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1

Electronic Resource

Spatial variability of energy fluxes in suburban terrain (1991)

Schmid, H. P. ; Cleugh, H. A. ; Grimmond, C. S. B. ; [et al.]

Springer

Boundary layer meteorology 54 (1991), S. 249-276

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Energy fluxes over an area of “homogeneous” suburban residential land-use in Vancouver, B.C., Canada are shown to vary by up to 25–40% within horizontal scales on the order of 102–103 m. Previously, variability of this magnitude has been expected to occur only at larger scales, between land-use zones or as urban-rural differences. In view of these findings, it is recognized that microadvective interaction between surface types at small scales may be important and can affect the energy balance even at larger scales. The present study discusses the small-scale spatial variability of energy fluxes and shows that it varies greatly for each term in the surface energy balance. Net radiation shows a relatively conservative behaviour (via albedo-surface temperature feedback) with little spatial variability. The turbulent fluxes (measured by eddy correlation at 28 m height), on the other hand, show a link between their temporal and spatial variability as the result of a temporally shifting source area which contains varying combinations of surface cover (using the dynamical source area concept of Schmid and Oke, 1990). As a result, part of the measured temporal variation is attributable to spatial differences in surface cover. Anthropogenic heat flux and storage heat flux (both modelled using a high resolution surface data-base) exhibit temporally varying spatial distributions. Their spatial pattern, however, is governed by nested scales of urban morphology (blocks, streets, properties, etc.). These differences in the source of variability between each component flux suggest a difficulty in the interpretation of the energy balance over urban areas, unless each term is spatially-averaged over the principal morphological units occurring in the area.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00183956

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2

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Suburban-rural energy balance comparisons in summer for Vancouver, B.C. (1986)

Cleugh, H. A. ; Oke, T. R.

Springer

Boundary layer meteorology 36 (1986), S. 351-369

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Simultaneous energy balance observations at a rural and a suburban site in Vancouver, B.C. during the summer of 1983 are presented. The study is a follow-up to that conducted in 1980. Many of the 1980 results were unexpected and the present study seeks to assess their representativeness. The net radiant, turbulent sensible, and rural soil heat flux densities were measured directly. The suburban heat storage was parameterized and the turbulent latent heat flux densities were resolved as residuals in the energy balances. The 1983 average diurnal energy partitioning for both sites was typical of those quoted in the literature, suggesting that the 1980 results represent an extreme case. Suburban-rural differences showed the suburban area to have a 4% increase in net radiation, a 51% increase in turbulent sensible heat, and a 46% decrease in turbulent latent heat flux density. The values of the average daytime Bowen ratio were 0.46 and 1.28 for the rural and suburban areas, respectively. The sensible heat flux density exhibited relatively large values in the late afternoon and remained directed upward on many summer evenings. Large day-to-day variability in the relative magnitude of the suburban turbulent fluxes may have been due to synoptic influences. In this environment, the turbulent surface and mixed layers are closely coupled because of the low aerodynamic resistance over the rough surface.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00118337

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3

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Urban heat storage derived as energy balance residuals (1987)

Oke, T. R. ; Cleugh, H. A.

Springer

Boundary layer meteorology 39 (1987), S. 233-245

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract The hourly heat storage changes of a suburban area in Vancouver B.C. are evaluated by residual after determining all other terms in the surface energy balance. Despite the errors and uncertainties associated with this approach, the resulting diurnal cycle of heat storage uptake and release appears to behave in a fashion similar to that of less complex systems. When plotted against the net all-wave radiation, the heat storage values display a hysteresis loop with the peak storage preceding that of radiation. Therefore, although a linear parameterization of heat storage is shown to produce reasonable first-order estimates of storage, a form which includes the time derivative of the net radiation is probably better.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00116120

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4

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Effects of windbreaks on airflow, microclimates and crop yields (1998)

Cleugh, H. A.

Springer

Agroforestry systems 41 (1998), S. 55-84

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ISSN: 1572-9680

Keywords: evaporation ; turbulence

Source: Springer Online Journal Archives 1860-2000

Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Notes: Abstract The mechanisms by which a porous windbreak modifies airflow, microclimates and hence crop yields are addressed, based upon recent wind tunnel experiments, field observations and numerical modelling. This paper is thus an update to the excellent reviews in Brandle (1988). It shows how a turbulent mixing layer initiated at the top of the windbreak dominates the airflow behind a windbreak. This mixing layer spreads vertically as it moves downwind, growing at a rate determined by the turbulence in the approach flow and the windbreak's ‘permeability’. The roughness of the terrain and land-cover upwind, windbreak height and porosity are thus the main controls on the amount and extent of shelter provided by a windbreak. The changes in temperature, humidity, heat and evaporation fluxes given these changes in turbulence are then described. Based on the turbulent mixing layer model, the highly sheltered ‘quiet zone’ will be typically warmer and more humid while further downwind in the ‘wake zone’, cooler and drier conditions would be expected. The careful experimental studies needed to verify these theoretical predictions have not yet been published. Shade is also shown to modify the heating in the quiet zone and, depending on the orientation of the windbreak, can offset the warming in the quiet zone. Lastly, the mechanisms affecting plant productivity are described in light of these airflow and microclimate changes. A major effect of a windbreak is to reduce the incidence of low frequency, high magnitude damage events such as sandblasting or lodging. Microclimate effects, however, do not always improve productivity. For example, while shelter may improve water-use efficiency in irrigated crops by increasing yields and reducing water-use, this may not be the case in dryland agriculture.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1006019805109

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Direct mechanical effects of wind on crops (1998)

Cleugh, H. A. ; Miller, J. M. ; Böhm, M.

Springer

Agroforestry systems 41 (1998), S. 85-112

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ISSN: 1572-9680

Keywords: abrasion ; leaf stripping ; lodging ; sandblasting ; thigmorphogenesis ; windbreaks

Source: Springer Online Journal Archives 1860-2000

Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Notes: Abstract This review describes those mechanisms by which wind directly affects crop growth rates and hence yields. Wind-induced plant movement is capable of altering growth rates and leaf morphology, although this is unlikely to be a major cause of growth differences between sheltered and unsheltered crops grown outdoors. The wind's force can tear leaves or strip them from the plant. Dense plant canopies may suffer abrasion through intermittent or constant rubbing. Soil particles lifted into suspension by the wind have the potential to abrade and damage plant tissue. The wind's force can physically knock plants over, making crops difficult to harvest. Each of these mechanisms operates at a particular time of the growing season. Recovery, and hence final yield, depends on the growth stage and soil/plant moisture status when the damage occurred, the particular species and variety as well as the preceding and subsequent weather. The fact that damage effects are so dependent on the crop and the past weather makes modelling and any simple synthesis of direct wind effects difficult. The most common forms of damage likely in Australia's agricultural regions are from sandblasting and lodging. These damage events will be intermittent – their frequency depending on the local climate. Leaf tearing is likely in broad-leafed horticultural crops, and growth effects are also likely in any windy location. It is not possible to predict what the impact of this damage, and other direct effects, will be on final yields, Based on the results in the literature, protection from damage offered by windbreaks may have as large an effect on yields as incremental microclimate benefits.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1006067721039

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