Effect of mountain breeze in ventilating the city under calm and neutral atmospheric environment: Interaction of airflow structures and ventilation efficiency

Many modern cities locate in the mountainous areas, like Hong Kong, Phoenix City and Los Angles. It is confirmed in the literature that the mountain wind system developed by differential heating or cooling can be very beneficial in ventilating the city nearby and alleviating the UHI effect. However, the direct interaction of mountain wind with the natural-convection circulation due to heated urban surfaces has not been studied, to our best knowledge. This kind of unique interaction of two kinds of airflow structures under calm and neutral atmospheric environment is investigated in this paper by CFD approach. A physical model comprising a simple mountain and three long building blocks (forming two street canyons) is firstly developed. Different airflow structures are identified within the conditions of different mountain-building height ratios (R=Hm/Hb) by varying building height but fixing mountain height. It is found that the higher ventilation rate in the street canyons is expected in the cases of smaller mountain-building ratios, indicating the stronger natural convection due to increasing heated building surfaces. However, there is the highest air change rate (ACH) in the lowest-building-height case and most of the air is advective into the street canyon through the top open area, highlighting the important role played by the mountain wind. In terms of the ventilation efficiency, it is shown that the smallest R case enjoys the best air change efficiency followed by the highest R case, while the worst ventilative street canyons occur at the middle R case. In the end, a gap across the streets is introduced in the modeling. The existence of the gap can greatly channel the mountain wind and distribute the air into streets nearby. Thus the ACH can be doubled and air quality can be significantly improved.

Ensuring the right to the city: pro-poor housing, urban development and tenure legalization in São Paulo, Brazil

São Paulo is one of Latin America’s most modern and developed cities, yet around one-third of its 10 million inhabitants live in poor-quality housing in sub-standard settlements. This paper describes the response of the São Paulo municipal government that took office in 2001. Through its Secretariat of Housing and Urban Development, it designed a new policy framework with a strong emphasis on improving the quantity and quality of housing for low-income groups. Supported by new legislation, financial instruments and partnerships with the private sector, the mainstays of the new policy are integrated housing and urban development, modernization of the administrative system, and public participation in all decision-making and implementation processes. The programmes centre on upgrading and legalizing land tenure in informal settlements, and regeneration of the city centre. The new focus on valuing the investments that low-income groups have already made in their housing and settlements has proved to be more cost-effective than previous interventions, leading to improvements on an impressive scale.

Passive urban ventilation by combined buoyancy-driven slope flow and wall flow: Parametric CFD studies on idealized city models

This paper reports the results of a parametric CFD study on idealized city models to investigate the potential of slope flow in ventilating a city located in a mountainous region when the background synoptic wind is absent. Examples of such a city include Tokyo in Japan, Los Angeles and Phoenix in the US, and Hong Kong. Two types of buoyancy-driven flow are considered, i.e., slope flow from the mountain slope (katabatic wind at night and anabatic wind in the daytime), and wall flow due to heated/cooled urban surfaces. The combined buoyancy-driven flow system can serve the purpose of dispersing the accumulated urban air pollutants when the background wind is weak or absent. The microscopic picture of ventilation performance within the urban structures was evaluated in terms of air change rate (ACH) and age of air. The simulation results reveal that the slope flow plays an important role in ventilating the urban area, especially in calm conditions. Katabatic flow at night is conducive to mitigating the nocturnal urban heat island. In the present parametric study, the mountain slope angle and mountain height are assumed to be constant, and the changing variables are heating/cooling intensity and building height. For a typical mountain of 500 m inclined at an angle of 20° to the horizontal level, the interactive structure is very much dependent on the ratio of heating/cooling intensity as well as building height. When the building is lower than 60 m, the slope wind dominates. When the building is as high as 100 m, the contribution from the urban wall flow cannot be ignored. It is found that katabatic wind can be very beneficial to the thermal environment as well as air quality at the pedestrian level. The air change rate for the pedestrian volume can be as high as 300 ACH.

CO intake fraction and city ventilation rate in Hong Kong

Given the background of serious urban pollution in Hong Kong, the intake fraction (iF) of carbon monoxide due to mobile vehicles in urban area of Hong Kong is investigated and estimated to be 600 per million, much higher than those in US urban areas, Helsinki and even Beijing, indicating the high exposure level to urban pollutants in Hong Kong. The dependence of iF to the metrological factors is also discussed. Easterly and northerly winds contribute most to the total iF value. A new method of predicting ventilation rate for a city based on iF concept is proposed. City ventilation rates for different cities are calculated and compared. It is found that Hong Kong has to face the fact that it has the lowest ventilation rate and ACH.

Briefing: re-engineering the city 2020–2050 – urban foresight and transition management

Cities may be responsible for up to 70% of global carbon emissions and 75% of global energy consumption and by 2050 it is estimated that 70% of the world's population could live in cities. The critical challenge for contemporary urbanism, therefore, is to understand how to develop the knowledge, capacity and capability for public agencies, the private sector and multiple users in city regions systemically to re-engineer their built environment and urban infrastructure in response to climate change and resource constraints. Re-Engineering the City 2020–2050: Urban Foresight and Transition Management (Retrofit 2050) is a major new interdisciplinary project funded under the Engineering and Physical Science Research Council's (EPSRC) Sustainable Urban Environments Programme which seeks to address this challenge. This briefing describes the background and conceptual framing of Retrofit 2050 project, its aims and objectives and research approach.

