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T. R. Oke

Abstract

The variation of the nondimensional transfer coefficients, k * and K *, and the Monin-Obukhov function ϕ in strong stability, are examined. The exchange coefficients for heat, water vapor and momentum are shown to be approximately equal even at large Ri. The relationships suggest that the decay of turbulent motion in the lower atmosphere begins at Ri=0.1, and completely ceases at Ri>0.3. In these nonturbulent conditions the profiles of wind and water vapor show very little variation with height, but the temperature profile reflects the increasing importance of infrared radiation.

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T. R. Oke

Abstract

No abstract available.

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M. Roth
and
T. R. Oke

Abstract

This study uses observational data from a suburban site in Vancouver, British Columbia, Canada, to investigate the relative facility with which heat, water vapor, and momentum are transported by turbulence in the unstable surface layer. The ratios of linear correlation coefficients −rwT /ruw and −rwq /ruw increase approximately linearly with instability and are generally smaller than typical rural values due to bluff-body effects. The ratio rwT /rwq is greatest near neutral and larger than unity at all stabilities. This inequality may be caused by the complex source/sink patterns of the urban surface, cloud effects on the radiative forcing, and the unusually well-developed interaction between the surface and the upper portions of the urban boundary layer. Inequality of transfer between T and q will make it difficult to measure turbulent fluxes for cities using standard gradient approaches.

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K. E. Runnalls
and
T. R. Oke

Abstract

A new method to detect errors or biases in screen-level air temperature records at standard climate stations is developed and applied. It differs from other methods by being able to detect microclimatic inhomogeneities in time series. Such effects, often quite subtle, are due to alterations in the immediate environment of the station such as changes of vegetation, development (buildings, paving), irrigation, cropping, and even in the maintenance of the site and its instruments. In essence, the technique recognizes two facts: differences of thermal microclimate are enhanced at night, and taking the ratio of the nocturnal cooling at a pair of neighboring stations nullifies thermal changes that occur at larger-than-microclimatic scales. Such ratios are shown to be relatively insensitive to weather conditions. After transforming the time series using Hurst rescaling, which identifies long-term persistence in geophysical phenomena, cooling ratio records show distinct discontinuities, which, when compared against detailed station metadata records, are found to correspond to even minor changes in the station environment. Effects detected by this method are shown to escape detection by current generally accepted techniques. The existence of these microclimatic effects are a source of uncertainty in long-term temperature records, which is in addition to those presently recognized such as local and mesoscale urban development, deforestation, and irrigation.

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J. A. Voogt
and
T. R. Oke

Abstract

An observation program using ground and airborne thermal infrared radiometers is used to estimate the surface temperature of urban areas, taking into account the total active surface area. The authors call this the complete urban surface temperature. This temperature is not restricted by the viewing biases inherent in remote sensors used to estimate surface temperature over rough surfaces such as cities. Two methods to estimate the complete surface temperature are presented. Results for three different land-use areas in the city of Vancouver, British Columbia, Canada, show significant differences exist between the complete, nadir, and off-nadir airborne estimates of urban surface temperature during daytime. For the sites and times studied, the complete surface temperature is shown to agree with airborne off-nadir estimates of the apparent surface temperature of the most shaded walls. Some implications of using the complete surface temperature to estimate screen level air temperature and to calculate surface sensible heat flux are given.

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I. D. Stewart
and
T. R. Oke

The effect of urban development on local thermal climate is widely documented in scientific literature. Observations of urban–rural air temperature differences—or urban heat islands (UHIs)—have been reported for cities and regions worldwide, often with local field sites that are extremely diverse in their physical and climatological characteristics. These sites are usually described only as “urban” or “rural,” leaving much uncertainty about the actual exposure and land cover of the sites. To address the inadequacies of urban–rural description, the “local climate zone” (LCZ) classification system has been developed. The LCZ system comprises 17 zone types at the local scale (102 to 104 m). Each type is unique in its combination of surface structure, cover, and human activity. Classification of sites into appropriate LCZs requires basic metadata and surface characterization. The zone definitions provide a standard framework for reporting and comparing field sites and their temperature observations. The LCZ system is designed primarily for urban heat island researchers, but it has derivative uses for city planners, landscape ecologists, and global climate change investigators.

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M. Nunez
and
T. R. Oke

Abstract

This study investigates the energy exchanges occurring within an urban canyon. It considers not only the energy balances of each of the canyon component surfaces (walls and floor), but also the balance of the canyon system and of the air volume contained therein. The results are based on measurements conducted in a specially instrumented canyon during a period of fine anticyclonic summer weather in Vancouver, B.C. The timing and magnitude of the energy regime of the individual canyon surfaces are shown to be very different from each other, each being strongly affected by the influence of the canyon geometry on the radiation exchanges. The diurnal course of the canyon system energy balance is relatively smooth and symmetric. By day the canyon system radiative surplus is mainly dissipated by turbulent transfer, and the remaining 25–30% is stored in the canyon materials. In contrast, the nocturnal radiative deficit is almost entirely balanced by the release of subsurface heat storage. Advective contributions to the air volume energy balance are shown to depend upon wind direction and speed, as well as the nature of the surrounding thermal environment.

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D. Yap
and
T. R. Oke

Abstract

The use of the eddy correlation technique is demonstrated for the measurement of sensible heat transfer in an urban area. The problems of time and space sampling (in the horizontal and vertical) are investigated. Based on 27 summer days of observations from a roof-top site in the central built-up part of Vancouver, the diurnal variation of sensible heat transfer above an urban area is described. The flux of heat at 1.2, 4 and 20 m above roof level largely reflected time and magnitude changes in the net radiation field. While being in phase with net radiation, the sensible heat flow commonly exhibited unusually high values in the late afternoon. Nocturnal urban sensible heat flow was quite unlike the normal rural pattern, often being directed into the atmosphere. Also at night the existence of flux divergence is suggested.

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C. S. B. Grimmond
and
T. R. Oke

Abstract

Previous measurements of urban energy balances have been restricted to a small number of cities. This paper presents directly measured energy balance fluxes for suburban areas in four cities within the United States: Tucson, Sacramento, Chicago, and Los Angeles. They represent a range of synoptic regimes and surface morphologies (built and vegetative). Ensemble diurnal patterns and ratios of fluxes for clear, cloudy, and all sky conditions are presented. Consideration is given to both the mean and the variability of the fluxes. As expected, the magnitudes of the fluxes vary between cities; however, in general, the diurnal trends of flux partitioning are similar in terms of the timing of the peaks and changes in sign. Chicago is slightly different due to frequent wetting by rain. In the other cities, it seems that daytime Bowen ratios are inversely related to the area irrigated.

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C. S. B. Grimmond
and
T. R. Oke
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