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• 基金项目：

• 陈长和
• 兰州大学

• 黄建国
• 兰州大学

• 施介宽
• 兰州大学

• 何玉贵
• 兰州大学

• 摘要：1980年1月和6月在位于黄河河谷内的兰州市上空200m高度释放了大量平衡气球。平衡气球用氢和二氧化碳混合气体充灌于测风气球内制成,用双经纬仪跟踪气球。对符合下列条件的平衡气球资料进行了分析:沿轨迹的平均风速小于1m/s,定位误差小,气球不通过风的垂直切变层。 平衡气球的轨迹说明,静风条件下仍然存在明显的平均运动,特征是路径曲折,低频摆动大。混合层内的垂直速度脉动在 0.46-0.63m/s之间,垂直运动范围与混合层深度相接近,通常在500m左右。静风条件下的σ_z和σ_y约为Briggs城市地区扩散参数的三倍。中性条件下扩散接近于各向同性,稳定条件下σ_x大于σ_y和σ_z. σ_x与下风距离同一量级,由于静风条件下平均运动的存在和σ_x不可忽略,建议用移动烟团积分扩散模式计算污染物浓度。用试验数据计算了移动烟团积分模式与静止烟团积分模式、高斯烟羽模式的浓度分布,并进行了比较。

• Abstract：A number of balanced balloons have been released in the months of January and June, 1981 at a height of 200m over Lanzhou, which is situated in a Yellow River valley. The balloons consist of pilot balloons filled with a mixture of hydrogen and carbon dioxide, and are tracked by means of a double-theodolite system. Analysis is conducted for information obtained which has to meet the following conditions: average wind speed along the trajectory being less than 1 m/s, small error of balloon positioning, and the balloon itself not passing the shear layer of wind.From the trajectories analysis, it is obvious to find that still there is average motion in near-calm, but characteristic of showing meandering path and an apparent low frequency oscillation. The fluctuations of vertical velocity vary from 0.46 m/s to 0.63 m/s in mixing layer, and the range of vertical motion is close to the depth of. the mixing layer, normally of about 500 m. Three dimensional diffusion parameters have been calculated according to the formula σ_i²=υ′_i²(ι,T)•T². with the result that σ_y and σ_z under near-calm are three times large compared with those based on Briggs (1973) interpolation formulas for flow over urban areas. Diffusion is almost isotropic under neutral atmosphere and σ_x is larger than σ_y and σ_x under stable conditions. is of the same order of magnitude with downwind distance. It is thus suggested to integrate diffusion model by a moving puff in order to calculate the pollutant concentration under near-calm, in view of the fact that there is existence of average motion under the conditions and σ_x is not to be neglected.

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