河谷城市通风系数研究

通风系数是科学确定污染物排放总量的基础.利用WRF模式模拟的边界层高度和风速计算了兰州新区2014年4个季节的通风系数，探讨了风速的季节性变化和日变化特征.结果表明:①WRF模式模拟得到的兰州新区的混合层平均风速呈夜间高、日间低的特征，日间混合层内平均风速最大值出现在20:00左右，这与地面风速积分法确定的平均风速具有较高相关性，验证了利用模式模拟边界层内平均风速特征的能力.②混合层高度季节变化呈现春夏季高、秋冬季低的特征；受太阳辐射的影响，日间混合层高度明显高于夜间.③通风系数具有明显的季节性变化特征（4个季节的通风系数分别为4 607.6、5 424.1、1 316.4、706.9 m2/s），夏季高，冬季小，这与混合层高度和混合层内平均风速的季节性变化特征一致；日变化呈现单峰型的变化规律，冬季的峰值出现在15:00，而其他3个季节的峰值则出现在17:00左右.研究显示，WRF模式的模拟结果可以较好地反映混合层平均风速的基本特征，利用WRF模式模拟的结果计算得到的河谷地形的通风系数较为合理，不同季节的通风系数差异较大. 

Ventilation Coefficient in River Valley Terrain

The ventilation coefficient is important for estimating air pollutants capacity scientifically. The ventilation coefficient over Lanzhou New District in 2014 has been computed based on the planetary boundary layer height (PBLH) and wind velocity simulated by WRF model, the seasonal and diurnal variation of wind velocity in the mixed layer was discussed. The results are shown as follows:(1) Wind velocity in the mixed layer was calculated both by WRF simulation and integration method based on the observed surface windvelocity. Two results showed a strong correlation, and represented the characteristic of lower in daytime and higher at night. (2) The mixed layer height in spring and summer was higher than that in autumn and winter. For diurnal fluctuations, the mixied layer was higher in daytime than at night which dominated by solar radiation. (3) The ventilation coefficient showed significant seasonal variations with ventilation coefficients of 4607.6, 5424.1, 1316.4 and 706.9 m²/s in spring, summer, autumn and winter in 2014, respectively, which was consistent with the characteristic of PBLH or average wind speed in mixed layers; For diurnal variations, the ventilation coefficient showed characteristic of the single peak. Besides, the peak value occured at 15:00 in winter, and at about 17:00 for other seasons. The study suggested that:the simulation results by WRF model can represent the basic characteristic of the average wind speed in the mixed layer. The ventilation coefficient computed by the WRF model was reasonable in river valley terrain, and it varied greatly in different seasons.