| Dust aerosol is one of the main aerosols in the troposphere,which plays an important role in the energy balance and ecological environment of the earth system.Dust aerosols are emitted by dust sources that are distributed around the world.The distribution characteristics of dust sources are important parameters that affect the simulation accuracy of dust physical process.The correct identification of the location of dust sources is a prerequisite for accurate simulation of dust cycle in numerical model.However,most of the current weather and climate models ignore the influence of vegetation dynamic change on the dust emission,which leads to a large error in the simulation of dust results.In this paper,we construct a dynamic dust source data set with various underlying surfaces and seasonal changes based on various satellite observations over the past 20 years.Based on this data set,we reveal the evolution of global desertification.Coupled the dynamic dust source into WRF-Chem model,the uncertainty analysis of the dynamic dust source on the simulation of dust emission was discussed,and the most suitable distribution of the dynamic dust source in East Asia was selected based on the observation data set.The Mongolia dust source is an important source of sandstorms in East Asia.Compared with the static dust source,the dynamic dust source is more reasonable to describe the Mongolia dust source.Therefore,this paper further quantifies the contribution of Mongolia dust transport to East Asia dust based on the Mongolia dynamic sand source.The main results obtained in this paper are as follows:(1)Characteristics of global desertification evolutionBased on the dynamic dust source and NDVI data set in the past 20 years,this paper analyzes the evolution characteristics of global greening and desertification.The results showed that the global greening was occurring in the past 20 years,and China and India contributed the most,with the NDVI rising rate up to 0.08/10 yr.However,the land surface contribution of greening was quite different between China and India.Grassland,desert,savanna,farmland and forest are the major contributors to greening in China.In India,farmland is the main source of greening,accounting for 70%.In the trend of global greening,desertification decreased in all regions except Central Asia and Australia,especially in the deserts of northern China and India,up to-0.12/10 yr.In the northern Sahara Desert,the Arabian Desert,the Turkestan Desert,the southeastern Australian desert and the North American desert,desertification also showed a decreasing trend,but the trend was much less than that in the northern China and India deserts,the decreasing trend was-0.03/10 yr.However,desertification increases significantly in the northern Karakum Desert of Central Asia and the western desert of Australia,up to 0.09/10 yr.In summer and autumn,the main contribution of desertification in Central Asia was grassland,accounting for more than 80%.In spring and winter,the contribution of grassland decreased to 63% and 36%,while that of desert increased significantly to 24% and 31%,respectively.In Australia,the contribution of desertification area in different seasons was not obvious.The main contribution of desertification in four seasons was open shrub land,which was more than 70%,followed by grassland,which was 15%-22%.(2)Construction of dynamic dust source and its uncertainty analysis for the simulation of dust physical processes in East AsiaIn this paper,NDVI was used to calculate the global surface bareness distribution map and high-precision topographic data to calculate the topographic characteristics.By combining the two data,a dynamic dust source with seasonal changes was constructed.The dynamic dust source was coupled to WRF-Chem model,which significantly improved the simulation effect of AOD in the numerical model.The AOD results of static dust source simulation commonly used in previous studies are underestimated compared with MODIS,MISR and AERONET observations,while the AOD results of dynamic dust source simulation are quite consistent with those of MODIS,MISR and AERONET observations.The sensitivity experiment shows that the surface bareness and topographic characteristics jointly control the distribution range and value of dynamic dust source.Among them,surface bareness controls the spatial distribution range of dynamic dust source,and topographic characteristics controls the value.In addition,the results of different dust variables simulated by WRF-Chem coupling with different dynamic dust sources are different from each other,and the variation value and distribution range are consistent with the variation of dynamic dust sources.It shows that the changes of surface bareness and topographic characteristics can affect the changes of different dust variables(such as dust AOD,dust emission,dust column concentration,dust concentration at different height layers and dust dry deposition)by influencing the dynamic dust source.(3)The distribution characteristics of Mongolia dust emission and its contribution to East Asian dustThis paper optimizes the Mongolia dust source in WRF-Chem model and simulated the dust physical process of Mongolia and East Asia in the spring(March,April and May)of 2018-2020.The results showed that the high value of the dust source function in March concentrated in the Taklamakan and the Gobi Desert.The dust source function of the Gobi Desert in the southern part of the Mongolian Plateau is enhanced and can reach 0.6 in April and May due to the decrease of the frozen soil area in the middle-high latitude Mongolian Plateau,the deepening of the East Asian trough and the strengthening of the westerly jet.This also resulted in the dust emission in March,and the dust transport flux and concentration in 700 hpa and 500 hpa were weaker than those in April and May.The Mongolian dust emission accounted for about 21.68% of the East Asian dust emission in March,and the proportion increased from April to May,accounting for 27.02% and 26.23%,respectively,and about 25% in the whole spring.Compared with the whole East Asia,the proportion trend of the dust column concentration and dry deposition in Mongolia is similar in each region,but there are differences in the value.On the whole,in spring,the dust column concentration and dry deposition amount in Mongolia are 23.11% and 26% of those in East Asia,respectively.Due to the large area of frozen soil on the Mongolian Plateau in March,the proportion of dust column concentration and dry deposition amount in each region is about 3-26%,lower than that in April and May.Compared with the dust column concentration and dry deposition,the 500 h Pa dust transport flux proportion is much lower.The 500 h Pa dust transport flux proportion in Northeast China is the highest,but it only accounts for33.23% in spring,and the proportion in other regions is lower than 25%. |
