172 research outputs found
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Wind-driven mixing below the oceanic mixed layer
This study describes the turbulent processes in the upper ocean boundary layer forced by a constant surface stress in the absence of the Coriolis force using large-eddy simulation. The boundary layer that develops has a two-layer structure, a well-mixed layer above a stratified shear layer. The depth of the mixed layer is approximately constant, whereas the depth of the shear layer increases with time. The turbulent momentum flux varies approximately linearly from the surface to the base of the shear layer.
There is a maximum in the production of turbulence through shear at the base of the mixed layer. The magnitude of the shear production increases with time. The increase is mainly a result of the increase in the turbulent momentum flux at the base of the mixed layer due to the increase in the depth of the boundary layer. The length scale for the shear turbulence is the boundary layer depth. A simple scaling is proposed for the magnitude of the shear production that depends on the surface forcing and the average mixed layer current. The scaling can be interpreted in terms of the divergence of a mean kinetic energy flux.
A simple bulk model of the boundary layer is developed to obtain equations describing the variation of the mixed layer and boundary layer depths with time. The model shows that the rate at which the boundary layer deepens does not depend on the stratification of the thermocline. The bulk model shows that the variation in the mixed layer depth is small as long as the surface buoyancy flux is small
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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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Controls on boundary layer ventilation: Boundary layer processes and large-scale dynamics
Midlatitude cyclones are important contributors to boundary layer ventilation. However, it is uncertain how efficient such systems are at transporting pollutants out of the boundary layer, and variations between cyclones are unexplained. In this study 15 idealized baroclinic life cycles, with a passive tracer included, are simulated to identify the relative importance of two transport processes: horizontal divergence and convergence within the boundary layer and large-scale advection by the warm conveyor belt. Results show that the amount of ventilation is insensitive to surface drag over a realistic range of values. This indicates that although boundary layer processes are necessary for ventilation they do not control the magnitude of ventilation. A diagnostic for the mass flux out of the boundary layer has been developed to identify the synoptic-scale variables controlling the strength of ascent in the warm conveyor belt. A very high level of correlation (R-2 values exceeding 0.98) is found between the diagnostic and the actual mass flux computed from the simulations. This demonstrates that the large-scale dynamics control the amount of ventilation, and the efficiency of midlatitude cyclones to ventilate the boundary layer can be estimated using the new mass flux diagnostic. We conclude that meteorological analyses, such as ERA-40, are sufficient to quantify boundary layer ventilation by the large-scale dynamics
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The moist boundary layer under a mid-latitude weather system
Mid-latitude weather systems are key contributors to the transport of atmospheric
water vapour, but less is known about the role of the boundary layer in
this transport. We expand a conceptual model of dry boundary-layer structure under
synoptic systems to include moist processes, using idealised simulations of cyclone
waves to investigate the three-way interaction between the boundary layer, atmospheric
moisture and large-scale dynamics. Forced by large-scale thermal advection,
boundary-layer structures develop over large areas, analogous to the daytime convective
boundary layer, the nocturnal stable boundary layer and transitional regimes
between these extremes
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A global climatology of wind–wave 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
Turbulence and Wave Dynamics Across Gas–Liquid Interfaces
Mechanisms are reviewed here for the distortion of turbulent flows near thin density interfaces and their effects on transfer processes across them. Firstly the results of rapid distortion calculations show how the in homogeneous eddy structure depends on whether the turbulence is generated above or below the interface, or in both regions. The flow is unstratified and the buoyancy forces are stable and strong enough relative to the inertial forces that the interface is continuous. It is shown that as the surface blocks the vertical turbulent eddy motions, horizontal straining motions are induced which affect the surface viscous layers and can then induce motions some distance from the interface on the opposite side from where the turbulence is generated. Secondly the paper reviews the physical mechanisms controlling how wind flows over monochromatic and groups of surface waves. The results of triple deck theory for turbulent shear flows, i. e. combining sheltering and unsteady critical layer mechanisms, explain why groups are the most efficient mechanism for waves to extract energy from the wind and therefore enhance transfer properties between atmosphere and water bodies. The third section of the paper reviews the mechanisms for the generation of turbulence coherent roll structures in the ocean surface layer, resulting from surface shear turbulence (normal stress variations), wave-mean shear vortex stretching and rotation, i.e. Langmuir cells, and unstable buoyancy forces (i.e. cooling at the surface) plus mean shear also via vortex rotation. Since these mechanisms are generally additive - exceptional situations exist - they are effective in transporting fluxes downwards into the ocean surface layers
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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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Implications of climate change for expanding cities worldwide
This paper analyses the trends of the changing environmental effects within growing megacities as their diameters
exceed 50–100 km and their populations rise beyond 30 million people. The authors consider how these effects are
influenced by climate change, to which urban areas themselves contribute, caused by their increasing greenhouse gas emissions associated with rapidly expanding energy use. Other environmental and social factors are assessed,
quantitatively and qualitatively, using detailed modelling of urban mesoscale meteorology, which shows how these
factors can lead to large conurbations becoming more vulnerable to climatic and environmental hazards. The paper
discusses the likely changes in meteorological and hydrological hazards in urban areas, both as the climate changes and the sizes of urban areas grow. Examples are given of how these risks are being reduced through innovations in warning and response systems, planning and infrastructure design, which should include refuges against extreme natural disasters. Policies are shown to be more effective when they are integrated and based on substantial
community involvement. Some conclusions are drawn regarding how policies for the natural and artificial environment and for reducing many kinds of climate and hazard risk are related to future designs and planning of infrastructure and open spaces
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