The California heat island: seasonal trends in maximum and minimum temperatures

This paper summarizes progress in an ongoing study of California's temperature trends. It supplements studies reported at PACLIM in 1984, 1986, and 1987. ... Objectives of this study are twofold: to examine and map the trends in maximum and minimum temperatures for the warm and cool seasons separately, and to examine regional differences in maximum and minimum temperature trends in California.

Interactions between building integrated photovoltaics and microclimate in urban environments

BIPV (building integrated photovoltaics) has progressed in the past years and become an element to be considered in city planning. BIPV has significant influence on microclimate in urban environments and the performance of BIPV is also affected by urban climate. The thermal model and electrical performance model of ventilated BIPV are combined to predict PV temperature and PV power output in Tianjin, China. Then, by using dynamic building energy model, the building cooling load for installing BIPV is calculated. A multi-layer model AUSSSM of urban canopy layer is used to assess the effect of BIPV on the Urban Heat Island (UHI). The simulation results show that in comparison with the conventional roof, the total building cooling load with ventilation PV roof may be decreased by 10%. The UHI effect after using BIPV relies on the surface absorptivity of original building. In this case, the daily total PV electricity output in urban areas may be reduced by 13% compared with the suburban areas due to UHI and solar radiation attenuation because of urban air pollution. The calculation results reveal that it is necessary to pay attention to and further analyze interactions between BIPV and microdimate in urban environments to decrease urban pollution, improve BIPV performance and reduce cooling load. Copyright © 2006 by ASME.

Interactions between Building Integrated Photovoltaics and microclimate in urban environments

BIPV(Building Integrated Photovoltaics) has progressed in the past years and become an element to be considered in city planning. BIPV has influence on microclimate in urban environments and the performance of BIPV is also affected by urban climate. The effect of BIPV on urban microclimate can be summarized under the following four aspects. The change of absorptivity and emissivity from original building surface to PV will change urban radiation balance. After installation of PV, building cooling load will be reduced because of PV shading effect, so urban anthropogenic heat also decreases to some extent. Because PV can reduce carbon dioxide emissions which is one of the reasons for urban heat island, BIPV is useful to mitigate this phenomena. The anthropogenic heat will alter after using BIPV, because partial replacement of fossil fuel means to change sensible heat from fossil fuel to solar energy. Different urban microclimate may have various effects on BIPV performance that can be analyzed from two perspectives. Firstly, BIPV performance may decline with the increase of air temperature in densely built areas because many factors in urban areas cause higher temperature than that of the surrounding countryside. Secondly, the change of solar irradiance at the ground level under urban air pollution will lead to the variation of BIPV performance because total solar irradiance usually is reduced and each solar cell has a different spectral response characteristic. The thermal model and performance model of ventilated BIPV according to actual meteorologic data in Tianjin(China) are combined to predict PV temperature and power output in the city of Tianjin. Then, using dynamic building energy model, cooling load is calculated after BIPV installation. The calculation made based in Tianjin shows that it is necessary to pay attention to and further analyze interaction between them to decrease urban pollution, improve BIPV Performance and reduce colling load. Copyright © 2005 by ASME.

Simulation of urban microclimates

Urban microclimates are greatly affected by urban form and texture and have a significant impact on building energy performance. The impact of urban form on energy consumption in buildings mainly relates to the availability of the uses of solar radiation, daylighting and natural ventilation. The urban heat island (UHI) effect increases the risk of overheating in buildings as well as the maximum energy demand for cooling. A need has arisen for a robust calculation tool (using the first-cut calculation method) to enable planners, architects and environmental assessors, to quickly and accurately compare the impact of different urban forms on local climate and UHI mitigation strategies. This paper describes a tool for the simulation of urban microclimates, which is developed by integrating image processing with a coupled thermal and airflow model.

Urban microclimates and simulation

This chapter examines the workings of urban microclimates and looks at the associated causes and effects of the urban heat island (UHI). It also clarifies the relationship between urban form and the key climatic parameters (sun, daylight, wind, temperature). A particular section is devoted to the concepts of UHI intensity and sky view factor (SVF); these are useful indicators for researchers in this area. The challenge of how to model urban microclimates is covered, featuring the six archetypal urban forms familiar to analysts involved in using simulation software. The latter sections address the issue of urban thermal comfort, the importance of urban ventilation and finally what mitigating strategies can be implemented to curb negative UHI effects.

