C. S. B. Grimmond

Aerodynamic Properties of Urban Areas Derived from Analysis of Surface Form

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 (zd), roughness length (z0), depth of the roughness sublayer, and aerodynamic conductance. A sensitivity analysis is conducted on seven formulas to estimate zd and nine to estimate z0, 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 zd and z0 a...

Turbulent heat fluxes in urban areas: Observations and a local-scale urban meteorological parameterization scheme (LUMPS)

Abstract A linked set of simple equations specifically designed to calculate heat fluxes for the urban environment is presented. This local-scale urban meteorological parameterization scheme (LUMPS), which has similarities to the hybrid plume dispersion model (HPDM) scheme, requires only standard meteorological observations and basic knowledge of surface cover. LUMPS is driven by net all-wave radiation. Heat storage by the urban fabric is parameterized from net all-wave radiation and surface cover information using the objective hysteresis model (OHM). The turbulent sensible and latent heat fluxes are calculated using the available energy and are partitioned using the approach of de Bruin and Holtslag, and Holtslag and van Ulden. A new scheme to define the Holtslag and van Ulden α and β parameters for urban environments is presented; α is empirically related to the plan fraction of the surface that is vegetated or irrigated, and a new urban value of β captures the observed delay in reversal of the sign of...

Heat Storage in Urban Areas: Local-Scale Observations and Evaluation of a Simple Model

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 308 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.

The International Urban Energy Balance Models Comparison Project: First Results from Phase 1

A large number of urban surface energy balance models now exist with different assumptions about the important features of the surface and exchange processes that need to be incorporated. To date, no comparison of these models has been conducted; in contrast, models for natural surfaces have been compared extensively as part of the Project for Intercomparison of Land-surface Parameterization Schemes. Here, the methods and first results from an extensive international comparison of 33 models are presented. The aim of the comparison overall is to understand the complexity required to model energy and water exchanges in urban areas. The degree of complexity included in the models is outlined and impacts on model performance are discussed. During the comparison there have been significant developments in the models with resulting improvements in performance (root-mean-square error falling by up to two-thirds). Evaluation is based on a dataset containing net all-wave radiation, sensible heat, and latent heat flux observations for an industrial area in Vancouver, British Columbia, Canada. The aim of the comparison is twofold: to identify those modeling approaches that minimize the errors in the simulated fluxes of the urban energy balance and to determine the degree of model complexity required for accurate simulations. There is evidence that some classes of models perform better for individual fluxes but no model performs best or worst for all fluxes. In general, the simpler models perform as well as the more complex models based on all statistical measures. Generally the schemes have best overall capability to model net all-wave radiation and least capability to model latent heat flux.

Local-scale fluxes of carbon dioxide in urban environments: methodological challenges and results from Chicago

Much attention is being directed to the measurement and modeling of surface-atmosphere exchanges of CO2 for different surface types. However, as yet, few measurements have been conducted in cities, even though these environments are widely acknowledged to be major sources of anthropogenic CO2. This paper highlights some of the challenges facing micrometeorologists attempting to use eddy covariance techniques to directly monitor CO2 fluxes in urban environments, focusing on the inherent variability within and between urban areas, and the importance of scale and the appropriate height of measurements. Results from a very short-term study of CO2 fluxes, undertaken in Chicago, Illinois in the summer of 1995, are presented. Mid-afternoon minimum CO2 concentrations and negative fluxes are attributed to the strength of biospheric photosynthesis and strong mixing of local anthropogenic sources in a deep mixed layer. Poor night-time atmospheric mixing, lower mixed layer depths, biospheric respiration, and continued missions from mobile and fixed anthropogenic sources, account for the night-time maxima in CO2 concentrations. The need for more, longer-term, continuous eddy covariance measurements is stressed.

The energy balance of central Mexico City during the dry season

Abstract The first measurements of the energy balance fluxes of a dry, densely built-up, central city site are presented. Direct observation of the net radiation, sensible and latent heat flux densities above roof-top in the old city district of Mexico City allow the heat storage flux density to be found by residual. The most important finding is that during daytime, when evaporation is very small ( 8), the uptake of heat by the buildings and substrate is so large (58%) that convective heating of the atmosphere is reduced to a smaller role than expected (38%). The nocturnal release of heat from storage is equal to or larger than the net radiation and sufficient to maintain an upward convective heat flux throughout most nights. It is important to see if this pattern is repeated at other central city, or dry urban sites, or whether it is only found in districts dominated by massive stone structures. These findings have implications for the height of the urban mixing layer and the magnitude of the urban heat island.

An Urban Parameterization for a Global Climate Model. Part I: Formulation and Evaluation for Two Cities

Abstract Urbanization, the expansion of built-up areas, is an important yet less-studied aspect of land use/land cover change in climate science. To date, most global climate models used to evaluate effects of land use/land cover change on climate do not include an urban parameterization. Here, the authors describe the formulation and evaluation of a parameterization of urban areas that is incorporated into the Community Land Model, the land surface component of the Community Climate System Model. The model is designed to be simple enough to be compatible with structural and computational constraints of a land surface model coupled to a global climate model yet complex enough to explore physically based processes known to be important in determining urban climatology. The city representation is based upon the “urban canyon” concept, which consists of roofs, sunlit and shaded walls, and canyon floor. The canyon floor is divided into pervious (e.g., residential lawns, parks) and impervious (e.g., roads, par...

AN EVAPOTRANSPIRATION-INTERCEPTION MODEL FOR URBAN AREAS

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.

Comparison of Heat Fluxes from Summertime Observations in the Suburbs of Four North American Cities

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.

An objective urban heat storage model and its comparison with other schemes

Abstract An objective hydteresis model to predict the storage heat flux in urban areas is presented. A review of observational and theoretical work reveals this approach to be more approproate than the linear relation between soil heat flux and net all-wave radiation. A scheme to implement the model in any urban area is developed. In essence the model only requires land cover and net all-wave radiation as input, but it can be further refined to include anthropogenic heat release, the three-dimensional form of the surface, and can allow for changes in source area. Tests against energy balance data from a site in Vancouver, BC indicate the model simulates most aspects of measured storage heat flux values for a suburban site in both winter and summer. Comparison with the results of a study in Bonn, Germany involving the use of heat flux plates and detailed heat content change calculations gives good agreement except for a phase difference of about 1 h. There is evidence to suggest that the spatial variation of intra-urban heat storage may be relatively conservative.