Studies On Spatiotemporal Characteristics And Impact Factors Of The Winter Visibility In North China

North China is one of the most developed regions in china.In recent years,low-visibility incidents occur in winter in this region frequently,which seriously affect people's production,daily life and health.Therefore,it is of great significance to study spatiotemporal characteristics and impact factors of the winter visibility in North China for understanding the change of atmospheric visibility and improving the prediction accuracy of visibility in this region.Based on the conventional meteorological observation data of 55 stations in North China,NCEP/NCAR reanalysis data and recent environmental quality data of some stations,the spatial distribution and long-term variation characteristics of winter visibility in North China are analyzed,and the influencing factors are discussed.Then,it takes 6 stations in North China as examples to analyze the influence of environmental meteorological elements and surface circulation on winter visibility.The main conclusions are as follows:?1?The mean winter visibility in North China shows a meridional distribution,it is higher in the northern region and lower in the southern region.The southern Hebei,central Shanxi,western Shandong and central Henan are 4 centers of low mean visibility.The overall mean visibility of 55 stations decreases at a rate of-0.24 km/10a,but the trend is not significant,its fluctuation characteristics are obvious.?2?The spatial distribution of low-visibility days in winter is similar to the mean visibility.In the low-visibility days,the plateau areas in northern North China is dominated by sand days,the coastal areas in eastern North China is dominated by fog days,most of Shanxi and stations along Hohhot-Zhangjiakou-Beijing-Chengde are dominated by haze days,in other areas,the proportions of fog and haze are about the same.The long-term change of overall mean low-visibility days of 55 stations has 4obvious stages.Fog and haze days play a dominant role in the 4 stages,while the effect of sand days is not significant.?3?Since the mid-1990s,the winter visibility of Beijing and its surrounding stations has experienced a rising process,while the winter visibility has changed in the opposite direction in most other areas of North China.This change is significantly related to the decrease in pollutant emissions of the former and the increase in emissions of the latter.In addition,the winter visibility in North China has a low-frequency fluctuation characteristic with a time scale of 12a,which is significantly and positively correlated with the winter monsoon intensity.The high-frequency fluctuation with a time scale of 5a is mainly affected by the long-term changes of local meteorological elements such as temperature,relative humidity and wind speed,and this effect is different between the north and south sides of the Hohhot-Zhangjiakou-Chengde line.?4?There are 9 main surface circulation types in North China in winter,of which C,E and SE are the all-region low visibility circulation types,A,N and AN are the all-region high visibility circulation types,NE,AE and ANE have differences in their effects on visibility at different representative stations.Relative humidity and PM_2.5concentration are the main factors that affect visibility directly.Low visibility circulation types correspond to high humidity and high PM_2.5 concentration,while high visibility circulation type corresponds to low humidity and low PM_2.5concentration.Discriminant analysis based on the surface circulation indexes shows that when the visibility is divided into 5 levels,the overall accuracy for 6representative stations is about 60%,and the discriminant effect is ordinary,when the visibility is divided into 2 levels,the overall accuracy is about 80%,and the discriminant effect is better.For stations with more low-visibility days,the discriminant model has stronger recognition ability for low-visibility.