A fieldwork study on the diurnal changes of urban microclimate in four types of ground cover and urban heat island of Nanjing, China

摘要

Records of the past years showed that the climate of built-up regions differs significantly from rural regions and one of the most obvious and important modifying effects of urbanization on local climate is the urban heat island (UHI). In this paper, four types of land cover, namely urban bare concrete cover, urban woods or the shade of trees, urban water areas and urban lawn, were selected to study their microclimate, and the UHI was also analyzed using air temperature data measured at four fixed observation spots in Nanjing, China, during hot weather from July to September, 2005. Dry and wet bulb temperature data were obtained by whirling psychrometers, and wind speed data by cup anemometers. Our observed data focused on the detailed statistical analysis of the microclimate variation in the four types of land cover during the whole day. The results showed that: (1) the microclimate of these four types of land cover had significant differences among different observation sites. In general, the air temperature of these four types of land cover complied with the order during daytime: bare concrete cover > lawn > water areas > woods or the shade of trees, with reversed order during nighttime when the air temperature of the lawn became the lowest. Compared with the bare concrete cover, the other three types of land cover showed the effect of dropping air temperature ranging between 0.2 and 2.9 ℃. There were some instant dynamic characteristics in detailed temporal series among these four types of cover in the different observation sites. (2) The UHI effect could be detected obviously by the air temperature difference between the urban center area and the rural area. The average UHI intensity during the monitoring period was between 0.5 and 3.5℃; however, there were also significant day-to-day variations. A strong UHI effect usually occurred around midnight; while about 2-3 h after sunrise the UHI began to decrease till midday time; and during 13:00-15:00, the UHI effect had a sudden increase and then decreased again; after sunset, a peak UHI effect was frequently observed during 18:00-21:00. (3) Finally, by means of the standard deviation (SD), this paper provides a concise and comprehensive understanding for the temporal and spatial microclimatic dynamics of these four kinds of urban cover in the four observation sites. Air temperature at the height of 1.5 m in Nanjing showed that the nocturnally horizontal temperature gradient was somewhat different from that reported in other large cities, and a marked heterogeneity in a smaller ground cover scale could be detected from the microclimatic spatial pattern. There is no doubt that the analysis of these four types of land cover presents the insight into possible countermeasures to decrease the high air temperature in hot summers, and is relevant to the urban planning redevelopment.