Street spaces lined with trees in a central business district (CBD) are micro-climatologically interesting, from the two viewpoints of the urban canyon phenomenon and greenspaces. The purposes of this study are to clarify the distribution of air temperature in an urban canyon with thick street trees, and to understand the climatological role of trees in a CBD.
Observations were conducted in and around Jozenjidori Avenue in the CBD of Sendai city (Fig. 1). It is oriented in an almost east-west direction and surrounded by mid- or high-rise buildings on its north and south sides; Zelkova serrata are planted in four lines on both sidewalks and on the median strip of the street (Fig. 2).
The results obtained are as follows;
1. The temperature of the street space lined with thick trees is about 1.0°C lower than that of the CBD around the street (Fig. 3-b). The deviations of temperature are greater in the afternoon and evening on sunny days (Fig. 4).
2. The relative humidity of the green street canyon is higher than that of the CBD in accordance with the lower temperature (Fig. 3-c). However, the vapor pressure is not always higher (Fig. 3-d).
3. The vertical cross-sections of daytime air temperature in the urban canyon differs according to the season. In the foliation period from May to October, high temperatures are observed around the tree canopy which acts as a heat source and receives strong insolation on sunny days, and an inversion layer is developed under the canopy. In the defoliation period from November to April, high temperatures are observed on the north side of the street, where insolation strikes at about noon (Fig. 5).
4. The distribution of daytime air temperature in the urban canyon is deformed by the winds blowing across the canyon. The deformation somewhat differs according to the season. In the defoliation period, winds are blocked by buildings, and backflows are supposed to transport the warm and cool air near the ground. In the foliation period, winds blow less under the tree canopy, so more warm air is left there, and little deformation of the temperature distribution is observed (Fig. 6).
5. The degree of temperature difference between the inside of the canyon and the roofs of buildings shows no seasonal difference. The vertical difference of air temperature in the canyon is significant in the foliation period, because the tree canopy layer is supposed to be a heat source in the daytime and a heat sink at night on sunny days (Fig. 7).

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Cool island (lowtemperature area) phenomena in urban green spaces were analyzed based on longterm continuous temperature observations. Cool island intensity (CII), which is defined as the temperature difference between the green space and its surrounding built-up areas, shows diurnal variations. In a grass area, CII is weak in the daytime and strong at night. On the other hand, CII is strong for both day and night in forest areas. The reason for this is that radiative cooling from the surface of grass and forest at night might be more effective in the green space than in the periferal built-up. It is also pointed out that CII shows little variability in either fine or cloudy weather.
Surface air temperature observations along a horizontal straight line in and around the green space reveal cooling in the peripheral built-up area caused by the advection of cooled air in a green space. Vertical temperature profiles in the green space show ground inversion which reaches 60 meters in height during the night. No clear ground inversion can be found in urban built-up areas. Based on these results, the nocturnal heat island in an urban area is expected to be reduced by the cool island effect in urban green spaces.

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Multiple wire-sonde observations (Fig. 1) were conducted in the urban area of Hiroshima City (Photo 1) on fine days in July of 1999 and 1991. In the present paper, vertical structure of wind velocity and urban heat island are documented, and the formative process of a daytime urban “heat dome” is discussed on the basis of observation data. Special attention has been paid to the discrimination between the urban canopy layer and urban boundary layer from the point of view of a vertical wind profile, and on the behavior of the heat island in both layers.
Average wind velocity profiles during the daytime along a through-urban section (DJM-HRU-NBR-HKS-GON), which is parallel to the direction of the prevailing sea breeze, were evaluated from wind data up to 50m (Fig. 2). A logarithmic low was applied to these wind profiles to detect the transformation of vertical wind velocity distribution in the urban area from the regression coefficients of estimated equations. Regression of wind profile to the logarithmic equation was carried out by the least square method above and below the height at which inclination of wind velocity profile varies (Fig, 3).
Wind velocity profiles of the upper and lower parts are regarded as that of the lower urban boundary layer and urban canopy layer, respectively (Fig. 4). The height of zero-plain displacement of upper profile (lower urban boundary layer) or the intersection of upper and lower profiles is considered to be a dividing height between urban boundary layer and urban canopy layer. The height of the urban canopy layer indicates that its maximum is in or slightly leeward of the urban core (NBR or HKS) and is approximately 0.7-0.8 times as high as that of typical buildings in the urban area (Fig. 5-a). The momentum of the intruded sea breeze is consumed by the friction against the roughness on the top of the urban canopy layer (Figs. 5-b, c), which could cause a decrease in wind velocity in the lower urban boundary layer and hence a readjustment of the vertical wind profile above the urban area.
Vertical sections of temperature distribution up to 80m along the through-urban section (DJM-HRU-NCR-GON) and the along-river section (DJM-SMY-MTM-GON) are shown in Figs. 6-a, b. During the nighttime, when a weak land breeze blows, a vertically isothermal condition is observed along both sections, and it is slightly warmer in the coastal area than in the inland area. In contrast, during the daytime (sea breeze duration) in the through-urban section, the air temperature at NBR is higher than that at comparable heights at other sites, and the isothermal lines are raised in the central urban area like a dome. The urban heat dome is most intensified between noon and 4 p. m. (Fig. 8), In the along-river section, however, warm air is observed only in the near surface level on the leeward side of the urban area. Decrease of wind velocity in the urban canopy layer (Fig. 7) should contribute to the appearance of a daytime urban heat dome. The temperature difference between NBR and its windward site HRU is larger above 15-20m than near surface level in the early afternoon (Fig. 10). This fact indicates that the daytime thermal effect of the urban area appears more clearly above the urban canopy layer than in the layer. Moreover, diurnal variation of heat island intensity in the lower urban boundary layer differs from that in the urban canopy layer (Fig. 9). Thus, it should be noted that the development of heat islands in the urban canopy layer and the lower urban boundary layer is controlled by varying factors.
During the daytime, the urban air mass is heated by surfaces whose surface temperature is much higher than the air temperature as a result of absorption of solar radiation. Rooftop surfaces are an elevated heat source which is peculiar to urban areas, and become particularly high temperature compared to other urban surfaces (Fig. 11-a).

