Characteristics Of AOD Variation In East China And Its Relationship With East Asian Monsoon Circulation

Due to the accelerated urbanization process in East China,a large number of aerosol particles are discharged into the atmosphere,resulting in increasingly serious aerosol pollution.East China is located in the East Asian monsoon area at the same time.The strength of the East Asian monsoon and the abnormal monsoon setup will be the main factors affecting the distribution of precipitation,especially the and drought and flood events in China.Aerosol and monsoon are considered to be two important factors affecting the climate and environment in East China.These two factors are not independent of each other,they do interrelate and interact with each other.Therefore,it is of great scientific and practical significance to clarify the relationship between aerosols and monsoon circulation in East China.Previous studies mainly used numerical simulation and data diagnostic analysis to study the relationship between aerosols and East Asian monsoon.Because the interaction process between aerosol and cloud is complex,it is difficult to fully understand the impact of aerosol,so the results of numerical simulation are still uncertain.Therefore,data analysis is a supplement to numerical simulation.However,in previous data analysis,the aerosol optical thickness inversion accuracy is not high because of the short observation time series.MODIS aerosol optical thickness data have been continuously accumulated,and the inversion accuracy is gradually improved,which provides a possibility for further study of the relationship between aerosols and monsoon.Based on the monthly aerosol optical depth(AOD)of the latest version of MODIS C06,NCEP/NCAR reanalysis data,CERES data and CMAP precipitation data,the seasonal variation characteristics of AOD in East China are analyzed,and the relationship between AOD and East Asian summer monsoon circulation is revealed by comparing AOD characteristics with those in India.Secondly,the relationship between AOD and East Asian monsoon circulation under different pollution levels is analyzed after classifying AOD according to its interannual anomalous variation characteristics.Finally,using REOF analysis method,the key regional distribution of AOD in eastern China is separated,and the local causes of AOD are further revealed.The main conclusions are as follows:(1)AOD values in East China are much larger than those in India,especially in spring.The seasonal variation characteristics of AOD in East China are obviously different from those in India.AOD values are the largest in spring and summer,followed by autumn and winter,while those in India are the largest in summer and smaller in spring,autumn and winter.On the scale of seasonal variation,AOD in East China is more closely related to the variation of surface wind speed,while precipitation has no obvious relationship with AOD distribution.The atmospheric circulation and stratification stability in the boundary layer affect the aerosol distribution a lot.The distribution characteristics of the upper and lower tropospheric circulations in East China and India are obviously different,and have different effects on AOD.The advection effect of lower tropospheric circulation on AOD and the influence of divergence and convergence on AOD distribution play different roles in maintaining AOD in East China and India.(2)According the interannual variations of AOD averaged over the eastern part of China,the AOD is divided into five different levels,which corresponds to five pollution grades.The results show that the anomalies of AOD in East China are larger when the surface wind speed is weaker together with the unfavorable aerosol diffusion condition of the anomalous monsoon circulation in the middle and high troposphere.When the aerosol pollution is strongest(weakest),variations of AOD is strongly related to the variations of the solar radiation at the earth surface,surface temperature,wind speed,and precipitation.At other AOD levels,the air pollution is significantly related to the surface solar radiation rather than other variables.When the air pollution is severe,the solar radiation at the earth surface is relatively lower,along with the lower surface temperature.The atmosphere in lower troposphere is cooling,and the wind speed is weakening.Meanwhile,the more precipitation occurs in the southern and less in the northern part of the eastern China.On the other hand,when the air pollution is very light,the solar radiation that reaches the earth surface will increase,and the surface air temperature gets higher.At this time,the surface wind will get stronger,and less precipitation will be received in the southern and more in the northern part of East China.(3)The interannual variation of AOD in summer in East China is also significant.The interannual variation of AOD is mainly concentrated in four key areas: the North China Plain,the middle and lower reaches of the Yangtze River,the Shanxi-Mongolia Mountains and the north of Beijing,Tianjin and Hebei,covering the whole eastern part of China.The interannual variation of AOD in each key region is significantly different,and the relationship between AOD and the circulation variable field in East Asian monsoon region is significantly different.On the whole,the correlation between AOD anomaly and circulation variable field is not high in the key area,but relatively high outside the key area.This fully shows that the influence of circulation variable field on the interannual variation of regional AOD distribution is mainly peripheral.The correlation between the anomalous variation of AOD distribution in four key areas of Eastern China and the East Asian summer monsoon circulation index is generally weak.(4)The regional average AOD anomaly in winter in eastern China has significant interannual variation characteristics,and is significantly different from that in summer.In winter,the regional average of AOD in East China has a good correlation with the climatic factor field.When AOD is abnormally high(low),the solar shortwave radiation flux decreases(increases),the surface temperature decreases(increases),the cooling(heating)of the earthatmosphere system and the surface precipitation show "less inland in North China and Jianghuai,more southeast in Jianghuai"("more in North China and Southwest Jianghuai,more in Jianghuai).The high AOD anomaly in winter in eastern China is related to the low wind speed anomaly,and the negative correlation is most obvious in North China: when the AOD anomaly is high(low),the surface wind speed in North China is small(large),and the surface wind speed in Jianghuai is large(small).A case study shows that the main reasons for the high AOD anomaly in winter of 2013 are the small zonal wind speed anomaly,the east wind anomaly and the weak Aleutian low.(5)The interannual variation of AOD in winter in eastern China is also significant,mainly concentrated in the south of the middle and lower reaches of the Yangtze River,the south of Beijing,Tianjin and Hebei and the Huaihe River Basin,the middle of Shanxi,Sichuan,Bazhong and Nanchong,and the north of Beijing,Tianjin and Hebei,Xilinhot and Liaoning.The relationship between AOD interannual anomaly and wind anomaly in each key area is closer than that in summer.Wind field has more significant influence on the regional distribution of AOD interannual anomaly in winter than in summer.The correlation between AOD anomalies and surface radiation anomalies in key areas in winter can not reveal the influence of aerosols on radiation.By comparing and analyzing the relationship between AOD anomaly and surface radiation in each key area in summer,it is found that the influence of cloud is an important factor that can not be ignored,and the relationship between AOD anomaly and ground long-wave radiation and ground temperature is relatively close to that of short-wave radiation.The relationship between AOD anomaly and surface relative humidity and specific humidity in key areas is related to the relationship between AOD anomaly and surface precipitation,that is,when the correlation between AOD and surface relative humidity and specific humidity is strong,the relationship between AOD and surface precipitation is strong,and vice versa.The above results are helpful for us to understand the spatial-temporal characteristics of aerosols in East China and their relationship with East Asian monsoon circulation as well as the mechanisms behind.