Effects of a drainage flow from surrounding mountains on an urban heat island of a city located at the bottom of a basin

Although the urban heat island intensity basically depends on the housing characteristics or the population density of the urban area, the geographical condition still affects especially in the mountainous area. Observation was made for a month in summer time at several spots and heights in and around the city located on the shore of the lake in the bottom of a basin, intended to obtain the interaction between the urban heat island and the drainage from the surrounding mountains. On most of calm days in the observed period, although upslope winds formed as a part of an entire circulation in the basin and continued high temperature until late afternoon, in the evening a drainage flow occurred along the cooled mountain slope although the circulation still existed and decreased the overall air temperature. Compared the city center temperature with the temperature of the field at the foot of the mountains, a marked difference produced by the formation of the drainage front was observed. The lag in the arrival time of the cooler drainage between the field at the foot of the mountains and the city center provides the instantaneous temperature difference. Thus, although air temperature was higher at the city center than the field throughout a day, the difference markedly increased while the drainage front was advancing between the field and the city center, because the temperature difference was large between inside and outside the drainage front and the wind speed extremely reduced due to clashing of the front with the still existing circulation. As the drainage temperature gradually decreased until midnight, the lag in the arrival time continued to produce temperature difference although it was almost negligible. Heating of the drainage air by the ground surface during the travelling over the city area also produced the temperature difference, although it soon deceased as the ground surface cooled. Temperature differences produced by these two mechanisms were obtained from the falling rate of the drainage temperature, travelling time of air parcel, and the heating rate from the ground surface. The sum of the estimated differences agreed well with the observed peak temperature difference between two spots, however, the estimated value was much less than the observed value after the passage of the front. This temperature difference was, however, in good agreement with the empirically obtained value for a Japanese city of this size. Thus, after the passage of the drainage front, the normal process for urban heat island was supposed to be predominant. This result indicates that in the city located at the bottom of the basin the urban heat island is intensified apparently by the formation of the local drainage flow from the surrounding mountains, which is almost inevitable on a calm summer day, thus the extremely high temperature difference may sometimes be obtained between the city center and the upwind surrounding area compared with cities in the flat plain.