Urban climatological studies can be historically divided into three stages: the first stage is a simple comparison of standard meteorological elements between a city and its immediate environs; the second is an investigation of the horizontal distribution of these meteorological elements in the city; and the third is an investigation of the three-dimensional aspects of the urban climate. The author, however, considers that comparative investigation of the underlying processes that produce these observed characteristics in the horizontal meteorological fields should also be very important. In this context, this study is directed towards a systematic understanding of the radiation balance of an urban environment by introducing examples obtained primarily in Canada and the United States.
The radiation balance at the earth's surface can be written as follows:
QN=
K↓-
K↑+
L↓-
L↑ (1) where Q
N is net all-wave radiation;
K↓, global solar radiation and
K↑, reflected solar radiation;
L↓, downward atmospheric radiation and
L↑, upward terrestrial radiation. Both
K↓ and
L↓ are dependent on atmospheric properties whereas KI and LT are influenced by the surface. For the former, scattering and absorption of aerosols are important and for the latter, albedo (α) and emissivity (ε) of the surface are important.
Among the radiation balance components, global solar radiation has received much attention because it is the most fundamental energy for life and because it can be readily and easily monitored. Notable radiation studies in urban areas were carried out in Montreal (East, 1968), Cincinnati (Bath and Patterson, 1970) and Toronto (Yamashita, 1973). In general, the reduction of solar radiation in an urban environment is about 15_??_20% (Landsberg, 1970) but this effect on the radiation balance is unique to each city. From a climatological viewpoint, the total solar energy is more important than the energy associated with a specific wavelength, despite the fact that the physical property of radiation is dependent on wavelength (the shorter the wavelength, the larger is the attenuation of solar radiation).
A considerable amount of solar radiation, scattered by aerosol layers in the urban atmosphere, is received at the surface as diffuse sky radiation. This was verified for Toronto (Yamashita, 1973) and for Hartford (Sprigg and Reifsnyder, 1972). Hence, turbidity factor may be useful to understand the dome-shaped structure of an urban atmosphere. Not only dust content but also water vapour content over a city must be investigated because these exert considerable influences on the radiation and energy regimes. Albedo is one of the most complex element in urban climatological studies. Albedo will be increased by removal of vegetation and frequent use of light coloured materials, while, on the other hand, will be decreased by multiple reflections in urban canyons. Numerical solution of albedo for the urban canyon by Craing and Lowry (1972) appears promising.
An aerosol layer over the city absorbs solar and terrestrial radiation and then emits this energy as longwave radiation. However, this process is not easily verified because of observational difficulty and small energy involved. Significant studies were made in Cincinnati, Montreal and Los Angeles. Upward terrestrial radiation also increases in an urban area, and this tends to offset the downward atmospheric radiation. Thus, emissivity of urban surfaces should be investigated in detail.
Lettau's climatonomy helps us to investigate the shortwave radiation balance of the urban atmosphere. Figure. 6 shows such an example derived, for Metropolitan Toronto (Yamashita, 1974).
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