................................................. 11 2.3.3. Solar Radiation Fluxes..................................................................................................................... 12 2.3.4. Precipitation...................................................................................................................................... 13 2.3.5. Surface Buoyancy Flux.................................................................................................................... 13 2.3.6. Gas Fluxes......................................................................................................................................... 14 3. OCEAN TURBULENCE MEASUREMENT.........................................................14 3.1. Measurement systems ................................................................................................................... 14 3.2. Sensor Features................................................................................................................................21 3.2.1. Temperature..................................................................................................................................... 21 3.2.2. Conductivity...................................................................................................................................... 21 3.2.3. Shear ................................................................................................................................................. 22 3.3. Parameter Estimation..................................................................................................................... 23 3.3.1. Turbulence Dissipation Rate .......................................................................................................... 23 3.3.2. Vertical Eddy Diffusivity for Mass .................................................................................................. 24 3.3.3. Dissipation Rate of Temperature Variance and Eddy Diffusivity for Heat............................... 24 3.3.4. Vertical Overturns............................................................................................................................ 25 3.4. Measurement Limitations and Platform Motion ................................................................... 26 3.4.1. Platform Motion and Vibration Correction Procedures ............................................................... 26 3.4.2. Despiking and Denoising ................................................................................................................ 30 4. TURBULENCE NUMERICAL MODELING .......................................................30 4.1. Direct Numerical Simulation........................................................................................................ 30 4.2. Large Eddy Simulation ................................................................................................................... 30 4.3. Statistical Turbulence Modeling ................................................................................................. 31 4.4. Empirical Turbulence Modeling................................................................................................... 31 5. SUMMARY ........................................................................................................31 6. REFERENCES....................................................................................................32 1 Abstract Exchange of mass, momentum and heat by turbulent processes is crucial in the air-sea interactions. In this report, state-of-the-art of turbulent kinetic energy dynamics in the upper ocean boundary layer is reviewed. Particular attention is given to the structure of the upper mixed layer, main turbulent processes in this layer, different methodology of near surface turbulence measurements, and complementary turbulence numerical models. 1. Introduction The complex air-sea interface sets the boundary conditions for physical and biogeochemical processes and plays a key role in the global atmosphere-ocean heat balance. Fluxes across this interface influence the weather, climate and spatial distribution and evolution of greenhouse gases. The upper water column below the air-sea interface comprises of an actively entraining upper ocean mixed layer, the upper 10-50 m well-mixed in temperature and salinity, bounded at its base by a sharp density discontinuity separating the layer from a stable, essentially non-turbulent pycnocline. The thermal and mechanical energy received from the atmosphere in the upper layer not only control the local dynamics, but the layer itself modulates the flux of this energy to the deeper water masses (Garwood 1976). This mixing process plays a major role for the dynamics of marine ecosystems. It provides food web, light and nutrients for phytoplankton population (Gargett 1997). The first classical study of the surface mixed layer and shelf sea waters was done by Sverdrup (1953) about the development of spring phytoplankton blooms as a function of light supply and the depth of vertical mixing. Blooming can occur only when the depth of the mixed layer is less than a compensation depth at which gain and loss of the population balance each other. Sverdrup’s theory thus necessitated a better understanding of the complex upper ocean dynamics, of erosion of the pycnocline, and especially of the turbulent mixing processes which control the exchange and variability of properties such as momentum, heat, mass and gases in the upper ocean. Furthermore, understanding turbulence and its role in ocean mixing is key in critical ocean processes (Nihoul 1980), and needed even in the studies of ocean circulations and the coupled ocean-atmosphere climate. Upper ocean turbulence has a broad spectrum of time and spatial scales, forced by surface fluxes of buoyancy, momentum and surface waves. The buoyancy loss at the surface due to surface cooling and evaporation (and due to ice freezing at high latitudes, which is not considered in this report) results in convection due to unstable surface density stratification. The overturning eddies will lead to entrainment of water from below the pycnocline. The mean shear due to loss of mean flow kinetic energy generated by wind stress over sea surface (and its associated currents) can lead to instability of Kelvin–Helmholtz billows with their axes oriented roughly at right angles to the shear (Li et al. 2005). The interaction between the wind-driven Stokes drift of surface waves and the mean shear generates counter-rotating vortices aligned with the wind direction, so-called the Langmuir circulation (Thorpe 2004, 2007). Langmuir circulation is a means of efficient energy transfer from the surface wave energy to turbulence mixing (Grant and Belcher 2009; Teixeira and Belcher 2010). Therefore, the upper ocean turbulence and associated mixing are sensitive to and dependent on the properties of the surface gravity waves (Ardhuin and Jenkins 2006). Deeper in the water column, internal wave breaking, dynamical Kelvin-Helmholtz instabilities at the pycnocline 2 and local frictional drag against the topography are other typical trigger mechanisms which contribute significantly to the vertical redistribution of both momentum and scalars in the ocean interior (Thorpe 2004, 2007). Understanding of the aforementioned processes at the sea surface and the structure
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