Boundary Layer Climates.

The energetic basis of the urban heat island

I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

Progress in urban climatology over the two decades since the first publication of the International Journal of Climatology is reviewed. It is emphasized that urban climatology during this period has benefited from conceptual advances made in microclimatology and boundary-layer climatology in general. The role of scale, heterogeneity, dynamic source areas for turbulent fluxes and the complexity introduced by the roughness sublayer over the tall, rigid roughness elements of cities is described. The diversity of urban heat islands, depending on the medium sensed and the sensing technique, is explained. The review focuses on two areas within urban climatology. First, it assesses advances in the study of selected urban climatic processes relating to urban atmospheric turbulence (including surface roughness) and exchange processes for energy and water, at scales of consideration ranging from individual facets of the urban environment, through streets and city blocks to neighbourhoods. Second, it explores the literature on the urban temperature field. The state of knowledge about urban heat islands around 1980 is described and work since then is assessed in terms of similarities to and contrasts with that situation. Finally, the main advances are summarized and recommendations for urban climate work in the future are made. Copyright © 2003 Royal Meteorological Society.

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Two decades of urban climate research: a review of turbulence, exchanges of energy and water, and the urban heat island

[1] We update the Goddard Institute for Space Studies (GISS) analysis of global surface temperature change, compare alternative analyses, and address questions about perception and reality of global warming. Satellite-observed night lights are used to identify measurement stations located in extreme darkness and adjust temperature trends of urban and periurban stations for nonclimatic factors, verifying that urban effects on analyzed global change are small. Because the GISS analysis combines available sea surface temperature records with meteorological station measurements, we test alternative choices for the ocean data, showing that global temperature change is sensitive to estimated temperature change in polar regions where observations are limited. We use simple 12 month (and n × 12) running means to improve the information content in our temperature graphs. Contrary to a popular misconception, the rate of warming has not declined. Global temperature is rising as fast in the past decade as in the prior 2 decades, despite year-to-year fluctuations associated with the El Nino-La Nina cycle of tropical ocean temperature. Record high global 12 month running mean temperature for the period with instrumental data was reached in 2010.

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Global surface temperature change

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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Turbulent Transport Near the Ground in Stable Conditions

Measurements of long-wave radiative flux divergence in the vertical were obtained from an above roof-level site in the urban area of Montreal, P.Q. Radiative flux divergence was observed in the 4–5.5 m layer above the roof on almost all occasions, but especially with weak air flow. Radiative cooling rates calculated from the radiation measurements were greater than actual air temperature cooling rates, and in reasonable agreement with previous rural results. The importance and implications of these findings in urban meteorology are discussed.

Long-wave radiative flux divergence and nocturnal cooling of the urban atmosphere

An observational study of the temperature profile over grass, snow and bare soil surfaces in strong stability is presented It was found that in nearly calm, clear conditions the minimum temperature occurred 1 to 50 cm above the surface The height and intensity of the minimum is strongly correlated with wind speed, surface roughness and stability A discussion of existing hypotheses suggests that the most likely explanation for the phenomenon should involve both turbulence and infra-red flux divergence

The temperature profile near the ground on calm clear nights

This paper investigates the applicability of standard boundary layer theory and observational methods to the urban system. It considers the spatial and temporal sampling problems posed by the large size of the roughness elements and the considerable horizontal spatial variability of the sources and sinks. The discussion is illustrated by reference to a series of studies conducted over suburban terrain in Vancouver, B.C., Canada. Special attention is paid to the validity of micrometeorological theory and method utilizing eddy correlation or profile methods to evaluate turbulent exchanges. It is concluded that despite the physical problems presented by the nature of the 'surface' it is possible to obtain valid areally-averaged fluxes from fixed-point observations provided that careful site selection, height of measurement and temporal sampling procedures are followed.

Evaluation of spatially-averaged fluxes of heat, mass and momentum in the urban boundary layer

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.

Timothy R. Oke

Papers

Turbulent Transport Near the Ground in Stable Conditions

Long-wave radiative flux divergence and nocturnal cooling of the urban atmosphere

The temperature profile near the ground on calm clear nights

Evaluation of spatially-averaged fluxes of heat, mass and momentum in the urban boundary layer

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