71 research outputs found
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Characteristics of Langmuir Turbulence in the Ocean Mixed Layer
This study uses large-eddy simulation (LES) to investigate the characteristics of Langmuir turbulence through the turbulent kinetic energy (TKE) budget. Based on an analysis of the TKE budget a velocity scale for Langmuir turbulence is proposed. The velocity scale depends on both the friction velocity and the surface Stokes drift associated with the wave field. The scaling leads to unique profiles of nondimensional dissipation rate and velocity component variances when the Stokes drift of the wave field is sufficiently large compared to the surface friction velocity. The existence of such a scaling shows that Langmuir turbulence can be considered as a turbulence regime in its own right, rather than a modification of shear-driven turbulence.
Comparisons are made between the LES results and observations, but the lack of information concerning the wave field means these are mainly restricted to comparing profile shapes. The shapes of the LES profiles are consistent with observed profiles. The dissipation length scale for Langmuir turbulence is found to be similar to the dissipation length scale in the shear-driven boundary layer. Beyond this it is not possible to test the proposed scaling directly using available data. Entrainment at the base of the mixed layer is shown to be significantly enhanced over that due to normal shear turbulence
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A global climatology of windâwave interaction
Generally, ocean waves are thought to act as a drag on the surface
wind so that momentum is transferred downwards, from the atmosphere
into the waves. Recent observations have suggested that when long
wavelength waves, characteristic of remotely generated swell,
propagate faster than the surface wind momentum can also be
transferred upwards. This upward momentum transfer acts to accelerate
the near-surface wind, resulting in a low-level wave-driven wind
jet. Previous studies have suggested that the sign reversal of the
momentum flux is well predicted by the inverse wave age, the ratio of
the surface wind speed to the speed of the waves at the peak of the
spectrum. ECMWF ERA-40 data has been used here to calculate the global
distribution of the inverse wave age to determine whether there are
regions of the ocean that are usually in the wind-driven wave regime
and others that are generally in the wave-driven wind regime. The
wind-driven wave regime is found to occur most often in the
mid-latitude storm tracks where wind speeds are generally high. The
wave-driven wind regime is found to be prevalent in the tropics where
wind speeds are generally light and swell can propagate from storms at
higher latitudes. The inverse wave age is also a useful indicator of
the degree of coupling between the local wind and wave fields. The
climatologies presented emphasise the non-equilibrium that exists
between the local wind and wave fields and highlight the importance of
swell in the global oceans
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Dispersion of a passive scalar within and above an urban street network
The transport of a passive scalar from a continuous point-source release in an urban street network is studied using direct numerical simulation (DNS). Dispersion through the network is characterized by evaluating horizontal fluxes of scalar within and above the urban canopy and vertical exchange fluxes through the canopy top. The relative magnitude and balance of these fluxes are used to distinguish three different regions relative to the source location: a near-field region, a transition region and a far-field region. The partitioning of each of these fluxes into mean and turbulent parts is computed. It is shown that within the canopy the horizontal turbulent flux in the street network is small, whereas above the canopy it comprises a significant fraction of the total flux. Vertical fluxes through the canopy top are predominantly turbulent. The mean and turbulent fluxes are respectively parametrized in terms of an advection velocity and a detrainment velocity and the parametrization incorporated into a simple box-network model. The model treats the coupled dispersion problem within and above the street network in a unified way and predictions of mean concentrations compare well with the DNS data. This demonstrates the usefulness of the box-network approach for process studies and interpretation of results from more detailed numerical simulations
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Friction in mid-latitude cyclones: an Ekman-PV mechanism
The mechanism by which the atmospheric boundary layer reduces the intensity of mid-latitude cyclones is investigated. It is demonstrated that two alternative theories, Ekman pumping and the baroclinic potential vorticity (PV) mechanism, in fact act in union to maximize the spin-down. Ekman pumping aids the ventilation of PV from the boundary layer, and shapes the resulting PV anomaly into one of increased static stability. PV inversion techniques are used to demonstrate how this anomaly reduces the coupling between the upper- and lower-levels within the cyclone, reducing the growth rate
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Flow structure and near-field dispersion in arrays of building-like obstacles
Dispersion in the near-field region of localised releases in urban areas is difficult to predict because of the strong influence of individual buildings. Effects include upstream dispersion, trapping of material into building wakes and enhanced concentration fluctuations. As a result, concentration patterns are highly variable in time and mean profiles in the near field are strongly non-Gaussian. These aspects of near-field dispersion are documented by analysing data from direct numerical simulations in arrays of building-like obstacles and are related to the underlying flow structure. The mean flow structure around the buildings is found to exert a strong influence over the dispersion of material in the near field. Diverging streamlines around buildings enhance lateral dispersion. Entrainment of material into building wakes in the very near field gives rise to secondary sources, which then affect the subsequent dispersion pattern. High levels of concentration fluctuations are also found in this very near field; the fluctuation intensity is of order 2 to 5
