京津冀地区重污染天气过程的污染气象条件数值模拟研究

通过采用全球再分析格点资料的统计分析和WRF中尺度数值模拟,从天气学和大气边界层气象学角度分析了2013年12月和2014年2月两次重污染过程中京津冀地区天气尺度大气停滞气象条件和大气污染扩散气象条件的特征及其作用,并根据WRF模式精细化模拟结果分析了太行山和燕山对京津冀地区城市大气污染形成的作用.研究结果表明,两次重污染天气过程中京津冀地区500 h Pa等压面上的平均风速均表现为明显的气候异常特征,500 h Pa平均风速较近10年同期分别下降了约30.8%和50.4%,大气停滞系数较近5年同期分别偏高10%和20%以上;京津冀地区发生严重污染时,WRF模式模拟的日平均混合层高度低于200 m,日平均地面10 m风速低于2 m·s-1,日平均通风量可降低到1000 m2·s-1以下,空气质量指数与日平均通风量成负相关,重污染期间的平均通风量比近5年同期平均通风量偏低29.3%~52.8%,这些不利于污染扩散的天气条件持续数日,导致了重污染天气的发生.此外,太行山对西风气流的阻挡是河北中南部地区大气污染加剧的一个重要原因,而当主导风向为偏南风时,偏南气流遇燕山后或转向回流、或爬坡,导致近地面风速减小,不利于污染物扩散,亦加剧了京津冀地区中南部城市的大气污染.

Numerical simulation on the air pollution potential in the severe air pollution episodes in Beijing-Tianjin-Hebei Region

Based on the statistical analysis of global reanalysis data and WRF mesoscale simulation, the characteristics and effect of synoptic scale air stagnation and meteorological condition on air pollution diffusion during two severe air pollution episodes in December 2013 and February 2014 in Beijing-Tianjin-Hebei Region were studied from the perspective of synoptic meteorology and atmospheric boundary layer meteorology. The terrain effect of Taihang mountain and Yanshan mountain on the formation of air pollution in Beijing-Tianjin-Hebei Region was studied as well by WRF modeling in high resolution. The results showed obvious climatic anomalies of mean wind speed at the height of 500 hPa during the air pollution episodes. Wind speeds on 500 hPa decreased by about 30.8% and 50.4% respectively compared with the mean wind speeds in the same period of the past ten years. Air stagnation indexes increased by 10% and 20% respectively compared with the mean air stagnation indexes over the past 5 years. The simulated results from WRF showed that the mean mixing height was lower than 200 m, the mean wind speed at 10m height above ground level was less than 2 m · s^-1, and the daily mean ventilation index was less than 1000 m² · s^-1 during the episodes, indicating a decrease by about 29.3%~52.8% compared with the mean air stagnation indexes in the same period of the past 5 years. There is a negative correlation between AQI (air quality index) and the daily mean ventilation index. Lower ventilation index lasted for several days favored air pollution accumulation and caused the severe air pollution. In addition, Taihang mountain blocked the west flow therefore leading to severe air pollution in central and south Hebei. When the southerly wind dominated, it was blocked or climbed up the Yanshan mountain which led to decreased surface wind, leading to increased air pollution in south and central parts of Beijing-Tianjin-Hebei Region.