排序方式： 共有59条查询结果，搜索用时 500 毫秒
41.
Aurore Porson Ian N. Harman Sylvia I. Bohnenstengel Stephen E. Belcher 《Boundary-Layer Meteorology》2009,132(1):107-128
We investigate the question of how many facets are needed to represent the energy balance of an urban area by developing simplified
3-, 2- and 1-facet versions of a 4-facet energy balance model of two-dimensional streets and buildings. The 3-facet model
simplifies the 4-facet model by averaging over the canyon orientation, which results in similar net shortwave and longwave
balances for both wall facets, but maintains the asymmetry in the heat fluxes within the street canyon. For the 2-facet model,
on the assumption that the wall and road temperatures are equal, the road and wall facets can be combined mathematically into
a single street-canyon facet with effective values of the heat transfer coefficient, albedo, emissivity and thermodynamic
properties, without further approximation. The 1-facet model requires the additional assumption that the roof temperature
is also equal to the road and wall temperatures. Idealised simulations show that the geometry and material properties of the
walls and road lead to a large heat capacity of the combined street canyon, whereas the roof behaves like a flat surface with
low heat capacity. This means that the magnitude of the diurnal temperature variation of the street-canyon facets are broadly
similar and much smaller than the diurnal temperature variation of the roof facets. Consequently, the approximation that the
street-canyon facets have similar temperatures is sound, and the road and walls can be combined into a single facet. The roof
behaves very differently and a separate roof facet is required. Consequently, the 2-facet model performs similarly to the
4-facet model, while the 1-facet model does not. The models are compared with previously published observations collected
in Mexico City. Although the 3- and 2-facet models perform better than the 1-facet model, the present models are unable to
represent the phase of the sensible heat flux. This result is consistent with previous model comparisons, and we argue that
this feature of the data cannot be produced by a single column model. We conclude that a 2-facet model is necessary, and for
numerical weather prediction sufficient, to model an urban surface, and that this conclusion is robust and therefore applicable
to more general geometries. 相似文献
42.
The mean flow profile within and above a tall canopy is well known to violate the standard boundary-layer flux–gradient relationships.
Here we present a theory for the flow profile that is comprised of a canopy model coupled to a modified surface-layer model.
The coupling between the two components and the modifications to the surface-layer profiles are formulated through the mixing
layer analogy for the flow at a canopy top. This analogy provides an additional length scale—the vorticity thickness—upon
which the flow just above the canopy, within the so-called roughness sublayer, depends. A natural form for the vertical profiles
within the roughness sublayer follows that overcomes problems with many earlier forms in the literature. Predictions of the
mean flow profiles are shown to match observations over a range of canopy types and stabilities.
The unified theory predicts that key parameters, such as the displacement height and roughness length, have a significant
dependence on the boundary-layer stability. Assuming one of these parameters a priori leads to the incorrect variation with stability of the others and incorrect predictions of the mean wind speed profile. The
roughness sublayer has a greater impact on the mean wind speed in stable than unstable conditions. The presence of a roughness
sublayer also allows the surface to exert a greater drag on the boundary layer for an equivalent value of the near-surface
wind speed than would otherwise occur. This characteristic would alter predictions of the evolution of the boundary layer
and surface states if included within numerical weather prediction models. 相似文献
43.
