Wave Breaking and Near-Surface Turbulence

<p>The stable boundary layer is typically characterized by weak and sometimes intermittent turbulence, particularly under very stable conditions. In mountain valleys, nocturnal temperature inversions and cold-air pools form frequently under synoptically undisturbed and clear-sky conditions, which will dampen turbulence. On the other hand, thermally driven slope and valley winds form under the same conditions, which interact with each other and are both characterized by jet-like wind profiles, thus resulting in both horizontal and vertical wind shear, which creates a persistent source for turbulence production. Data will be presented from six flux towers in the Austrian Inn Valley, which are part of the i-Box measurement platform, designed to study near-surface turbulence in complex, mountainous terrain. The six sites are located within an approximately 6.5-km long section of the 2-3-km wide valley approximately 20 km east of Innsbruck. The data are analyzed to characterize the strength and intermittency of turbulence kinetic energy and turbulent fluxes across the valley and to determine whether the persistent wind shear associated with thermally driven flows is sufficient to generate continuous turbulence.</p>

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THEORY OF NEAR SURFACE TURBULENCE APPLIED TO WIND SPEED PROFILES, DRY DEPOSITION, AIR-WATER EXCHANGE, AND CANOPY EFFECTS

Air Dispersion Modeling ◽

10.1002/9781118723098.app3 ◽

2013 ◽

pp. 607-628

Keyword(s):

Wind Speed ◽

Dry Deposition ◽

Water Exchange ◽

Near Surface ◽

Surface Turbulence

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Influence of Complex Terrain on Near-Surface Turbulence Structures over Loess Plateau

Atmosphere ◽

10.3390/atmos11090930 ◽

2020 ◽

Vol 11 (9) ◽

pp. 930

Author(s):

Jiening Liang ◽

Qi Guo ◽

Zhida Zhang ◽

Min Zhang ◽

Pengfei Tian ◽

...

Keyword(s):

Loess Plateau ◽

Complex Terrain ◽

Longitudinal Velocity ◽

Complex Topography ◽

Horizontal Scale ◽

Flux Footprint ◽

Near Surface ◽

Turbulence Structures ◽

Vertical Scale ◽

Surface Turbulence

To study the influence of complex terrain with different scales on the structure of near-surface turbulence, the turbulence observational data from Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) were analyzed. SACOL is located in typical Loess Plateau topography. The terrain around the site varies greatly with the direction. Representative data from the Northeast, Southeast, Southwest, and Northwest were selected to study the structure characteristics of the near surface turbulence. The complex topography within the flux footprint enhances the vertical scale of turbulence σw and thereby increases the vertical–longitudinal ratio of turbulence Ar. While the turbulent horizontal scale σu is also significantly affected by the distant terrain beyond the flux footprint. The upwind terrain undulation increases σu and reduces Ar. Affected by the complex terrain, the ratio of the spectrum of the vertical velocity to that of the longitudinal velocity, Sw(n)/Su(n), is far less than 4/3 in the southwest direction, and the turbulence is significantly anisotropic.

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The Effects of Small-Scale Turbulence on Air–Sea Heat Flux

Journal of Physical Oceanography ◽

10.1175/2010jpo4491.1 ◽

2011 ◽

Vol 41 (1) ◽

pp. 205-220 ◽

Cited By ~ 17

Author(s):

Fabrice Veron ◽

W. Kendall Melville ◽

Luc Lenain

Keyword(s):

Field Experiments ◽

Renewal Theory ◽

Small Scale ◽

Surface Renewal ◽

Near Surface ◽

Turbulence Measurements ◽

Renewal Time ◽

Surface Turbulence ◽

The Mean ◽

Renewal Events

Abstract The air–sea exchange of heat is mainly controlled by the molecular diffusive layer adjacent to the surface. With an order of magnitude difference between the kinematic viscosity and thermal diffusivity of water, the thermal sublayer is embedded within its momentum analog: the viscous sublayer. Therefore, the surface heat exchange rates are greatly influenced by the surface kinematics and dynamics; in particular, small-scale phenomena, such as near-surface turbulence, have the greatest potential to affect the surface fluxes. Surface renewal theory was developed to parameterize the details of the turbulent transfer through the molecular sublayers. The theory assumes that turbulent eddies continuously replace surface water parcels with bulk fluid, which is not in equilibrium with the atmosphere and therefore is able to transfer heat. The so-called controlled-flux technique gives direct measurements of the mean surface lifetime of such surface renewal events. In this paper, the authors present results from field experiments, along with a review of surface renewal theory, and show that previous estimates of air–sea scalar fluxes using the controlled-flux technique may be erroneous if the probability density function (PDF) of surface renewal time scales is different from the routinely assumed exponential distribution. The authors show good agreement between measured and estimated heat fluxes using a surface renewal PDF that follows a χ distribution. Finally, over the range of forcing conditions in these field experiments, a clear relationship between direct surface turbulence measurements and the mean surface renewal time scale is established. The relationship is not dependent on the turbulence generation mechanism. The authors suggest that direct surface turbulence measurements may lead to improved estimates of scalar air–sea fluxes.

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A Model of Strongly Forced Wind Waves

Journal of Physical Oceanography ◽

10.1175/2009jpo4155.1 ◽

2009 ◽

Vol 39 (10) ◽

pp. 2502-2522 ◽

Cited By ~ 2

Author(s):

Alexey V. Fedorov ◽

W. Kendall Melville

Keyword(s):

Wind Stress ◽

Wave Breaking ◽

Wind Waves ◽

Surface Wind ◽

Finite Width ◽

Hydraulic Jumps ◽

Turbulent Layer ◽

Near Surface ◽

Layer Interface ◽

Forced Waves

Abstract A model of surface waves generated on deep water by strong winds is proposed. A two-layer approximation is adopted, in which a shallow turbulent layer overlies the lower, infinitely deep layer. The dynamics of the upper layer, which is directly exposed to the wind, are nonlinear and coupled to the linear dynamics in the deep fluid. The authors demonstrate that in such a system there exist steady wave solutions characterized by confined regions of wave breaking alternating with relatively long intervals where the wave profiles change monotonically. In the former regions the flow is decelerated; in the latter it is accelerated. The regions of breaking are akin to hydraulic jumps of finite width necessary to join the smooth “interior” flows and have periodic waves. In contrast to classical hydraulic jumps, the strongly forced waves lose both energy and momentum across the jumps. The flow in the upper layer is driven by the balance between the wind stress at the surface, the turbulent drag applied at the layer interface, and the wave drag induced at the layer interface by quasi-steady breaking waves. Propagating in the downwind direction, the strongly forced waves significantly modify the flow in both layers, lead to enhanced turbulence, and reduce the speed of the near-surface flow. According to this model, a large fraction of the work done by the surface wind stress on the ocean in high winds may go directly into wave breaking and surface turbulence.

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