INFLUENCE OF URBANIZED ATMOSPHERE ON LONG-WAVE RADIATION FIELD AT NIGHT

Abstract

In recent years, numerous climatological studies have been made on the basic parameters of urban heat island. However, these parameters of the actual energy exchange in urban environments are not sufficiently understood so far. Very few observations are, in particular, available for the long-wave radiation balance components in urban environments. Considering a long-wave radiation field as a model, it should be noted that radiatioe energy is basically redistributed on the ground-surface in the rural area, and in the urban area it is redistributed on the roof-level and the ground-surface. The urban roof-level is exposed to the additional long-wave radiation from the aerosol layer and the warm vertical temperature structure of urban atmosphere besides the background longwave radiation. The urban ground-surface is, moreover, under the influence of the screen effects due to buildings. This suggests that the restraint of radiatioe cooling resulting from the double screen effects on the urban ground-surface is one of the main causes of the urban heat island. The existence of the screen effects due to buildings on the urban ground-surface can be confirmed in the previous paper (Kobayashi, 1979). The purposes of this paper are to clarify the urban-rural differences for the long-wave radiation balance components on the roof-level, and to discuss the influence of urbanized atmosphere on the long-wave radiation field on the clear night. Observations were carried out on the roof-level in and around Tokyo on the clear nights of the winter from 1969 to 1970, when the heat island phenomena were developing well. The radiation sensors used in this experiment were polyethylene-shielded net radiometers (C. S. I. R. O., Net Radiometer Model CN-1 and CN-2). Downward long-wave radiation flux (Ri) was computed by R↓RN/e+σTg4 (1) where RN, Tg, σ and are net long-wave radiation flux, the measured surface temperature, the Stefan-Boltzman constant and the infrared emissivity respectively. Since all the observation points have the concrete surfaces, a uniform value of e=0.92 is used in calculating R↓ in and around the city. Observations are restricted to nights, and so the radiation balance equation is expressed in a simple form; RN=R↓-R↑ (2) Upward long-wave radiation flux (R↑) was computed by the equation (2). The results are as follows: 1. The intensity of urban heat island under the condition of the warm-moist polluted atmosphere so called the urbanized atmosphere, in the urban area is about double as much as that under the condition of the clean atmosphere in and around the urban area. 2. The urban-rural difference for the net long-wave radiation (RN) presents a striking con trast according as whether the urbanized atmosphere exists or not. That is to say, when it does not exist, RN of the urban area is more than that of the rural area; when it exists the urban area apparently receives a decrease in RN. This is one of the causes of the intensity of urban heat island. 3. When the urbanized atmosphere does not exist, R↑ is a little larger in the urban areaa than in the rural area, but RI is approximately equal in and around the urban area. 4. When the urbanized atmosphere exists, the urban increases of R↑ and R↓ are distinct. The urban increase of RI is especially larger and its average increase amounts to no less than 22. 3 mly/min (6.9%). 5. The causes for the urban increase of R↓ were analysed by using theoretically calculated values and so on.