城市大气边界层的综合观测研究--实验介绍与近地层微气象特征分析

Comprehensive observation research on urban atmospheric boundary layer——Description of field experiment and analysis of micrometeorological properties in the surface layer

介绍了南京大学大气科学系依托国家自然科学基金重点项目《城市边界层三维结构研究》在南京进行的两次城市边界层综合观测实验(时间分别在2005年夏季与2006年冬季),对实验所获观测资料进行了初步分析,获得了一些城市近地层微气象学的基本特征,并对结果进行了讨论.主要结果为:市区气温明显高于郊区,体现出城市热岛效应的存在,且市区与郊区的气温差异随高度的增加而减小;2006年冬季观测期间,市区的风速比郊区约小1 m/s,反映出城市建筑物对气流的摩擦、阻尼和拖曳等作用,这些作用使得平均风速在城市上空衰减,并使一部分气流平均动能转化为湍流动能;市区白天的感热通量与郊区相当,但潜热通量远小于郊区,因此市区的鲍恩比远大于郊区.市区下垫面储热项在地表能量分配中占有较大份额,成为城市热岛效应的主要成因;夏季观测期间,市区摩擦速度均值比郊区高34%;冬季观测期间,市区摩擦速度均值比郊区高23%.这种夏季大于冬季、市区大于郊区的现象表明,由于冬/夏热力学性质和城/郊下垫面动力学性质的不同而引起湍流活动的差异;为了定量地衡量气流平均场转化为湍流场的效率,表述动量输送的强弱,我们引入了一个新的变量:归一化摩擦速度.对冬/夏两季和城/郊两地的比较发现,归一化摩擦速度具有很好的相似性和一致性,它表现为一个不随季节和时间变化的常数;通过比较水平湍流方差2uσ+2vσ与垂直湍流方差2σw的方法,来更直观地研究湍流能量在水平和垂直两个方向上的分配特征.在2006年冬季观测期间,市区的2σw/(2uσ+2vσ)平均值为0.16,郊区为0.14,市区湍流能量在垂直方向所占份额略高于郊区.

Two field observation experiments in Nanjing aimed at urban boundary layer researches were described. The observation data were analyzed and some micrometeorological properties in the urban surface layer were obtained. The main results are. Air temperature in the urban area is sigmticantly higher than in the suburban area, which embodies the presence of urban heat island effect, and the differences of air temperature between the urban and suburban areas decrease with height. During the observation period in the winter of 2006, wind speed in the urban area is about 1 m/s less than in the suburban area, which reflects the damping, friction and drag effects of urban buildings on the airflow. Sensible heat flux in the daytime in the urban area is approximately equivalent to that in the suburban area, but latent heat flux in the urban area is far less than in the suburban area, thus the Bowen ratio in the urban area is far larger than in the suburban area. In the summer, friction velocity in the urban area is 34% larger than in the suburban area, and is 230% larger in the winter. In this paper, a new variable, normalized friction velocity, was introduced, of which the mean value was found a constant not varying with time and season. It is found that the proportion of vertical turbulent energy to total turbulent energy in the urban area is somewhat larger than in the suburban area during the winter of 2006.