Showing papers by "Timothy R. Oke published in 1991"

Simulation of surface urban heat islands under `ideal' conditions at night part 2: Diagnosis of causation

Abstract: A simple energy balance model which simulates the thermal regime of urban and rural surfaces under calm, cloudless conditions at night is used to assess the relative importance of the commonly stated causes of urban heat islands. Results show that the effects of street canyon geometry on radiation and of thermal properties on heat storage release, are the primary and almost equal causes on most occasions. In very cold conditions, space heating of buildings can become a dominant cause but this depends on wall insulation. The effects of the urban ‘greenhouse’ and surface emissivity are relatively minor. The model confirms the importance of local control especially the relation between street geometry and the heat island and highlights the importance of rural thermal properties and their ability to produce seasonal variation in the heat island. A possible explanation for the small heat

An evapotranspiration‐interception model for urban areas

Abstract: A model to calculate evapotranspiration from urban areas over a wide range of meteorological conditions is presented. An evapotranspiration-interception approach is used because it is necessary to cope with the changing water availability on the surface, during and following rainfall or irrigation. The model is applicable to areas ranging from the size of city blocks to land use zones and time periods of one hour and longer. The modeled evaporation is compared with that from micrometeorological measurements conducted from January to June 1987 in a suburb of Vancouver, British Columbia, Canada. The results show that this approach to modeling urban evapotranspiration provides realistic hourly and daily estimates of the areally averaged latent heat flux and surface water state.

Spatial variability of energy fluxes in suburban terrain

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.

Simulation of surface urban heat islands under ‘IDEAL’ conditions at night part 1: Theory and tests against field data

Abstract: Observations show that the urban heat island in the atmospheric layer below roof level is most strongly developed during calm, cloudless conditions at night. This paper outlines two versions of a numerical model to describe the cooling of rural and street canyon surfaces under these conditions using surface thermal and radiative properties and the radiative geometry of the canyons. One version uses a full system of differential equations and the other the simpler force-restore approach. The two approaches are shown to be in general agreement and the output of the simpler model is shown to give a faithful representation of cooling of rural and urban surfaces, and therefore heat islands, when compared with field observations.

Validation of an Urban Canyon Radiation Model for Nocturnal Long-wave Fluxes

Abstract: The urban canyon radiation model of Arnfield (1976, 1982) is validated using measurements of long-wave fluxes taken within a scaled down urban canyon constructed from concrete building blocks. A custom-designed traversing system allowed miniature radiometers to be automatically moved around the perimeter of a canyon cross-section, thereby providing for the validation of individual model grid-points. The agreement between measured and modelled radiation is generally very good. Some differences between the two over the canyon walls are attributed to difficulties in achieving precise instrument orientation. Model results derived from the measured surface temperature data are compared to results using various approximation schemes more likely to be used in practice. Approximations based on canyon surface temperatures are superior to those using air temperature.

Climate of Cities

Abstract: In a review of Helmut Landsberg’s book The Urban Climate (L1981-d), I referred to him as ‘urban climatology’s most respected doyen’. The subject was enormously enhanced by him having lent his name and interest. During the thirty years of his most direct influence, he helped bring the field from a largely descriptive status to one with a much fuller physical understanding. But more than that he kept a much needed balance between research ideals and practical relevance through his gentle urgings and reminders.