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  • 1
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    Oceanic Turbulent Energy Budget using Large-Eddy Simulation of a Wind Event during DYNAMO (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-02-24
    Description: The dominant processes governing ocean mixing during an active phase of the Madden–Julian oscillation are identified. Air–sea fluxes and upper-ocean currents and hydrography, measured aboard the R/V Revelle during boreal fall 2011 in the Indian Ocean at 0°, 80.5°E, are integrated by means of a large-eddy simulation (LES) to infer mixing mechanisms and quantify the resulting vertical property fluxes. In the simulation, wind accelerates the mixed layer, and shear mixes the momentum downward, causing the mixed layer base to descend. Turbulent kinetic energy gains due to shear production and Langmuir circulations are opposed by stirring gravity and frictional losses. The strongest stirring of buoyancy follows precipitation events and penetrates to the base of the mixed layer. The focus here is on the initial 24 h of an unusually strong wind burst that began on 24 November 2011. The model shows that Langmuir turbulence influences only the uppermost few meters of the ocean. Below the wave-energized region, shear instability responds to the integrated momentum flux into the mixed layer, lagging the initial onset of the storm. Shear below the mixed layer persists after the storm has weakened and decelerates the surface jet slowly (compared with the acceleration at the peak of the storm). Slow loss of momentum from the mixed layer extends the effect of the surface wind burst by energizing the fluid at the base of the mixed layer, thereby prolonging heat uptake due to the storm. Ocean turbulence and air–sea fluxes contribute to the cooling of the mixed layer approximately in the ratio 1:3, consistent with observations.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Baroclinic Frontal Instabilities and Turbulent Mixing in the Surface Boundary Layer. Part II: Forced Simulations (2017)
    American Meteorological Society
    Details
    Publication Date: 2017-10-01
    Description: Generation of ocean surface boundary layer turbulence and coherent roll structures is examined in the context of wind-driven and geostrophic shear associated with horizontal density gradients using a large-eddy simulation model. Numerical experiments over a range of surface wind forcing and horizontal density gradient strengths, combined with linear stability analysis, indicate that the dominant instability mechanism supporting coherent roll development in these simulations is a mixed instability combining shear instability of the ageostrophic, wind-driven flow with symmetric instability of the frontal geostrophic shear. Disruption of geostrophic balance by vertical mixing induces an inertially rotating ageostrophic current, not forced directly by the wind, that initially strengthens the stratification, damps the instabilities, and reduces vertical mixing, but instability and mixing return when the inertial buoyancy advection reverses. The resulting rolls and instabilities are not aligned with the frontal zone, with an oblique orientation controlled by the Ekman-like instability. Mean turbulence is enhanced when the winds are destabilizing relative to the frontal orientation, but mean Ekman buoyancy advection is found to be relatively unimportant in these simulations. Instead, the mean turbulent kinetic energy balance is dominated by mechanical shear production that is enhanced when the wind-driven shear augments the geostrophic shear, while the resulting vertical mixing nearly eliminates any effective surface buoyancy flux from near-surface, cold-to-warm, Ekman buoyancy advection.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    Cold Pools and Their Influence on the Tropical Marine Boundary Layer (2017)
    American Meteorological Society
    Details
    Publication Date: 2017-04-01
    Description: Cold pools dominate the surface temperature variability observed over the central Indian Ocean (0°, 80°E) for 2 months of research cruise observations in the Dynamics of the Madden–Julian Oscillation (DYNAMO) experiment in October–December 2011. Cold pool fronts are identified by a rapid drop of temperature. Air in cold pools is slightly drier than the boundary layer (BL). Consistent with previous studies, cold pools attain wet-bulb potential temperatures representative of saturated downdrafts originating from the lower midtroposphere. Wind and surface fluxes increase, and rain is most likely within the ~20-min cold pool front. Greatest integrated water vapor and liquid follow the front. Temperature and velocity fluctuations shorter than 6 min achieve 90% of the surface latent and sensible heat flux in cold pools. The temperature of the cold pools recovers in about 20 min, chiefly by mixing at the top of the shallow cold wake layer, rather than by surface flux. Analysis of conserved variables shows mean BL air is composed of 51% air entrained from the BL top (800 m), 22% saturated downdrafts, and 27% air at equilibrium with the ocean surface. The number of cold pools, and their contribution to the BL heat and moisture, nearly doubles in the convectively active phase compared to the suppressed phase of the Madden–Julian oscillation.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 4
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    Modeling the Transient Response of Tropical Convection to Mesoscale SST Variations (2019)
    American Meteorological Society
    Details
    Publication Date: 2019-05-01
    Description: A cloud-resolving model coupled to a mixed layer ocean with an initial 500-km-wide, +3-K sea surface temperature (SST) patch is used to demonstrate the relationship between tropical mesoscale SST gradients and convection under different wind speeds. On these scales, boundary layer convergence toward hydrostatic low surface pressure is partially responsible for triggering convection, but convection subsequently organizes into cells and squall lines that propagate away from the patch. For strong wind (12 m s−1), enhanced convection is shifted downstream from the patch and consists of relatively small cells that are enhanced from increased moist static energy (MSE) flux over the patch. Convection for weak wind (6 m s−1) develops directly over the patch, merging in larger-scale coherent squall-line systems that propagate away from the patch. Squall lines decay after approximately 1 day, and convection redevelops over the patch region after 2 days. Decreasing patch SST from ocean mixing in the coupled simulations affects the overall strength of the convection, but does not qualitatively alter the convective behavior in comparison with cases with a fixed 3-K SST anomaly. In all cases, increased fluxes of heat and moisture, along with latent heating from shallow convection, initially generate lower pressure over the patch and convergence of the boundary layer winds. Within about 1 day, secondary convective circulations, such as surface cold pools, act to spread the effects of the convection over the model domain and overwhelm the effect of low pressure. SST anomalies (1 and 0.5 K) generate enhanced convection only for winds below 6 m s−1.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 5
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    The LatMix Summer Campaign: Submesoscale Stirring in the Upper Ocean (2015)
    American Meteorological Society
    Details
    Publication Date: 2015-08-01
    Description: Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.
