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

Abstract

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.

The reliability of global and hemispheric surface temperature records

“…Numerous papers have addressed urban climates and found large differences between city centre sites and rural neighbours for individual day and night temperatures (see ref-erences in Arnfield, 2003) but these studies are generally not relevant to the global-scale data bases described here. This is because most of these comparisons only consider days that maximise the urban/rural difference and so are not directly relevant in the context of long-term monthly averages for typical (non-city-centre) weather stations.…”

“…Such urban growth is likely to affect temperature records from urban sites, and warming trends from such sites are likely, on average, to be larger than if the city or town were not there (see review by Arnfield, 2003). In climatology, this issue is referred to as the urbanization effect or the urban heat island.…”

An Urban Parameterization for a Global Climate Model. Part II: Sensitivity to Input Parameters and the Simulated Urban Heat Island in Offline Simulations

“…Urban ecosystems can significantly alter the radiative, thermal, moisture, and aerodynamic characteristics of the land surface (Landsberg 1981;Oke 1987;Bonan 2002;Arnfield 2003). As a consequence of these changes, urban climates can differ significantly from surrounding natural ecosystems, often resulting in urban heat islands (e.g., Landsberg 1981).…”

“…As a consequence of these changes, urban climates can differ significantly from surrounding natural ecosystems, often resulting in urban heat islands (e.g., Landsberg 1981). The simulation of urban climate impacts requires two major components: 1) the representation of the physical processes controlling energy and water fluxes and 2) the characterization of urban morphology and urban materials with respect to aerodynamic, radiative, and heat transfer properties (e.g., Terjung and O'Rourke 1980;Arnfield 2000;Masson 2000;Grimmond and Oke 2002;Martilli et al 2002;Best 2005;Oleson et al 2008, hereinafter Part I). We have developed an urban parameterization to simulate urban systems on a global scale (Part I) under a wide variety of climate and surface conditions.…”

Improved method of ultraviolet radiation reflection measurement for non-horizontal urban surfaces

“…Reflected electromagnetic radiation is important for UV exposure modeling studies in urban environments, which require information on reflected radiation in order to account for incident irradiance not directly from the sun, such as heat island studies [6,7] and urban radiation distribution studies [8]. Whilst most of these studies tend towards long wave radiation research, the same modeling tools could potentially be used to create specific shortwave radiation models in order to predict "hot spots" in UV radiation in an urban environment that would be useful in helping the public monitor their UV exposure in the urban environment such as a study by Kumar, Skidmore and Knowles [9].…”