Effects of urban ventilation patterns on the carbon monoxide concentration in a high-rise mega city

Carbon monoxide (CO) concentration data from 1999–2006, monitored at 5 different pollution stations in a high-rise mega city (Hong Kong), were collected and investigated. The spatio-temporal characteristics of urban CO concentration profiles were obtained. A new approach was put forward to examine the relationship between urban CO concentration and different wind flow patterns. Rather than relying on the meteorological data from a single weather station, usually adopted in previous studies, four weather stations on the boundary of Hong Kong territory were used in the present study so as to identify 16 different wind flow patterns, among which a typical urban heat island circulation (UHIC) can be distinguished. Higher concentrations were observed to be associated with the flow pattern of an inflow from Lau Fau Shan (LFS) station which is located in the northwest of Hong Kong. This suggests that the ability of dilution for north-to-west wind is relatively weak due to the pollutants carried from outside Hong Kong. The effectiveness of wind speed on the alleviation of urban concentration is dependent on the initial concentration of the approaching wind. The increase of wind speed of north-to-west wind from 0 m/s to 6 m/s has little effect on the reduction of urban CO concentration, especially on the non-roadside stations. By contrast, for the southerly marine wind, pollution concentration decreases sharply with an increase in the wind speed. It was also found that urban heat island circulation (UHIC) is conducive of the accumulation of pollutants, especially at night. There exists a positive correlation between CO concentration and UHI intensity. This correlation is much stronger at night compared to during the day. Keywords: urban pollution monitoring, urban ventilation pattern, urban heat island circulation, mega city

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.

Impact of anthropogenic heat emissions on London's temperatures

We investigate the role of the anthropogenic heat flux on the urban heat island of London. To do this, the time-varying anthropogenic heat flux is added to an urban surface-energy balance parametrization, the Met Office–Reading Urban Surface Exchange Scheme (MORUSES), implemented in a 1 km resolution version of the UK Met Office Unified Model. The anthropogenic heat flux is derived from energy-demand data for London and is specified on the model's 1 km grid; it includes variations on diurnal and seasonal time-scales. We contrast a spring case with a winter case, to illustrate the effects of the larger anthropogenic heat flux in winter and the different roles played by thermodynamics in the different seasons. The surface-energy balance channels the anthropogenic heat into heating the urban surface, which warms slowly because of the large heat capacity of the urban surface. About one third of this additional warming goes into increasing the outgoing long-wave radiation and only about two thirds goes into increasing the sensible heat flux that warms the atmosphere. The anthropogenic heat flux has a larger effect on screen-level temperatures in the winter case, partly because the anthropogenic flux is larger then and partly because the boundary layer is shallower in winter. For the specific winter case studied here, the anthropogenic heat flux maintains a well-mixed boundary layer through the whole night over London, whereas the surrounding rural boundary layer becomes strongly stably stratified. This finding is likely to have important implications for air quality in winter. On the whole, inclusion of the anthropogenic heat flux improves the comparison between model simulations and measurements of screen-level temperature slightly and indicates that the anthropogenic heat flux is beginning to be an important factor in the London urban heat island.

Urban climates and global climate change

Cities and global climate change are closely linked: cities are where the bulk of greenhouse gas emissions take place through the consumption of fossil fuels; they are where an increasing proportion of the world’s people live; and they also generate their own climate – commonly characterized by the urban heat island. In this way, understanding the way cities affect the cycling of energy, water, and carbon to create an urban climate is a key element of climate mitigation and adaptation strategies, especially in the context of rising global temperatures and deteriorating air quality in many cities. As climate models resolve finer spatial-scales, they will need to represent those areas in which more than 50% of the world’s population already live to provide climate projections that are of greater use to planning and decision-making. Finally, many of the processes that are instrumental in determining urban climate are the same factors leading to global anthropogenic climate change, namely regional-scale land-use changes; increased energy use; and increased emissions of climatically-relevant atmospheric constituents. Cities are therefore both a case study for understanding, and an agent in mitigating, anthropogenic climate change. This chapter reviews and summarizes the current state of understanding of the physical basis of urban climates, as well as our ability to represent these in models. We argue that addressing the challenges of managing urban environments in a changing climate requires understanding the energy, water, and carbon balances for an urban landscape and, importantly, their interactions and feedbacks, together with their links to human behaviour and controls. We conclude with some suggestions for where further research is needed.

An integrated study of urban microclimates in Chongqing, China: historical weather data, transverse measurement and numerical simulation

Chongqing is the largest central-government-controlled municipality in China, which is now under going a rapid urbanization. The question remains open: What are the consequences of such rapid urbanization in Chongqing in terms of urban microclimates? An integrated study comprising three different research approaches is adopted in the present paper. By analyzing the observed annual climate data, an average rising trend of 0.10◦C/decade was found for the annual mean temperature from 1951 to 2010 in Chongqing,indicating a higher degree of urban warming in Chongqing. In addition, two complementary types of field measurements were conducted: fixed weather stations and mobile transverse measurement. Numerical simulations using a house-developed program are able to predict the urban air temperature in Chongqing.The urban heat island intensity in Chongqing is stronger in summer compared to autumn and winter.The maximum urban heat island intensity occurs at around midnight, and can be as high as 2.5◦C. In the day time, an urban cool island exists. Local greenery has a great impact on the local thermal environment.Urban green spaces can reduce urban air temperature and therefore mitigate the urban heat island. The cooling effect of an urban river is limited in Chongqing, as both sides of the river are the most developed areas, but the relative humidity is much higher near the river compared with the places far from it.

