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Stable Stratification (stable + stratification)
Selected AbstractsFlow and pollution transport during Wagerup 2006: a case studyMETEOROLOGICAL APPLICATIONS, Issue 3 2010Charles Retallack Abstract As part of a broader field campaign dubbed Wagerup 2006, a case study was carried out to determine the overnight pollution transport mechanisms and flow characteristics near Wagerup, Western Australia. The ambient conditions were characterized by stable stratification with little synoptic influence in the lower boundary layer. An elevated jet intrusion originating on a nearby escarpment slope was found to induce sufficient mixing causing elevated pollution plumes to reach ground level. Onset of mixing was accurately predicted by non-linear critical Richardson number estimates obtained in previous laboratory work. The increase in ground level temperatures brought about by shear induced mixing later prompted a sea-breeze like gravity current that was completely blocked by the escarpment and as a result pollutants were trapped against the escarpment slope. A notable side effect of the topographic blocking was the subsequent steady 360° rotation of ground level winds within an area of influence described by the Rossby deformation radius. Copyright © 2009 Royal Meteorological Society [source] Turbulence energetics in stably stratified geophysical flows: Strong and weak mixing regimesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 633 2008S. S. Zilitinkevich Abstract Traditionally, turbulence energetics is characterised by turbulent kinetic energy (TKE) and modelled using solely the TKE budget equation. In stable stratification, TKE is generated by the velocity shear and expended through viscous dissipation and work against buoyancy forces. The effect of stratification is characterised by the ratio of the buoyancy gradient to squared shear, called the Richardson number, Ri. It is widely believed that at Ri exceeding a critical value, Ric, local shear cannot maintain turbulence, and the flow becomes laminar. We revise this concept by extending the energy analysis to turbulent potential and total energies (TPE, and TTE = TKE + TPE), consider their budget equations, and conclude that TTE is a conservative parameter maintained by shear in any stratification. Hence there is no ,energetics Ric', in contrast to the hydrodynamic-instability threshold, Ric,instability, whose typical values vary from 0.25 to 1. We demonstrate that this interval, 0.25 < Ri < 1, separates two different turbulent regimes: strong mixing and weak mixing rather than the turbulent and the laminar regimes, as the classical concept states. This explains persistent occurrence of turbulence in the free atmosphere and deep ocean at Ri , 1, clarifies the principal difference between turbulent boundary layers and free flows, and provides the basis for improving operational turbulence closure models. Copyright © 2008 Royal Meteorological Society [source] Variability in the summertime coastal marine atmospheric boundary-layer off California, USATHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004Linda Ström Abstract An analysis of boundary-layer structure and surface-layer turbulence from measurements off the California coast is presented from data collected by research aircraft during two field experiments: the Coastal Waves 1996 (CW96) and the Monterey Area Ship Track (MAST) experiments. CW96 covers the near-coast region, in particular in the vicinity of major headlands, whereas MAST extends offshore. Along the US west coast, coastal modification of the along-coast flow occurs on two main horizontal scales. Firstly, a large-scale variability is due to the interplay between the shallow near-coast marine atmospheric boundary-layer (MABL) and the coastal terrain, typically higher than the MABL depth. The MABL depth decreases smoothly towards the coast while the wind speed increases to a coastal jet in response to the sloping MABL inversion. Secondly, the flow is affected by supercritical flow dynamics. As the wind speed increases and the MABL depth decreases towards the coast, the MABL flow becomes supercritical in a shallow-water sense. As supercritical shallow-water flow interacts with major headlands, expansion fans form, affecting both the wind speed and the MABL depth. The combination of CW96 and MAST data reveals significant differences between the flow along the coast and that far offshore. MABL winds are stronger near the coast whereas aloft the winds are weaker than offshore. The near-coast MABL is also better mixed. Turbulence increases towards the coast while the sensible-heat flux decreases and often changes sign, leading to stable stratification near the coast. A length-scale determining the across-coast influence of the expansion fans is defined from simple inviscid and irrotational shallow-water theory. Data from four days of the CW96 experiment show that this simple theory describes the low-level wind speed adequately. Surface-layer turbulence also scales with this simple length-scale, but the results are complicated by upwelling of cold water, giving rise to strong internal boundary-layers. From terms in the turbulent-energy budgets it appears that dissipation of turbulence exceeds local production. Copyright © 2004 Royal Meteorological Society [source] Effects of shear sheltering in a stable atmospheric boundary layer with strong shearTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 596 2004Ann-Sofi Smedman Abstract Data from two marine field experiments in the Baltic Sea with stable stratification have been analysed. The purpose was to test the concept of the ,detached' or ,top-down' eddies and the ,shear-sheltering' mechanism in the presence of a low-level wind speed maximum in the atmosphere. Data used include turbulence and profile measurements on two 30 m towers and concurrent wind profiles throughout the boundary layer obtained from pilot-balloon soundings. Measurements show that large eddies are being suppressed when there is a low-level wind speed maximum present somewhere in the layer 40,300 m above the water surface and when the stratification is slightly stable. The effect is seen both in normalized standard deviations of the velocity components and in corresponding component spectra. In previous work it was argued that the relatively large eddies, which dominate the low wave number spectra in measurements in the surface layer, are detached or top-down eddies generated higher up in the boundary layer, that interact with the surface layer. The low-level wind maximum introduces a distinct layer with strong vorticity which, according to the shear-sheltering hypothesis, prevents these eddies from penetrating downwards. In the limit of the wind maximum occurring at a very low height (less than about 100 m), usual turbulence statistics characteristic of the ,canonical' boundary layer are found. Combining all the statistics, it is demonstrated that the wavelength of maximum spectral energy is locally related to a turbulence length-scale, which shows that for values of the Richardson number of unity or less the effect of the local wind gradient is greater than that of static stability. The reduction of length-scale with the strength of a low-level wind maximum, explains the observed reduction (by a factor of two) of the turbulent flux of sensible heat at the surface. This result indicates that the shear-sheltering mechanism is likely to play an important role in the turbulent exchange process at the surface in sea areas where low-level wind maxima are a frequently occurring phenomenon, such as the Baltic and other large water bodies surrounded by landmasses. Copyright © 2004 Royal Meteorological Society [source] | ||||||||||