    Print ISSN: 0003-0007
    Electronic ISSN: 1520-0477
    Topics: Geography , Physics
    Published by American Meteorological Society
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  • 6
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    Cloud-Resolving Large-Eddy Simulation of Tropical Convective Development and Surface Fluxes (2015)
    American Meteorological Society
    Details
    Publication Date: 2015-07-01
    Description: Cloud-resolving large-eddy simulations (LES) on a 500 km × 500 km periodic domain coupled to a thermodynamic ocean mixed layer are used to study the effect of large-scale moisture convergence M on the convective population and heat and moisture budgets of the tropical atmosphere, for several simulations with M representative of the suppressed, transitional, and active phases of the Madden–Julian oscillation (MJO). For a limited-area model without an imposed vertical velocity, M controls the overall vertical temperature structure. Moisture convergence equivalent to ~200 W m−2 (9 mm day−1) maintains the observed temperature profile above 5 km. Increased convective heating for simulations with higher M is partially offset by greater infrared cooling, suggesting a potential negative feedback that helps maintain the weak temperature gradient conditions observed in the tropics. Surface evaporation decreases as large-scale moisture convergence increases, and is only a minor component of the overall water budget for convective conditions representing the active phase of the MJO. Cold pools generated by evaporation of precipitation under convective conditions are gusty, with roughly double the wind stress of their surroundings. Consistent with observations, enhanced surface evaporation due to cold pool gusts is up to 40% of the mean, but has a small effect on the total moisture budget compared to the imposed large-scale moisture convergence.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 7
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    Estimated Heat Budget During Summer Melt of Arctic First-Year Sea Ice (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-11-08
    Description: Melt ponds on summer Arctic sea ice control surface albedo, governing energy and mass balance of the ice. The date ponds first form has been connected to interannual variations in ice retreat. Here we evaluate the surface energy balance that governs this critical pond formation date. A three-dimensional sea ice model with resolved melt ponds is used to diagnose pond onset date at a coastal site across years with observed surface fluxes but incomplete pond observations. Results show that the combined sensible and latent heat flux is the best predictor of pond formation date. This finding supports the hypothesis that synoptic weather events transporting warm, moist air into the Arctic are key to initiating pond formation, triggering albedo feedbacks, and, by extension, ice retreat. Changes in timing and frequency of spring warm air incursions may have significant implications on the ice cover and provide predictive power over seasonal ice retreat. ©2018. American Geophysical Union. All Rights Reserved.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 8
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    Simulation of the melt season using a resolved sea ice model with snow cover and melt ponds (2015)
    American Geophysical Union
    Details
    Publication Date: 2015-07-01
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 9
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    Observations and numerical simulations of large-eddy circulation in the ocean surface mixed layer (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 7584–7590, doi:10.1002/2014GL061637.
    Description: Two near-surface dye releases were mapped on scales of minutes to hours temporally, meters to order 1 km horizontally, and 1–20 m vertically using a scanning, depth-resolving airborne lidar. In both cases, dye evolved into a series of rolls with their major axes approximately aligned with the wind and/or near-surface current. In both cases, roll spacing was also of order 5–10 times the mixed layer depth, considerably larger than the 1–2 aspect ratio expected for Langmuir cells. Numerical large-eddy simulations under similar forcing showed similar features, even without Stokes drift forcing. In one case, inertial shear driven by light winds induced large aspect ratio large-eddy circulation. In the second, a preexisting lateral mixed layer density gradient provided the dominant forcing. In both cases, the growth of the large-eddy structures and the strength of the resulting dispersion were highly dependent on the type of forcing.
    Description: Support for the 2004 field experiment was provided by the Cecil H. and Ida M. Green Technology Innovation Fund and Coastal Ocean Institute grant 27001545, both through Woods Hole Oceanographic Institution, and by Office of Naval Research grant N00014-01-1-0984. Support for the 2011 field experiments was provided by ONR grants N00014-09-1-0194, N00014-09-1-0175, N00014-11-WX-21010, N00014-12-WX-21031, and N00014-09-1-0460 and NSF grants OCE-0751734 and OCE-0751653. Simulations were supported under grant N00014-09-1-0268.
    Description: 2015-05-06
    Keywords: Large-eddy circulation ; Ocean surface mixed layer ; Lidar ; Fluorescent dye ; Numerical model
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 10
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    The LatMix summer campaign : submesoscale stirring in the upper ocean (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 96 (2015): 1257–1279, doi:10.1175/BAMS-D-14-00015.1.
    Description: Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.
    Description: The bulk of this work was funded under the Scalable Lateral Mixing and Coherent Turbulence Departmental Research Initiative and the Physical Oceanography Program. The dye experiments were supported jointly by the Office of Naval Research and the National Science Foundation Physical Oceanography Program (Grants OCE-0751653 and OCE-0751734).
    Description: 2016-02-01
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/vnd.google-earth
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