| Urban heat island effect, which is an important issue related to urban environment, urban development planning, local climate changing and even global climate warming, is acting as a hot topic in urban meteorological research.By using the data of2010Nanjing Summer heat island three-dimensional observation test, temporal and spatial distribution of summer heat island in Nanjing and its causes were analyzed in methods of data analysis and numerical simulation. The results showed that:(1) Daily UHI intensity on typically sunny day was above1℃in summer2010in Nanjing. UHI was stronger and more stable in nighttime than it was in daytime, with average UHI intensity reaching1.63℃and UHI intensity&distribution corresponding well to urban land use status at night. The strong heat island existed in sunny, windless weather conditions. In case that wind speed was small in summer, UHI intensity was weakened with increasing wind speed. Wind direction had significant influence on geographical distribution of UHI when the leeward of city was warming by the tail flow of city, resulting in the thermal region extending to down wind direction.(2) During daytime, mixed layer in urban developed faster and higher than that in suburb. During nighttime, unstable stratification of the urban atmosphere was easy to form a mixed layer. The strong convective mixing in urban atmosphere exacerbated the development of boundary layer, so that the night inversion layer and gale area uplifted. Urban mixed layer height was estimated as300m in nighttime and1300m in daytime in summer2010in Nanjing. Due to the strong heat island at night, mixing layer height in nighttime was growing rapidly in recent years. Owing to high heat storage and strong turbulent transport of urban underlying surface, heat island took shape in urban boundary layer, with UHI intensity decreasing with height. Heat island in boundary layer extended up to900m in daytime and maintained at300m in nighttime.(3) Simulation results showed that:In the summer time when the weather was fine, heat island took shape in boundary layer all day in Nanjing. In02:00pm, sensible heat flux reached350W/m2in city, twice as that in suburbs, and soil heat flux reached200W/m2in city as four times that of the suburbs. Urban underlying surface stored a lot of heat, providing the heat base of UHI. At the same time, TKE in city reached the higher value of1.2m2/s2in200-700m, twice as that in suburbs. Strong turbulent motion in daytime promoted the development of urban mixed layer, and UHI in boundary layer formed within the height700m, which was higher than that at night. In02:00am, sensible heat flux and TKE reduced significantly both in urban and suburban, when urban soil heat flux was up to30-35W/m2, twice as that in suburbs. Urban surface continuously released heat storage, to maintain higher groud UHI intensity in nighttime. Compared with daytime, urban atmosphere at night was relatively stable, so that groud UHI was of wider distribution and greater strength and UHI developped at low altitudes of100m. |
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