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Urban Sustainability: the Role of Micrometeorology by Dan Li (CEE) |
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Figure 1. The relationship between the average electric load and the maximum daily temperature in New Orleans, LA. The figure is directly downloaded from http://www.epa.gov/hiri/impacts/index.htm. |
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Urban areas are emerging as the nexus of the energy, water, health and climate challenges facing humanity in this century and beyond. More than 50% of the world population is currently living in cities, and this percentage is projected to continue growing rapidly. In addition, climate change arguably poses another dimension of challenge on cities for example by increasing the frequency deadly heat waves. Understanding and assessing the combined impact of urbanization and global climate change on various issues including health risks, water/energy sustainability, and pollution management in urban areas is becoming increasingly important. Urban sustainability has become one of those interesting and important topics that span a wide range of disciplines. Two common standards of sustainability are: (1) the ability to improve the quality of human life while living within the capacity of ecosystem support [1]; and (2) the ability to meet contemporary needs without compromising the ability of future generations to meet their needs [2]. According to Wikipedia, a sustainable city is defined as “a city designed with consideration of environmental impact, inhabited by people dedicated to minimization of required inputs of energy, water and food, and waste output of heat, air pollution and water pollution” [3]. Micrometeorology is a discipline that studies the meteorological conditions in the natural and built environments at micro-scales (~10 to 1000m)[4]. Understanding the micrometeorology in urban terrain has enormous socio-economic implications. For example, some of the problems in the area of urban micrometeorology include urban heat islands (UHI) and building energy consumptions. In the following sections, I’ll focus on the urban heat island problem and present some of the recent studies including those accomplished in our group (http://efm.princeton.edu/), with a specific focus on its impact on energy consumption and the green roof mitigation strategy. Cities are generally warmer than the surrounding rural areas [5], which is termed the “urban heat island” effect. The urban heat island effect has direct impacts on the energy consumption in urban areas. For instance, studies have shown that the average electricity load is almost linearly corrected with the maximum daily temperature in New Orleans, LA. We recently conducted a study to examine the urban heat island effect under heat wave conditions. A heat wave is identified as a sustained period of excessively hot days during which the temperature is significantly higher than the average climatological mean [6]. Cities are more vulnerable to heat waves than rural areas because of the pre-existing urban heat island (UHI) effect. We found that not only does the heat wave increase the ambient temperature; it also increases the difference between the urban and rural temperatures. That is, the hotter is becoming even hotter.
Figure 2. 2m air temperatures in urban and rural areas around the Baltimore city simulated by WRF. The temperatures are high during the heat wave period. 
Figure 3. 2m air temperature differences between the urban and rural areas around the Baltimore city simulated by WRF. The temperature differences are boosted during the heat wave period. Figure 2 shows the spatially-averaged air temperature for urban and rural areas over the Baltimore city from June 5 to June 14, 2008, which are obtained from a regional climate model (the Weather Research Forecasting model [7]). As can be seen, it includes a heat wave period during which the air temperatures in both urban areas and rural areas are higher than the periods before and after. Figure 3 shows air temperatures differences (i.e., the urban heat island effect). As can be seen, the differences also reach the maximum during the heat wave conditions, suggesting that cities are more vulnerable to heat waves as compared to the rural areas due to the synergistic interactions between urban heat islands and heat waves. Green roof strategy is always considered as one of the solutions to the urban heat island problem, particularly under a changing climate that favors more frequent and intense heat waves. Figure 4 shows changes in the air temperatures due to changes in the green roof fraction in the Baltimore city on June 9, 2008 (during the heat wave period). As can be seen, the daytime maximum temperature is significantly reduced by both increasing the green roof fraction. During nighttime, since the interiors are likely to be hotter than the exteriors and green roofs act as an insulation layer which prevents heat in the buildings from escaping, the surface temperatures are higher than those without green roofs. 
Figure 4. Changes in the surface temperature over Baltimore city due to increases in the green roof fraction (the reference green roof fraction is 0). In order to estimate the effect of these strategies on energy reduction, the relationship between the average electric load and the maximum daily temperature that is shown in Figure 1is used. Assume this relationship holds for the Baltimore-Washington area and the daytime maximum temperature in the summer is within the range of 80ºF (27ºC) to 100ºF(38ºC), it is easy to conclude from Figure 6 that the average electric load would reduce by 500MWh (from 1000 MWh to 500 MWh) if the daily maximum temperature is reduced by 20ºF (from 100ºF to 80ºF) or 11ºC (from 38ºC to 27ºC). As a result, the effect of the green roof strategy on the electric load in the summertime can be quantified, as shown in Table 1. For example, a green roof fraction of 50% can result in a reduction in the daily average electric load by 126MWh (or 0.4GBtu) for the Baltimore-Metropolitan area. It is noted that the relationship used in the above calculations between the average electric load and the maximum daily temperature may not hold for the Baltimore-Washington area, most likely because the Baltimore-Washington metropolitan area has a higher daily average electric load as compared to New Orleans, LA. However, the fact that the average electric load will increase linearly with the maximum daily temperature and the slope may still hold and the above calculations will remain valid. Table 1 Effect of the green roof strategy in reducing electric load in summertime
In conclusion, under the pressure of surging urban population and climate change, urban sustainability has been the subject of extensive research. From the energy perspective, the urban heat island effect has significant impacts on energy consumption in the urban areas, in particular under heat wave conditions. The green roof’s impact on reducing energy consumption is evaluated in a fairly simple way. This short essay underlines the important role of micrometeorology in promoting urban sustainability and calls for more studies and creative ideas.
Reference
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Copyright 2012 by Princeton University China Energy Group. All rights reserved. |
