基于多种边界层方案的天津PM_(2.5)集合预报试验

Ensemble Forecast Experiments of PM_(2.5) Based on Multiple Boundary Layer Schemes in Tianjin

基于大气化学模式WRF/Chem采用4种边界层方案(YSU,BL,MYJ和MYN3)模拟2015年全年天津地区细颗粒物质量浓度演变趋势,分析不同边界层方案对天津地区PM_(2.5)质量浓度模拟和预报的影响,并构建多种边界层方案的集合预报产品,以期提高天津地区PM_(2.5)质量浓度预报效果。结果表明:大气化学模式的4种边界层方案在空气质量模拟中均有较好的适用性,PM_(2.5)模拟值与观测值相关系数达到0.76左右,晴空和大风天气BL方案表现较优,阴天和小风天气YSU和MYJ方案表现优于其他两种方案,综合所有过程没有任何单一方案具有显著优势。基于上述原因,该文开展多种边界层方案的天津空气质量集合预报试验,经过对2015年全年模拟值分析,多边界层方案和多气溶胶机制扰动的集合预报可以减小PM_(2.5)质量浓度预报的相对误差和均方根误差,降低重污染天气预报的漏报率,提升空气质量等级预报能力,在计算资源得到保证的基础上,是一种可以提升数值模式PM_(2.5)质量浓度预报能力的有效手段。

Based on the atmospheric chemical model WRF/Chem, four kinds of boundary layer schemes （YSU, BL, MYJ and MYN3） are used to simulate the evolution of PM2.5 mass concentration of Tianjin in 2015. Effects of different boundary layer schemes on the simulation and prediction of PM2.5 mass concentration are analyzed, and a set of prediction products with various boundary layer schemes are constructed to improve forecast effects. Results show that the best boundary layer scheme for the near surface temperature simulation is BL scheme, the best boundary layer scheme for relative humidity simulation is MYN scheme and YSU scheme, the best boundary layer scheme for wind speed simulation is YSU scheme, and four boundary layer schemes of atmospheric chemical model have good applicability in simulation of air quality. The correlation coefficient between the simulated value and the actual value can reach 0.76, and the relative error is between 31.7% and 33% . Among four boundary layer schemes, MYN scheme leads to highest simulated boundary layer height, and the simulated boundary layer height of BL scheme is the lowest. As for the correlation coefficient between boundary layer height and PM2.5 mass concentration, BL scheme is the highest （0.64）, comparing to 0.62 with YSU scheme and MYJ scheme, and only 0.5 with MYN scheme. No single scheme has significant advantages. BL scheme is better in sunny and windy weather, while YSU and MYJ schemes perform better in cloudy and breeze weather. The simulated PM2.5 mass concentration in Tianjin shows a significant disturbance characteristic with different boundary layer schemes. The standard deviation of daily average PM2.5 concentration is about 5.2 μg · m-3 , accounting for 8% of the mean, and its maximum can reach 23 μg · m-3. The hourly standard deviation reaches 11.8%, which is greater than the daily standard deviation, especially in the process of mutual transformation between stable boundary layer and unstable boundary layer. To overcome these problems, air quality ensemble prediction tests of multiple boundary layer schemes are carried out in Tianjin. Based on the analysis of forecast value in 2015, the ensemble prediction of multiple boundary layer schemes and disturbance of multiple aerosol mechanisms can decrease the relative error and root mean square error of PM2.5 mass concentration prediction by about 25%. It can also reduce the false negative rate of heavy pollution weather from 44% to 30% and improve the forecasting capabilities of air quality level by 3%- 6%. When the computing re- sources are sufficient, it is also an effective means to enhance the forecast ability of PM2.5 mass concentration.