City futures: exploring urban retrofit and sustainable transitions

Cities are responsible for up to 70% of global carbon emissions and 75% of global energy consumption. By 2050 it is estimated that 70% of the world's population will live in cities. The critical challenge for contemporary urbanism, therefore, is to understand how to develop the knowledge, capacity and capability for public agencies, the private sector and multiple users in city-regions (i.e. the city and its wider hinterland) to re-engineer systemically their built environment and urban infrastructure in response to climate change and resource constraints. To inform transitions to urban sustainability, key stakeholders' perceptions were sought though a participatory backcasting and scenario foresight process in order to illuminate challenging but realistic socio-technical scenarios for the systemic retrofit of core UK city-regions. The challenge of conceptualizing complex urban transitions is explored across multiple socio-technical ‘regimes’ (housing, non-domestic buildings, urban infrastructure), scales (building, neighbourhood, city-region), and domains (energy, water, use of resources) within a participatory process. The development of three archetypal ‘guiding visions’ of retrofit city-regional futures developed through this process are discussed, along with the contribution that such foresight processes might play in ‘opening up’ the governance and strategic navigation of urban sustainability.

Nature of vegetation and building morphology characteristics across a city: influence on shadow patterns and mean radiant temperatures in London

Vegetation and building morphology characteristics are investigated at 19 sites on a north-south LiDAR transect across the megacity of London. Local maxima of mean building height and building plan area density at the city centre are evident. Surprisingly, the mean vegetation height (zv3) is also found to be highest in the city centre. From the LiDAR data various morphological parameters are derived as well as shadow patterns. Continuous images of the effects of buildings and of buildings plus vegetationon sky view factor (Ψ) are derived. A general reduction of Ψ is found, indicating the importance of including vegetation when deriving Ψ in urban areas. The contribution of vegetation to the shadowing at ground level is higher during summer than in autumn. Using these 3D data the influence on urban climate and mean radiant temperature (T mrt ) is calculated with SOLWEIG. The results from these simulations highlight that vegetation can be most effective at reducing heat stress within dense urban environments in summer. The daytime average T mrt is found to be lowest in the densest urban environments due to shadowing; foremost from buildings but also from trees. It is clearly shown that this method could be used to quantify the influence of vegetation on T mrt within the urban environment. The results presented in this paper highlight a number of possible climate sensitive planning practices for urban areas at the local scale (i.e. 102- 5 × 103 m).

Energy exchange in a dense urban environment Part II: impact of spatial heterogeneity of the surface

The centre of cities, characterised by spatial and temporal complexity, are challenging environments for micrometeorological research. This paper considers the impact of sensor location and heterogeneity of the urban surface on flux observations in the dense city centre of London, UK. Data gathered at two sites in close vicinity, but with different measurement heights, were analysed to investigate the influence of source area characteristics on long-term radiation and turbulent heat fluxes. Combining consideration of diffuse radiation and effects of specular reflections, the non-Lambertian urban surface is found to impact the measurements of surface albedo. Comparisons of observations from the two sites reveal that turbulent heat fluxes are similar under some flow conditions. However, they mostly observe processes at different scales due to their differing measurement heights, highlighting the critical impact of siting sensors in urban areas. A detailed source area analysis is presented to investigate the surface controls influencing the energy exchanges at the different scales

The role of location in knowledge creation and diffusion: evidence of centripetal and centrifugal forces in the City of London financial services agglomeration

In this paper we report on a major empirical study of centripetal and centrifugal forces in the City of London financial services agglomeration. The study sheds light on (1) the manner and magnitude of firm interaction in the agglomeration; (2) the characteristics of the agglomeration that aid the competitiveness of incumbent firms; and (3) the problems associated with agglomeration. In addressing these issues, we use the data to (1) test emerging theory that explains the high productivity and innovation of agglomerations in terms of their ability to generate and diffuse knowledge; and (2) evaluate the ‘end of geography’ thesis.

Creativity, entrepreneurship and economic development: city-level evidence on creativity spillover of entrepreneurship

We examine the black box of creativity, entrepreneurship and economic development by asking about the mechanisms through which creativity can influence economic development in cities. We propose that, like the knowledge spillover theory of entrepreneurship, creativity spillovers occur and can be slowed by a creativity filter. We examine how creativity and entrepreneurship, and creativity and a melting pot environment, interact to influence urban economic development. Using data on 187 cities in 15 European countries for the period 1999–2009, we advance the extant literature by providing evidence on the existence and dynamics of a creativity filter.

The urban cool island phenomenon in a high-rise high-density city and its mechanisms

The urban heat island (UHI) phenomenon has been studied extensively, but there are relatively fewer reports on the so-called urban cool island (UCI) phenomenon. We reveal here that the UCI phenomenon exists in Hong Kong during the day, and is associated with the UHI at night under all wind and cloud conditions. The possible mechanisms for the UCI phenomenon in such a high-rise compact city have been discovered using a lumped urban air temperature model. A new concept of urban cool island degree hours (UCIdh) to measure the UCI intensity and duration is proposed. Our analyses reveal that when anthropogenic heat is small or absent, a high-rise and high-density city experiences a significant daytime UCI effect. This is explained by an intensified heat storage capacity and the reduced solar radiation gain of urban surfaces. However, if anthropogenic heat in the urban area increases further, the UCI phenomenon still exists, yet UCIdh decrease dramatically in a high-rise compact city. In a low-rise, low-density city, the UCI phenomenon also occurs when there is no anthropogenic heat, but easily disappears when there is little anthropogenic heat, and the UHI phenomenon dominates. This probably explains why the UHI phenomenon is often observed, but the UCI phenomenon is rarely observed. The co-existence of urban heat/cool island phenomena implies reduction of the daily temperature range (DTR) in such cities, and its dependence on urban morphology also implies that urban morphology can be used to control the urban thermal environment.