Unsteady Thermally Driven Flows on Gentle Slopes

Hunt, J. C. R.; Fernando, Harindra J. S.; Princevac, Marko

doi:10.1175/1520-0469(2003)060<2169:utdfog>2.0.co;2

Cited by 63 publications

(68 citation statements)

References 26 publications

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“…Following the analysis of Hunt et al (1988Hunt et al ( , 2003a, if the heating applied at t ¼ 0 is strong enough that h/ |L MO | ) 1, the root mean square turbulent velocity (u¢) is determined by the buoyancy forces, so that, for t >ât MO ; u 0 $ w Ã , where w Ã $ ðgF s h=h 0 Þ 1=3 . The convection time scale h/w Ã is smaller than the shearing time-scale h/u Ã , and the mean shear generates Reynolds stresses (Rao and Narasimha, 1996).…”

Section: Strong Surface Heat Fluxmentioning

confidence: 99%

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Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

Owinoh

Hunt

Orr

et al. 2005

Boundary-Layer Meteorol

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We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u Ã when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t h less than about h/(10u Ã ). Standard Monin-Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h/|L MO | over the time scale t MO~| L MO |/(10u Ã ) (where L MO and t MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h/ |L MO | () 1), over the time scale t MO . The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A lowlevel wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.

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Section: Strong Surface Heat Fluxmentioning

confidence: 99%

“…In a subsequent paper, we will use similar analysis and numerical simulations to study the effect of the changing boundary layer over small slopes, which previous authors (e.g. Scorer, 1954;Hunt et al, 2003a) have shown are significant.…”

Section: Introductionmentioning

confidence: 99%

Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

Owinoh

Hunt

Orr

et al. 2005

Boundary-Layer Meteorol

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“…Air currents move along slopes and riparian areas in a predictable manner during the day, generally flowing upstream (anabatic winds) as ambient air temperatures increase and warm air moves upwards along drainages, and then downstream (katabatic winds) as temperatures drop and cold air sinks from surrounding slopes (e.g., Gazzilli et al 2001;Hunt et al 2003). Movement of air across the opening of enclosed bird nests has been suggested to create a small vacuum that circulates air and creates cooler internal temperatures beneficial to a desert nesting species (Facemire et al 1990).…”

Section: Discussionmentioning

confidence: 99%

Nest orientation of the Spotted Barbtail, Premnoplex brunnescens, is strongly correlated with stream flow

Greeney

2008

J Ethol

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The placement and orientation of bird nests may strongly influence reproductive success. For many species, nest orientation is related to the cardinal directions and has to do with prevailing winds or solar exposure. Nest orientation differs among species in different environments, variously cooling or warming nests to maintain a nest microclimate optimal for successful development of eggs and young. The Spotted Barbtail (Premnoplex brunnescens) builds mossy, enclosed nests along shaded streams in Neotropical cloud forests. It shows a unique pattern of nest orientation, whereby nests are oriented in relation to the direction of water flowing below the nest entrance, rather than compass bearing. Nests face in one of three directions in relation to the stream: downstream, upstream, or perpendicular to the flow of the water. I discuss the ability of various hypotheses to explain this pattern in the context of factors affecting nest orientation in other species. While the reasons for this pattern of orientation remain unclear, orientation is likely driven by the need to create and maintain an optimal internal nest microclimate.

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“…Evening and morning transition physics are beginning to be modeled (e.g., Hunt et al 2003), but due to a lack of sound theoretical guidance, previous observational programs have failed to employ diagnostics essential to capture some interesting features. Examples include formation of intrusions during morning transition and fronts during evening transition identified during theoretical and laboratory work.…”

Section: Terrain Inhomogeneitiesmentioning

confidence: 99%

Whither the Stable Boundary Layer?

Fernando¹,

Weil²

2010

Bull. Amer. Meteor. Soc.

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109

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C learly no other part of the atmosphere is more important to Earth's ecosystems than its lowest layer, known as the atmospheric boundary layer (ABL). The land surface exchanges heat, mass, and momentum with the free atmosphere through the ABL, and naturally the ABL is affected by orography, land use, external forcing (e.g., radiation), and Earth's rotation. Environmental changes, whether due to slowly evolving global warming or rapidly dispersing atmospheric releases, permeate through to living organisms via the ABL. During the daytime, the ABL is driven by surface heating [the convective boundary layer (CBL)], whereas radiative cooling near the ground at night leads to the stable boundary layer (SBL). The nocturnal boundary layer (NBL) is the most common manifestation of SBL, with notable exceptions being areas where the urban heat island eliminates the near-surface stable stratification and polar regions where the SBL can persist continuously for days. The SBL breaks down into a CBL during the "morning transition," and the CBL collapses to an SBL during the "evening transition."Over the past half century, the progress in understanding the CBL has far outpaced the SBL; the much stronger forcing in the CBL makes measurement and modeling of turbulence therein much easier. Conversely, the SBL encapsulates a unique mix of processes that are generally much weaker (at least in total) and often difficult to measure at their scales of influence (let alone over multiple scales), study in isolation, or parameterize robustly. These processes interact in nonlinear way such that emerging new phenomena overshadow the contributing processes, and direct parameterizations of the former based on an understanding of the latter may not be viable. A greater emphasis is therefore needed on the interactions of SBL processes and the resulting modification of heat, mass, and momentum fluxes. Modeling of commonly sought meteorological and air quality indicators-surface temperature and wind speed/direction, fog, air pollution, and dispersion of chemical, biological, and radiological contaminants-relies heavily on  Detail of a shallow SBL in a mountain valley. For complete photo, see Fig. 1.

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Unsteady Thermally Driven Flows on Gentle Slopes

Cited by 63 publications

References 26 publications

Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

Nest orientation of the Spotted Barbtail, Premnoplex brunnescens, is strongly correlated with stream flow

Whither the Stable Boundary Layer?

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