Arash Nemati Hayati Rob Stoll J. J. Kim Todd Harman Matthew A. Nelson Michael J. Brown Eric R. Pardyjak 《Boundary-Layer Meteorology》2017,164(2):217-247
Three computational fluid dynamics (CFD) methods with different levels of flow-physics modelling are comprehensively evaluated against high-spatial-resolution wind-tunnel velocity data from step-down street canyons (i.e., a short building downwind of a tall building). The first method is a semi-empirical fast-response approach using the Quick Urban Industrial Complex (QUIC-URB) model. The second method solves the Reynolds-averaged Navier–Stokes (RANS) equations, and the third one utilizes a fully-coupled fluid-structure interaction large-eddy simulation (LES) model with a grid-turbulence inflow generator. Unlike typical point-by-point evaluation comparisons, here the entire two-dimensional wind-tunnel dataset is used to evaluate the dynamics of dominant flow topological features in the street canyon. Each CFD method is scrutinized for several geometric configurations by varying the downwind-to-upwind building-height ratio (\(H_\mathrm{d}/H_\mathrm{u}\)) and street canyon-width to building-width aspect ratio (S / W) for inflow winds perpendicular to the upwind building front face. Disparities between the numerical results and experimental data are quantified in terms of their ability to capture flow topological features for different geometric configurations. Overall, all three methods qualitatively predict the primary flow topological features, including a saddle point and a primary vortex. However, the secondary flow topological features, namely an in-canyon separation point and secondary vortices, are only well represented by the LES method despite its failure for taller downwind building cases. Misrepresentation of flow-regime transitions, exaggeration of the coherence of recirculation zones and wake fields, and overestimation of downwards vertical velocity into the canyon are the main defects in QUIC-URB, RANS and LES results, respectively. All three methods underestimate the updrafts and, surprisingly, QUIC-URB outperforms RANS for the streamwise velocity component, while RANS is superior to QUIC-URB for the vertical velocity component in the street canyon. 相似文献
44.
Rebecca Cunningham Christopher Cvitanovic Thomas Measham Brent Jacobs Anne-Maree Dowd Ben Harman 《Climate Policy》2016,16(7):894-908
There has been a growing recognition regarding the use of social networks to engage communities in government actions. However, despite increasing awareness of social networks, there is very limited evidence for their application in relation to climate policy. This study fills this gap by assessing the potential of social networks for engaging local communities in climate adaptation policy, drawing on a case study of the Shoalhaven region in Australia. Participants from key representative groups were recruited using a purposive snowball sampling technique (N?=?24). By mapping knowledge acquisition and diffusion networks in relation to climate adaption at the local scale, this study identified key nodes within the networks. Findings demonstrate that although climate adaptation information was acquired from a diverse range of sources, the sharing knowledge networks were far more dispersed. Furthermore, although 165 knowledge sources were identified, three nodes had coverage cross the entire network, and as such acted as boundary spanners within the sharing network. This research demonstrates the utility of social network analysis to reveal the underlying knowledge networks and structures that influence community engagement pathways and in doing so outlines key implications in relation to engaging local communities in climate policy and action.Policy relevanceThe rapid development of adaptation as a mainstream strategy for managing the risks of climate change has resulted in the emergence of a broad range of adaptation policies and management strategies globally. However, the success of these initiatives is largely dependent on their acceptance and uptake by local communities, which to date remains a significant challenge. Accordingly, policy makers require novel approaches to overcome barriers to community engagement so as to enhance the likely success of community engagement pathways. This article demonstrates the value of using social network analysis to reveal the underlying knowledge network structures. This approach makes it possible to identify key individuals within a community who can disseminate adaptation information quickly across broad geographic ranges. By utilizing this approach, policy makers globally will be able to increase the extent to which adaption initiatives are accepted and adhered to by local communities, thus increasing their success. 相似文献
45.
The advance of a chemical weathering front into the bedrock of a hillslope is often limited by the rate weathering products that can be carried away, maintaining chemical disequilibrium. If the weathering front is within the saturated zone, groundwater flow downslope may affect the rate of transport and weathering—however, weathering also modifies the rock permeability and the subsurface potential gradient that drives lateral groundwater flow. This feedback may help explain why there tends to be neither “runaway weathering” to great depth nor exposed bedrock covering much of the earth and may provide a mechanism for weathering front advance to keep pace with incision of adjacent streams into bedrock. This is the second of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory. Here, we show how a simplified kinetic model of 1‐D rock weathering can be extended to consider lateral flow in a 2‐D hillslope. Exact and approximate analytical solutions for the location and thickness of weathering within the hillslope are obtained for a number of cases. A location for the weathering front can be found such that lateral flow is able to export weathering products at the rate required to keep pace with stream incision at steady state. Three pathways of solute export are identified: “diffusing up,” where solutes diffuse up and away from the weathering front into the laterally flowing aquifer; “draining down,” where solutes are advected primarily downward into the unweathered bedrock; and “draining along,” where solutes travel laterally within the weathering zone. For each pathway, a different subsurface topography and overall relief of unweathered bedrock within the hillslope is needed to remove solutes at steady state. The relief each pathway requires depends on the rate of stream incision raised to a different power, such that at a given incision rate, one pathway requires minimal relief and, therefore, likely determines the steady‐state hillslope profile. 相似文献
46.