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Evolution of oceanic near surface stratification in response to an autumn storm
Understanding the processes that control the evolution of the ocean surface boundary layer (OSBL) is a prerequisite for obtaining accurate simulations of air-sea fluxes of heat and trace gases. Observations of the rate of dissipation of turbulent kinetic energy (É), temperature, salinity, current structure and wave-field over a period of 9.5 days in the NE Atlantic during the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS), are presented. The focus of this study is a storm which passed over the observational area during this period. The profiles of É in the OSBL are consistent with profiles from large eddy simulation (LES) of Langmuir turbulence. In the transition layer (TL), at the base of the OSBL, É was found to vary periodically at the local inertial frequency. A simple bulk model of the OSBL and a parametrisation of shear driven turbulence in the TL are developed. The parametrisation of É is based on assumptions about the momentum balance of the OSBL and shear across the TL. The predicted rate of deepening, heat budget and the inertial currents in the OSBL were in good agreement with the observations, as is the agreement between the observed value of É and that predicted using the parametrisation. A previous study reported spikes of elevated dissipation related to enhanced wind-shear alignment at the base of the OSBL after this storm. The spikes in dissipation are not predicted by this new parametrisation, implying that they are not an important source of dissipation during the storm
The contribution of surface and submesoscale processes to turbulence in the open ocean surface boundary layer
The ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which occur at small scales (0.1â10 km, hours to days) and therefore are not yet represented in most ocean models, may play critical roles in these turbulent exchanges. While observational support for such phenomena has been demonstrated in the vicinity of strong current systems and littoral regions, relatively few observations exist in the openâocean environment to warrant representation in Earth system models. We use novel observations and simulations to quantify the contributions of surface and submesoscale processes to turbulent kinetic energy (TKE) dissipation in the openâocean surface boundary layer. Our observations are derived from moorings in the North Atlantic, December 2012 to April 2013, and are complemented by atmospheric reanalysis. We develop a conceptual framework for dissipation rates due to surface and submesoscale processes. Using this framework and comparing with observed dissipation rates, we find that surface processes dominate TKE dissipation. A parameterization for symmetric instability is consistent with this result. We next employ simulations from an ocean frontâresolving model to reestablish that dissipation due to surface processes exceeds that of submesoscale processes by 1â2 orders of magnitude. Together, these results suggest submesoscale processes do not dramatically modify vertical TKE budgets, though such dynamics may be climatically important owing to their ability to remove energy from the ocean
Reply to âComments on âLangmuir Turbulence and Surface Heating in the Ocean Surface Boundary Layerââ
The differences between the conclusions of Noh and Choi and of Pearson et al., which are largely a result of defining different length scales based on different quantities, are discussed. This study shows that the layer over which Langmuir turbulence mixes (nominally hTKE) under a stabilizing surface buoyancy flux should be scaled by a combination of the Langmuir stability length LL and initial/nocturnal boundary layer depth h0 rather than by the Zilitinkevich length
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Seasonality of submesoscale flows in the ocean surface boundary layer
A signature of submesoscale flows in the upper ocean is skewness in the distribution of relative vorticity. Expected to result for high Rossby-number flows, such skewness has implications for mixing, dissipation and stratification within the upper ocean. An array of moorings deployed in the Northeast Atlantic for one year as part of the OSMOSIS experiment reveals that relative vorticity is positively skewed during winter even though the scale of the Rossby number is less than 0.5. Furthermore, this skewness is reduced to zero during spring and autumn. There is also evidence of modest seasonal variations in the gradient Rossby number. The proposed mechanism by which relative vorticity is skewed is that the ratio of lateral to vertical buoyancy gradients, as summarized by the inverse gradient Richardson number, restricts its range during winter but less so at other times of the year. These results support recent observations and model simulations suggesting the upper ocean is host to a seasonal cycle in submesoscale turbulence
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Langmuir turbulence and surface heating in the ocean surface boundary layer
This study uses large-eddy simulation to investigate the structure of the ocean surface boundary layer (OSBL) in the presence of Langmuir turbulence and stabilizing surface heat fluxes. The OSBL consists of a weakly stratified layer, despite a surface heat flux, above a stratified thermocline. The weakly stratified (mixed) layer is maintained by a combination of a turbulent heat flux produced by the wave-driven Stokes drift and downgradient turbulent diffusion. The scaling of turbulence statistics, such as dissipation and vertical velocity variance, is only affected by the surface heat flux through changes in the mixed layer depth. Diagnostic models are proposed for the equilibrium boundary layer and mixed layer depths in the presence of surface heating. The models are a function of the initial mixed layer depth before heating is imposed and the Langmuir stability length. In the presence of radiative heating, the models are extended to account for the depth profile of the heating
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