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The difference in long-wave radiation balance between rural and urban areas is one of the main causes of the urban heat island. However, causes other than the areal difference of radiative cooling are considered to generate urban heat islands when radiative cooling does not develop.
We analyzed development of heat islands in Tokyo on both cloudless and cloudy nights when the surface. receives enough solar radiation in the daytime.
First, comparison is made for the diurnal variation of the horizontal distributions of temperature under the two conditions. Second, we compare the diurnal variation of heat island intensity on a cloudless night with that on a cloudy night. We select the observational data for autumn because radiative cooling occurs more frequently in that season.
The results are as follows:
1. On cloudless nights, the heat island intensity increases from sunset, becomes maximum at midnight, and continues until early morning.
2. On cloudy nights, the heat island intensity increases from sunset, becomes maximum at midnight, and declines towards early morning.
3. On cloudless nights, the cooling rate in rural areas is faster than in urban areas from sunset to midnight. It is at the same level after midnight towards early morning.
4. On cloudy nights, the cooling rate in rural areas is faster than in urban areas from sunset to midnight. After midnight, however, it becomes faster in urban areas than in rural areas towards early morning.

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A prominent issue in climate research is whether the global climate is warming related to the greenhouse effect. The warming trend of the global or hemispheric surface air temperature has been noted (IPCC, 1990, 1992) as evidence of the “global warming”. However, it has also been argued that the urban effect might have contributed largely to this warming trend of surface temperature. It is essemtial to estimate quantitatively the urban effect component in the surface air temperature trend.
Based on the assumption that climatic changes due to different mechanisms or forcing may show different seasonal dependencies in the anomaly time series of meteorological elements, Yasunari (1986) proposed an original method of analysis of climatic change related to the seasonal pattern of anomalies for each climatic year.
In this study, based on Yasunari's method, seasonal patterns of surface air temperature anomaly and their changes during the past 90 years (1901-1990) are examined for analysis of the urban climatic component on the surface air temperature trend using principal component analysis for 42 Japanese weather stations (Fig. 1).
More than 50 percent of variance can be explained by the first component. The examination of the seasonal eigen vector patterns shows that the first component is characterized by anomalies with the same sign for all months but with relatively high values for winter at all stations (Fig. 2).
The time series (score) of the first component generally shows increasing trends since 1901 at all stations (Fig. 3). The first component strongly seems to explain the variation due to the warming by the urban climate, although this includes the long-term climatic trend on a hemispheric scale.
The linear increasing rate of temperature anomaly (a) of the first component shows a good correla-tion with the populations of the city as an index of urbanization for large cities with populations over 300, 000 (R2=0.655; level of significance 99%). Although the linear increasing rate of temperature anomaly is greater the larger the urban population is, regional differences are obvious. Table 1 shows that for cities with similar size populations, the linear increase rates of temperature anomalies of basin cities are larger than those of inland plain cities and coastal cities. The relationship seems to be better presented if we divide the samples into two groups because the trends bend at around 300, 000 population. The increment of the linear increase rate for cities with populations over 300, 000 is larger (1.0-3.0°C/100 years: for example, Tokyo, 2.7; Kyoto, 2.4; Fukuoka, 2.2; Sapporo, 2.0) than for cities smaller than 300, 000 (0.5-1.0°C/100 years) where there is no apparent relation between the linear increasing rate of temperature anomaly and population (Fig. 4).

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A two-dimensional building model has been developed to estimate the effect of a wet roof covered with water, of ventilation, and of air conditioning on the interior temperature and sensible heat flux on the roof of the building. Sensible heat flux on the wet roof drops to 25% of that on a dry roof, but the effect of ventilation and of air conditioning on sensible heat flux cannot be seen in a numerical study.
The simulated room temperature drops by about 5°C with both 80mh^-1 ventilation and the wet roof, and by 3.5°C with -46.4Wm-2 anthropogenic heat generated in the room.
The heat release generated by the outdoor heat exchanger for an air conditioning system is at its maximum value of 219 Wm-2 at 4 p. m, in a summer day when a constant room temperature of 24°C is maintained by the air conditioning; the heat amount corresponds to the sensible heat flux on the roof.

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