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- Author or Editor: T. R. Oke x
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Abstract
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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.
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.
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.
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.
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.
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.
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.
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.
Abstract
The flux density of sensible heat to or from storage in the physical mass of the city is determined for seven cities (Chicago, Illinois; Los Angeles, California; Mexico City, Distrito Federal; Miami, Florida; Sacramento, California; Tucson, Arizona; and Vancouver, British Columbia) in North America across a 30° latitudinal range. These cities have a variety of synoptic-scale climates and surface cover and structural morphologies. In all cases the “measured” storage heat flux is determined as the energy balance residual from direct observations of net all-wave radiation, and sensible and latent heat fluxes conducted using the same radiometer and eddy correlation techniques. Databases describing the surface characteristics around each site are developed from analysis of aerial photography and field surveys. Results indicate that storage heat flux is a significant component of the surface energy balance at all sites and is greatest at downtown and light industrial sites. Hysteresis behavior, of varying degrees, is seen at all locations. A simple objective hysteresis model (OHM), which calculates storage heat flux as a function of net all-wave radiation and the surface properties of the site, is found to perform well in the mean for most cases, with the notable exception of Tucson; but considerable scatter is observed at some sites. Some of this is attributed to the moisture, wind, and synoptic controls at each of the sites, and to hour-to-hour variability in the convective fluxes that the OHM does not simulate. Averaging over 2 to 3 h may be a more appropriate way to use the model. Caution should be used when employing the OHM in windy environments.
Abstract
The flux density of sensible heat to or from storage in the physical mass of the city is determined for seven cities (Chicago, Illinois; Los Angeles, California; Mexico City, Distrito Federal; Miami, Florida; Sacramento, California; Tucson, Arizona; and Vancouver, British Columbia) in North America across a 30° latitudinal range. These cities have a variety of synoptic-scale climates and surface cover and structural morphologies. In all cases the “measured” storage heat flux is determined as the energy balance residual from direct observations of net all-wave radiation, and sensible and latent heat fluxes conducted using the same radiometer and eddy correlation techniques. Databases describing the surface characteristics around each site are developed from analysis of aerial photography and field surveys. Results indicate that storage heat flux is a significant component of the surface energy balance at all sites and is greatest at downtown and light industrial sites. Hysteresis behavior, of varying degrees, is seen at all locations. A simple objective hysteresis model (OHM), which calculates storage heat flux as a function of net all-wave radiation and the surface properties of the site, is found to perform well in the mean for most cases, with the notable exception of Tucson; but considerable scatter is observed at some sites. Some of this is attributed to the moisture, wind, and synoptic controls at each of the sites, and to hour-to-hour variability in the convective fluxes that the OHM does not simulate. Averaging over 2 to 3 h may be a more appropriate way to use the model. Caution should be used when employing the OHM in windy environments.
Abstract
Several methods to determine the aerodynamic characteristics of a site through analysis of its surface form (morphometry) are considered in relation to cities. The measures discussed include zero-plane displacement length (z d ), roughness length (z 0), depth of the roughness sublayer, and aerodynamic conductance. A sensitivity analysis is conducted on seven formulas to estimate z d and nine to estimate z 0, covering a wide range of probable urban roughness densities. Geographic information systems developed for 11 sites in 7 North American cities are used to characterize their morphometry—the height, shape, three-dimensional area, and spatial distribution of their roughness elements (buildings and trees). Most of the sites are in residential suburbs, but one is industrial and two are near city centers. This descriptive survey of urban geometric form is used, together with the morphometric formulas, to derive the apparent aerodynamic characteristics of the sites. The resulting estimates of z d and z 0 are compared with values obtained from analysis of wind and turbulence observations. The latter are obtained from a survey of approximately 60 field studies and 14 laboratory studies of real and scale model cities. Despite the comprehensive nature of the survey, very few studies are found to be acceptable and their scatter is large, hence they do not provide a standard against which to test the morphometric algorithms. Further, the data show only weak relations between measured z d and z 0 and roughness density. The relative merits of morphometric and wind-based estimates of aerodynamic parameters are discussed. Recommendations are made concerning the choice of method to estimate z d and z 0 in urban areas and their most likely magnitude.
Abstract
Several methods to determine the aerodynamic characteristics of a site through analysis of its surface form (morphometry) are considered in relation to cities. The measures discussed include zero-plane displacement length (z d ), roughness length (z 0), depth of the roughness sublayer, and aerodynamic conductance. A sensitivity analysis is conducted on seven formulas to estimate z d and nine to estimate z 0, covering a wide range of probable urban roughness densities. Geographic information systems developed for 11 sites in 7 North American cities are used to characterize their morphometry—the height, shape, three-dimensional area, and spatial distribution of their roughness elements (buildings and trees). Most of the sites are in residential suburbs, but one is industrial and two are near city centers. This descriptive survey of urban geometric form is used, together with the morphometric formulas, to derive the apparent aerodynamic characteristics of the sites. The resulting estimates of z d and z 0 are compared with values obtained from analysis of wind and turbulence observations. The latter are obtained from a survey of approximately 60 field studies and 14 laboratory studies of real and scale model cities. Despite the comprehensive nature of the survey, very few studies are found to be acceptable and their scatter is large, hence they do not provide a standard against which to test the morphometric algorithms. Further, the data show only weak relations between measured z d and z 0 and roughness density. The relative merits of morphometric and wind-based estimates of aerodynamic parameters are discussed. Recommendations are made concerning the choice of method to estimate z d and z 0 in urban areas and their most likely magnitude.
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.
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.
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.
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.
