アスファルト舗装面の表面温度と熱収支の解析

試験地表面による地壌面との長期比較観測

抄録

The increase of heat storage by artificial materials is one of the important factors in heat balance as well as the decrease of evapotranspiration in urban areas. In this study, asphalt pavement was chosen as a typical urban land cover, and its surface temperature and heat balance were compared to that of soil surface. For this purpose, a twin apparatus was set closely and measured under the same view factor and wind conditions. These observations were continued for about three years, and all kinds of data, solar radiation, rainfall, heat flux, surface temperatures, air temperature and humidity, etc., were collected at intervals of 1 minute.
The daytime surface temperature of asphalt pavement is approximately proportional to the integrated solar radiation until that time (Fig. 4), and is also affected by averaged wind speed (Fig. 5). Moreover, since it is closely related to the solar radiation just before, even if its total value is constant, the history of radiation causes variation in asphalt surface temperature (Table 2).
The surface temperature difference in the early morning between asphalt (TA) and soil (TS) becomes noticeable around the end of March and ends in November (Fig. 6). The these large differences in warmer seasons are related to the large amount of solar radiation on the previous day (Fig. 7). In addition, the high surface temperature of asphalt pavement leads to a reduced possibility of dew condensation at that surface (Fig. 8). This means that the micro-meteorological water cycle that is, the repetition of condensation at night and evaporation after sunrise disappears at asphalt surface.
The surface temperature of bare land varies greatly with soil moisture content. In particular, its daytime maximum value increases due to the decrease of latent heat flux (Fig. 9-12). In contrast, that of pine (Pinus densiflora) forest measured by infrared thermometer is almost constant (Fig. 9), which suggests that these forests had no water deficit during the period of measurement.
The convective heat-transfer coefficient (α_c) at the experimental apparatus is calculated for fine days in summer. Its hourly averagedd values can be expressed as the function of wind speed 15 m above the roof (Fig. 14). In this connection, the heat balance at asphalt pavement is different in calm and windy conditions (Fig. 15). The ratio of G/Rn (G: conductive heat flux into asphalt layer, Rn: net radiation) is not constant, and varies from 55% to 20% with wind speed (Fig. 16).