Observation And Numerical Simulation Of Urban Heat Island Effect In Valley City Lanzhou

With the improvement of urbanization, urban heat island (UHI) effect and its associated impact on regional climate, environment and human health have received wide attention. In the recent fifty years, the urbanization of Lanzhou has been development rapidly. Accordingly, Lanzhou has experienced a significant UHI effect. However, the unique river valley topography and complexed surface properties, making the boundary layer structure and the surface radiation energy balance characteristics has great different with the other citys. Therefore, the study which focus on the temporal and spatial characteristics as well as the formation mechanism of UHI effect over Lanzhou, has very important practical significance.Based on the daily average temperature, maximum temperature and minimum temperature data from Lanzhou urban station and Yuzhong rural station, this paper investigate the long-term variation of UHI effect for the time period1956to2010in Lanzhou, and quantitative analysis the impact of UHI effect on temperature series in Lanzhou urban station. On the other hand, Weather Research Forecasting (WRF) numerical model and its coupled single-layer Urban Canopy Model (UCM) are used to simulate the UHI effect over Lanzhou during25-27January2005(winter case) and27-29June2009(summer case). In order to evaluate the impacts of urbanization over Lanzhou, the land use data that inversed from MODIS lkm resolution data is applied. A sensitivity experiment that without urban underlying is designed to investigate the effects of urban underlying on urban boundary layer structures, valley wind circulation, land surface radiation balance and land surface energy budget. The main results are as follows: (1) The UHI effect of Lanzhou is obvious, especially for minimum temperature. For the past55years, the interdecadal variability of lanzhou UHI intensity has obvious stage features, the largest rise in the1980s and1990s. The UHI intensity is strongest in summer, and weakest in winter. But the increasing rate of UHI in winter, that is much higher than in any other seasons, can reach3.0℃. The UHI intensity demonstrates distinctive diurnal variation, with UHI intensity being stronger and more stable in nighttime than it is in daytime. The urban cool island effect, in which surface temperature in urban areas is lower than that in rural areas, also occurs at the winter noon.During1956-2010, the impact of UHI effect on temperature series recorded in Lanzhou is significant. The warming rate of annual average UHI is0.34℃/10a, and its contribution to surface temperature warming trends is69.4%. Around the year1986, an apparent temperature jump occured in northwest China. Therefore, temperature changes during1986-2010in lanzhou is mainly caused by natural factors, while contribution rate of UHI effects caused by human factors is relatively reduced.(2) Mesoscale numerical model WRF can well simulate the boundary layer structrue, characteristics of land surface radiation balance and energy in the complex terrain conditions. The simulated results are compared with the observations provided by Semi-Arid Climate Observatory Laboratory (SACOL) of Lanzhou University. The comparison results reveal that the non-local scheme (YSU) with the entrainment flux proportional to the surface flux is favourable under unstable conditions. Under stable conditions, the local TKE closure schemes (MYJ) perform better than the first-order approaches.(3) Diurnal variation of UHI intensity in winter presents the double-peak structure, the peak value appeared at19:00and06:00, while the valley value appeared at13:00. But diurnal variation of UHI intensity in summer presents the single-peak structure, the peak value appeared at20:00, while the valley value appeared at10:00. The UHI effect in winter maintain longer than in summer. Surface air temperature on urban underlying is higher than on natural underlying, and showed the most obvious during the night. The temperature difference is about2.4℃in winter, while can reach4.0℃in summer. Surface wind speed of urban is less than natural underlying surface, the wind speed difference is about1.0-1.5m-s-1, which is caused by the resistance of building to wind. The contribution rate of urban underlying to Lanzhou UHI intensity is about40%. Accordingly, Other factors (e.g., anthropogenic heat emission) also play important role for UHI intensity.The urban surface air keep the characteristic of mixing layer at nighttime. Due to the heating effects of mountain peak, there is a inversion layer exist above the city400m-600m at winter daytime. During the summer daytime, that the vertical turbulent motion is severe, atmospheric boundary layer presents nearly neutral structures. Furthermore, the UHI effect also has a significant impact on valley breeze circulation. At afternoon, the weak upflow caused by UHI circulation could restrain the formation and development of the valley breeze.Different of the underlying attributes can lead to a greater difference in surface radiation balance between urban and rural areas. The main reason of UHI intensity stronger in nighttime than in daytime is heating effect of urban surface long-wave radiation. The trapping effect of urban underlying on solar radiation could lead to the net radiation in urban areas are more than that in non-urban areas. In summer, the magnitude of surface latent heat flux on natural underlaying is about10times as much as that in winter. Because of materials waterproofness of building, the latent heat flux is far less than sensible heat flux, while the heat storage capacity is obvious enhanced.