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Mike Gregg

Professor Emeritus, Oceanography

Professor, Oceanography

Email

gregg@apl.washington.edu

Phone

206-543-1353

Biosketch

The idea that the cumulative action of centimeter-scale mixing affects the ocean's largest scales guides Mike Gregg's research. Evolving technology now enables the mixing to be put into the context of the meter-to-kilometer-scale processes directly producing it, such as internal waves, bottom and surface boundary layers, thermohaline staircases and intrusions, and hydraulic responses to flow constrictions. Because large-scale models, particularly coupled climate models, have grid scales vastly larger than those of the mixing and even of the intermediate-scale processes producing it, it is a goal to always try to work toward parameterizations that can be used in these models.

Department Affiliation

Ocean Physics

Education

B.S. Physics, Yale University, 1961

Ph.D. Physical Oceanography, Scripps Institution of Oceanography, 1971

Publications

2000-present and while at APL-UW

Mixing efficiency in the ocean

Gregg, M.C., E.A. D'Asaro, J.J. Riley, and E. Kunze, "Mixing efficiency in the ocean," Annu. Rev. Mar. Sci., 10, 443-473, doi:10.1146/annurev-marine-121916-063643, 2018.

More Info

1 Jan 2018

Mixing efficiency is the ratio of the net change in potential energy to the energy expended in producing the mixing. Parameterizations of efficiency and of related mixing coefficients are needed to estimate diapycnal diffusivity from measurements of the turbulent dissipation rate. Comparing diffusivities from microstructure profiling with those inferred from the thickening rate of four simultaneous tracer releases has verified, within observational accuracy, 0.2 as the mixing coefficient over a 30-fold range of diapycnal diffusivities. Although some mixing coefficients can be estimated from pycnocline measurements, at present mixing efficiency must be obtained from channel flows, laboratory experiments, and numerical simulations. Reviewing the different approaches demonstrates that estimates and parameterizations for mixing efficiency and coefficients are not converging beyond the at-sea comparisons with tracer releases, leading to recommendations for a community approach to address this important issue.

Using an ADCP to estimate turbulent kinetic energy dissipation rate in sheltered coastal water

Greene, A.D., P.J. Hendricks, and M.C. Gregg, "Using an ADCP to estimate turbulent kinetic energy dissipation rate in sheltered coastal water," J. Atmos. Ocean. Techno., 32, 318-333, doi:10.1175/JTECH-D-13-00207.1, 2015.

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1 Feb 2015

Turbulent microstructure and acoustic Doppler current profiler (ADCP) data were collected near Tacoma Narrows in Puget Sound, Washington. Over 100 coincident microstructure profiles have been compared to ADCP estimates of turbulent kinetic energy dissipation rate (ε). ADCP dissipation rates were calculated using the large-eddy method with theoretically determined corrections for sensor noise on rms velocity and integral-scale calculations. This work is an extension of Ann Gargett's approach, which used a narrowband ADCP in regions with intense turbulence and strong vertical velocities. Here, a broadband ADCP is used to measure weaker turbulence and achieve greater horizontal and vertical resolution relative to the narrowband ADCP. Estimates of ε from the Modular Microstructure Profiler (MMP) and broadband ADCP show good quantitative agreement over nearly three decades of dissipation rate, 3 x 10-8 – 10-5 m2 s-3. This technique is most readily applied when the turbulent velocity is greater than the ADCP velocity uncertainty (σ) and the ADCP cell size is within a factor of 2 of the Thorpe scale. The 600-kHz broadband ADCP used in this experiment yielded a noise floor of 3 mm s-1 for 3-m vertical bins and 2-m along-track average (~four pings), which resulted in turbulence levels measureable with the ADCP as weak as 3 x 10-8 m2 s-3. The value and trade-off of changing the ADCP cell size, which reduces noise but also changes the ratio of the Thorpe scale to the cell size, are discussed as well.

Mode-2 hydraulic control of flow over a small ridge on a continental shelf

Gregg, M.C., and J.M. Klymak, "Mode-2 hydraulic control of flow over a small ridge on a continental shelf," J. Geophys. Res., 119, 8093-8108, doi:10.1002/2014JC010043, 2018.

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1 Nov 2014

Some of the most intense turbulence in the ocean occurs in hydraulic jumps formed in the lee of sills where flows are hydraulically controlled, usually by the first internal mode. Observations on the outer Texas–Louisiana continental shelf reveal hydraulic control of internal mode‐2 lasting more than 3 h over a 20 m high ridge on the 100 m deep continental shelf. When control began the base of the weakly stratified surface layer bulged upward and downward, a signature of mode‐2. As the westward flow producing control was lost, large‐amplitude disturbances, initially resembling a bore in the weakly stratified layer, began propagating eastward. Average dissipation rates inferred from density inversions over the ridge were 10-8 and 10-7 W kg-1, one to two decades above local background. Corresponding diapycnal diffusivities, Kp, were 10-4 to 10-3 M2s-1. Short‐term mixing averages did not evolve systematically with hydraulic control, possibly owing to our inability to observe small overturns in strongly stratified water directly over the ridge. To test the feasibility of our interpretation of the observations, hydrostatic runs with a three‐dimensional MITgcm simulated mode‐2 control and intense mixing over the ridge below the interface. Details differed from observations, principally because we lacked three‐dimensional density fields to initialize the model which was forced with currents observed by a bottom‐mounted ADCP several kilometers east of the ridge. Consequently, the model did not capture all flow features around the bank. The principal conclusion is that hydraulic responses to higher modes can dominate flows around even modest bathymetric irregularities.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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