石家庄近地层风场辐合与重污染关系研究

石家庄多发生以风场辐合为特征的局地污染事件，客观判识辐合区和辐合过程强弱对重污染预报指示意义重大.为研究石家庄风场辐合特征及其对局地污染的贡献，更好地为区域重污染天气预报预警提供客观指标参考，在引入固定高度层内矢量通风系数物理量的基础上，结合箱体模型提出一种定量计算近地层风场辐合强度系数的方法，并利用ERA-Interim再分析资料计算了京津冀地区矢量通风系数以及辐合强度系数，对2016年9月—2019年3月期间10次典型风场辐合污染过程进行分析，探讨了风场辐合形成的原因和对污染贡献的关系.结果表明:①受地形影响，石家庄易出现近地层风向、风速辐合，均可归结为区域内输入通风量大于输出通风量，西部太行山地形的阻挡加重了其辐合程度.②风场辐合使空气中的水汽和污染物汇聚，为二次反应提供高湿条件，这种正反馈作用促发PM_2.5爆发性增长，期间ρ（PM_2.5）平均每小时可上升25 μg/m3.③石家庄近地层辐合强度系数在26以上时对污染贡献作用明显，辐合区污染强度比周边偏高1~2个等级，ρ（PM_2.5）比区域背景值偏高85%~200%.研究显示，风场辐合对重污染贡献明显，是形成局部污染物浓度偏高、PM_2.5爆发性增长的重要气象指标. 

Relationship between Near Ground Wind Convergence and Heavy Pollution in Shijiazhuang

Local pollution events caused by wind convergence occur frequently in Shijiazhuang. It is important to objectively identify the area and intensity of wind convergence zone for forecasting heavy pollution events. In order to study the convergence characteristics of near ground wind in Shijiazhuang its contribution to local pollution, and to provide an objective indicator reference for the prediction and early warning of regional heavy pollution, a method based on the introduction of vector ventilation coefficient in the fixed height for quantitatively calculating intensity coefficient of wind convergence was proposed using the box model. The vector ventilation and convergence intensity coefficients in the Beijing-Tianjin-Hebei Region were calculated using ERA-Interim reanalysis data, and 10 typical pollution processes occurred from September 2016 to March 2019 associated with wind convergence were analyzed. The reasons for the formation of wind convergence and its contribution to pollution were discussed. The results showed that: (1) Affected by the terrain, Shijiazhuang was prone to wind convergence, which could be attributed to the fact that the input ventilation volume in this area was greater than the output ventilation volume, and the Taihang Mountains increased this convergence. (2) The convergence of wind field accumulated water vapor and pollutants, and provided high humidity conditions for the secondary chemical reactions between pollutants. The positive feedback promoted the explosive growth of PM_2.5, and the average increase of ρ(PM_2.5) was 25 μg/m3 per hour during the process. (3) Wind convergence had an obvious contribution to the pollution when the convergence intensity coefficient was above 26 in Shijiazhuang. Under these conditions, the pollution intensity in the convergence area was 1 to 2 levels higher than that in other regions, and ρ(PM_2.5) was 85%-200% higher than the background average. The results also showed that the near ground wind convergence contributed significantly to heavy pollution, and was an important meteorological indicator for the formation of high local pollution and explosive growth of PM_2.5.