This is the first of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory (also known as Dupuit or Boussinesq theory). Here, we examine the effect of lateral flow on the downward fluxes of water and solutes through perched groundwater at steady state. We derive analytical expressions describing the decline in the downward flux rate with depth. Using these, we obtain analytical expressions for water age in a number of cases. The results show that when the permeability field is homogeneous, the spatial structure of water age depends qualitatively on a single dimensionless number, Hi. This number captures the relative contributions to the lateral hydraulic potential gradient of the relief of the lower‐most impermeable boundary (which may be below the weathering front within permeable or incipiently weathered bedrock) and the water table. A “scaled lateral symmetry” exists when Hi is low: age varies primarily in the vertical dimension, and variations in the horizontal dimension x almost disappear when the vertical dimension z is expressed as a fraction z/H(x) of the laterally flowing system thickness H(x). Taking advantage of this symmetry, we show how the lateral dimension of the advection–diffusion‐reaction equation can be collapsed, yielding a 1‐D vertical equation in which the advective flux downward declines with depth. The equation holds even when the permeability field is not homogeneous, as long as the variations in permeability have the same scaled lateral symmetry structure. This new 1‐D approximation is used in the accompanying paper to extend chemical weathering models derived for 1‐D columns to hillslope domains. 相似文献
47.
Christopher Poëtte Barry Gardiner Sylvain Dupont Ian Harman Margi Böhm John Finnigan Dale Hughes Yves Brunet 《Boundary-Layer Meteorology》2017,163(3):393-421
Landscape discontinuities such as forest edges play an important role in determining the characteristics of the atmospheric flow by generating increased turbulence and triggering the formation of coherent tree-scale structures. In a fragmented landscape, consisting of surfaces of different heights and roughness, the multiplicity of edges may lead to complex patterns of flow and turbulence that are potentially difficult to predict. Here, we investigate the effects of different levels of forest fragmentation on the airflow. Five gap spacings (of length approximately 5h, 10h, 15h, 20h, 30h, where h is the canopy height) between forest blocks of length 8.7h, as well as a reference case consisting of a continuous forest after a single edge, were investigated in a wind tunnel. The results reveal a consistent pattern downstream from the first edge of each simulated case, with the streamwise velocity component at tree top increasing and turbulent kinetic energy decreasing as gap size increases, but with overshoots in shear stress and turbulent kinetic energy observed at the forest edges. As the gap spacing increases, the flow appears to change monotonically from a flow over a single edge to a flow over isolated forest blocks. The apparent roughness of the different fragmented configurations also decreases with increasing gap size. No overall enhancement of turbulence is observed at any particular level of fragmentation. 相似文献
48.
49.
D. J. Harman M. Bryce M. P. Redman A. J. Holloway 《Monthly notices of the Royal Astronomical Society》2003,342(3):823-829
The little studied PN, Sa 2-21 has been observed using the Manchester echelle spectrometer at the Anglo-Australian telescope. Narrow band, long-slit spectra were obtained at six positions over two perpendicular position angles in both the [N ii ]λ6584 Å and [O iii ]λ5007 Å emission lines. An [O iii ] halo has been detected for the first time. A morphological modelling program was used to help determine the geometry, structure and kinematics of this ellipsoidal PN. It is proposed that the structure includes a pair of mid-latitude rings of [N ii ] emission, not previously seen in elliptical PNe. Radial spokes of [O iii ] emission have been detected in the main nebular shell indicating the presence of dynamical instabilities. 相似文献
50.