Heat island of a small city and its influences on the formation of a cold air lake and radiation fog in Xishuangbanna, Tropical China

Abstract

In Xishuangbanna, Yunnan Province, China, cold air lakes and radiation fog are formed in basins at frequencies of 70–90% of winter days. In January, 1989, mobile observations were carried out in order to clarify the urban effects on these local climatic phenomena. The small city, Jinghong, is located in the northwestern part of the Jinghong basin. The basin bottom has an area of about 76 km2. The city had a population of about 40 000 in 1988. Heat islands with an intensity higher than 1 °C were clearly observed in the city. The formation of radiation fog was also observed simultaneously on the slope located in the northeastern part of the basin. It was found that radiation fog was formed first in an air layer at a height of several tens of meters above the city and then it spread over the whole basin within about one hour. It disappeared from the northwestern part of the basin, where the city is located. The mechanism of the urban effect on radiation fog is discussed.On the other hand, some previous studies have pointed out that the local climate in Xishuangbanna has been changing; it has become drier, sunshine duration has increased, and the number of fog-days and the duration of fog at night have decreased. By comparing the meteorological data in the 1960s, when the population of the city was less than 10 000, and in the 1980s, when it was about 40 000, the effects of urbanization and deforestation on the local climate phenomena, such as cold air lakes and radiation fog, are analyzed.