Urban Climate Modifications

Urbanization has a dynamic relationship with the physical environment. While urban growth affects the physical environment (usually negatively), urban environmental changes also affects the quality of life in these areas. The latter lead to biochemical, epidemiological and psychological responses in the urban dwellers. The debate over the relative importance of social vs. physical environmental factors on these responses is not yet settled.

My research activities center around climate-conscious urban design for the mitigation of the negative effects of urban climate modifications. I would like to concentrate on the equatorial tropics, although my empirical work up to date has primarily been on cities in the temperate zone.

Here is a gist of materials (mine and others) on the "what", "why" and "how" of Urban Microclimate Modifications.

Urban Climate

"Urbanization, due to its increased thermal capacity, lack of water for evapotranspiration, and the "canyon effect", tends to aggravate the negative effects of climate. Although the heat islands in the equatorial cities are not extensively researched, the few studies that do exis, tend to confirm this view. What seems to happen in equatorial cities is thermal discomfort during the night when thermal stress is already positive.

This problem acquires greater importance in the equatorial tropics where the nighttime rate of air movement is low. Furthermore, being in the "Doldrums" twice a year for extensive periods, the equatorial tropics do not have high wind velocities at the macrolevel. Most daily wind flows are local in nature (like the sealand breeze). The build up of nocturnal urban heat island tends to keep the air temperature over land almost as high as that over sea, thus weakening nighttime land breeze.

The "problem" of climate-conscious equatorial urban design is therefore twofold: Prevention of heat buildup as the day unfolds (so as to reduce nighttime thermal discomfort) and encouraging convective cooling at night.

condensed from Emmanuel (1993) See this article for full references.

Urban Heat Island

Inadvertent climate changes induced by urbanization are well documented. Such changes, are epitomized by the concept of "urban heat island" (UHI). They are usually measured by urban-rural difference method, city traverse method or remote sensing. Urban-rural difference method which compares climate data from an urban and a rural weather stations can be further divided into two sub-categories: Time Averaged Method (TAM) where differences in landscape between the two stations are assumed unimportant and Time Rate Change Method (TCM) where climate parameter differences between the stations are related to a measure of urbanization at the urban station.

Using one or more of these methods, recent research on UHIs has lead to better description of urban climate modifications, development of mathematical models of urban climate change and comparison of causes for urban climate modifications based on model simulation. The dominant causes for UHIs identified so far include, heat trapping by urban geometry, alterations to urban thermal properties, changes in vegetation cover and man-made (anthropogenic) heat input.

This explosion of new knowledge on the theoretical aspects of urban climate change is not well matched by practical applications. In particular, urban designers and planners are yet to utilize the current knowledge base to develop architectural and urban design strategies for the mitigation of the negative effects of UHI. This is in part due to weaknesses in current methods. For example, some of the problems associated with remote sensing techniques hinder the detection of air temperature heat island that directly affect human comfort as opposed to surface temperature heat island. These problems include, difficulties in "seeing" the vertical active surfaces, the not so well defined coupling of surface and air temperatures in urban areas and inhomogeniety of urban surfaces leading to a patch work of emissivity and albedo. The problem with urban-rural difference method in general is that it assumes weather over time remains constant. Furthermore, the intra-urban differences are ignored. As Nichol points out, it is the intra-urban climatic difference that is of value for urban planners and designers interested in mitigating the negative effects of UHIs. As for the shortcomings with TAM and TCM, they both assume that rural climate is somehow "natural" to the area. In the context of rapid global urbanization, there are very few rural areas remaining with their "natural" climates intact.

Condensed from Emmanuel (1997a) . See this article for full references.

Urban Design Parameters & Microclimate

Recently, I have completed some work on the correlation between urban design parameters such as building density, ground and tree canopy cover and urban microclimate modifications. CLICK HERE to link to the abstract of the research project

For a summary of the main findings of the study (including some images) CLICK HERE. (I apologize for the poor quality of images. They will soon be published in refereed journals (Energy & Buildings, J. American Planning Association). In the meanwhile, I try to keep my usage of the disk storage space as low as possible!)

Condensed from Emmanuel (1997b)

Urban Physical Environmental Quality

The urban metabolism concept (Wolman, 1965) indicates that environmental quality improvement in urban areas rests on the careful use and removal of energy and matter. In the urban design sense, this desire can be translated into the following goals:

Wolman, Abel, (1965). quoted by, White, R. & J. Whitney, (1992). "Cities and the Environment: An Overview". In, Stren, R., R. White & J. Whitney, (eds.), Sustainable Cities: Urbanization & the Environment in International Perspective, Boulder, CO.: Westview Press. pp. 8-51. 

Urban design protocol offers environment-conscious designers at least three tools for the realization of the goals of energy efficiency, transport reduction and water quality improvement.

Click here to see some of the possible uses of these tools and their connections to the three urban design goals for a hot-humid city specified in section 3. Some attempts at utilizing these tools for the purposes of energy and transportation need reduction have already been made (cf. Emmanuel, 1995). Although these attempts are from the temperate climate cities, they offer possible models for hot-humid cities.

Climate-conscious design of the "Commons"

The enhancement of the urban physical environmental; quality should be the major goal of climate-conscious design of the "commons". In order to achieve the design goals of energy efficiency, transportation reduction and water quality improvement, in the tropics, design strategies could take one of the following forms:

Building Form Activity Relationships for Comfortable Moving & Transport Reduction Relationship to Natural Features - Landscape Controls
Thes are excerpted from Emmanuel (1994).

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