Biotic effects of climate change in urban environments: The case of the grey-headed flying-fox (Pteropus poliocephalus) in Melbourne, Australia

Urban climates are known to differ from those of the surrounding rural areas, as human activities in cities lead to changes in temperature, humidity and wind regimes. These changes can in turn affect the geographic distribution of species, the behaviour of animals and the phenology of plants. The grey-headed flying-fox (Pteropus poliocephalus) is a large, nomadic bat from eastern Australia that roosts in large colonies known as camps. Historically a warm temperate to tropical species, P. poliocephalus recently established a year-round camp in the Royal Botanic Gardens Melbourne. Using a bioclimatic analysis, we demonstrated that on the basis of long-term data, Melbourne does not fall within the climatic range of other P. poliocephalus camp sites in Australia. Melbourne is drier than other summer camps, and cooler and drier than other winter camps. The city also receives less radiation, in winter and annually, than the other summer and winter camps of P. poliocephalus. However, we found that temperatures in central Melbourne have been increasing since the 1950s, leading to warmer conditions and a reduction in the number of frosts. In addition, artificial watering of parks and gardens in the city may contribute the equivalent of 590 mm (95% CI: 450–720 mm) of extra rainfall per year. It appears that human activities have increased temperatures and effective precipitation in central Melbourne, creating a more suitable climate for camps of the grey-headed flying-fox. As demonstrated by this example, anthropogenic climate change is likely to complicate further the task of conserving biological diversity in urban environments.

Building surfaces and their effect on the urban thermal environment

Use of high reflectance surfaces reduces the amount of solar radiation absorbed through building envelopes and urban structures and thus keeping their surfaces cooler. The cooling energy savings by using high reflectance surfaces have been well documented. Higher surface temperatures add to increasing the ambient temperature as convection intensity is higher. Such temperature increase has significant impacts on the air conditioning energy utilization in hot climates. This study makes use of numerical simulations to analyze the effect of commonly used building materials on the air temperature. A part of the existing CBD (Central Business District) area of Singapore was selected for the study. A series of Computational Fluid Dynamics (CFD) simulations have been carried out using the software CFX-5.6. It was found that at low wind speeds, the effect of materials on the air temperature was significant and the temperature at the middle of a narrow canyon increased up to 2.5[degrees]C with the facade material having lower reflectance.

Green façades for urban comfort improvement. Implementation in a extreme Continental Mediterranean climate

Green façades constitute constructive technologies with a positive influence on sustainability in cities and several urban climate parameters such as thermal comfort, air quality and water management. According to the current research, the implementation of urban greenery contributes to increase the cooling effect and mitigate the urban heat island (UHI) phenomenon. This paper focuses on the role of vegetation in improving the urban environment of Madrid (Spain). The simulation results show that green walls could be more effective in urban morphologies with narrow streets. During overheated periods, the streets with green walls have a higher relative humidity in the surrounding areas than those with trees. The air temperature is found to be a little lower. The reduction of wind speed means a positive effect on urban hygrothermal comfort. Therefore, green walls could be taken into account as suitable tools to modify the outdoor thermal environment in cities with an extreme Continental Mediterranean climate.

Potential urban distribution of Phlebotomus mascittii Grassi and Phlebotomus neglectus Tonn. (Diptera: Psychodidae) in 2021–50 in Budapest, Hungary

Background & Objective: The most northern populations of two sand fly species (Phlebotomus mascittii and Phlebotomus neclectus) in the Carpathian Basin are known from Central Hungary. The most important limiting factor of the distribution of Phlebotomus species in the region is the annual minimum temperature which may be positively affected by the urban heat island and the climate change in the future. Method: Based on the latest case reports of the species, Climate Envelope Model was done for the period 1961-1990 and 2025-2050 to project the potential urban distribution of the species. The climatic data were obtained from RegCM regional climate model and MODIS satellite images. Results: The recent occurrence of the species in Central Hungary indicates that Phlebotomus species can overwinter in non-heated shelters in the built environment. Interpretation & Conclusion: Jointly heat island and future climate change seem to be able to provide suitable environment for the studied species in urban areas in a great extent.

Surface thermal analysis of North Brabant cities and neighbourhoods during heat waves

The urban heat island effect is often associated with large metropolises. However, in the Netherlands even small cities will be affected by the phenomenon in the future (Hove et al., 2011), due to the dispersed or mosaic urbanisation patterns in particularly the southern part of the country: the province of North Brabant. This study analyses the average night time land surface temperature (LST) of 21 North-Brabant urban areas through 22 satellite images retrieved by Modis 11A1 during the 2006 heat wave and uses Landsat 5 Thematic Mapper to map albedo and normalized difference temperature index (NDVI) values. Albedo, NDVI and imperviousness are found to play the most relevant role in the increase of nighttime LST. The surface cover cluster analysis of these three parameters reveals that the 12 “urban living environment” categories used in the region of North Brabant can actually be reduced to 7 categories, which simplifies the design guidelines to improve the surface thermal behaviour of the different neighbourhoods thus reducing the Urban Heat Island (UHI) effect in existing medium size cities and future developments adjacent to those cities.