Modeling Study On Temporal And Spatial Variation And Regional Transport Of Dust Aerosols During A Typical Dust Storm Over The Tarim Basin

As one of the largest shifting sand dunes in the world,the Taklamakan Desert(TD)covers the wide central region of the Tarim Basin(TB)with the mean elevation of 1.1 km.Strong sandstorms occurred frequently.Dust emission,sedimentation,transport and spatiotemporal variations are controlled by the special boundary layer(BL)and unique terrain effect over the basin.Characterization of forming the unique spatiotemporal variations and regional transport of the dust aerosol over the TB is still a challenging issue in environment sciences.Aiming this scientific issue,this thesis,by employing the air quality model WRF-Chem(The Weather Research and Forecasting model with chemistry)simulates a dust storm event over the TB to investigate dust aerosol spatiotemporal variations,regional transport and its radiative forcing over the TB with the selecting appropriable PBL scheme over the desert region.The main research contents and results are summarized as follows:1.An assessment of atmospheric boundary layer schemes over the hinterland of TDDust emission,sedimentation and transport are controlled by the BL over the deserts.The observational data in desert areas limit the examination on BL simulations.In this study,the performances of five BL parameterizations of ACM2,BL,MYJ,MYNN2.5 and YSU in the mesoscale numerical model WRF were assessed by simulating the BL in the TD in April 2014,compared with the observations of a 80m tower and wind profile radar under sunny weather at Tazhong in the TD hinterland.The comparison results reveal that five PBL schemes could reasonably capture the diurnal variations of near-surface air temperature,surface temperature,boundary layer height,sensible heat,latent heat and surface heat flux,excepting for wind speed.Regarding the vertical BL structures,all the simulated temperature,wind and moisture profiles within BL can be well reproduced only with the underestimated simulations of wind speed during the daytime.The dry desert's thermal heat capacity is low to easily form convective unstable BL in sunny days.The non-local ACM2 scheme is a more appropriable BL scheme for simulating the BL in the desert area.2.Numerical simulation of spatiotemporal variations of dust aerosols during aspringtime dust storm over the TBThe air quality model WRF-Chem was employed to simulate a dust storm event in spring,2015 over the Taklamakan Desert of the Tarim Basin(TB),Northwest China.Based on the reasonable evaluations with the ground observations and CALIPSO satellite data of meteorology and aerosols,the simulation was used to characterize the three-dimensional distribution of dust aerosols for this springtime dust storm over the TB.Under the impact of unique basin topography on a cold frontal system,a large amount of cold air invading the TB in three main tracks of near-surface winds:(1)westerly winds across the Pamirs Plateau,(2)northerly air flows crossing the Tianshan Mountains,and(3)strong northeast winds passing the northeast mouth of TB,building three main dust plumes over the TB for the dust storm process.The strong dust plume swept southwestwards across the basin with dominating over two weak dust plumes,and the mechanical effect of Tibetan Plateau on the dust plumes could determine the surface dust aerosol distribution over the TB.Based on observation and simulation,the extremely thick boundary layer was identified with the height of 3-5km from the surface over the TB for the vertical distribution of dust aerosols.Dust aerosols were lifted up to 7km in the free troposphere over the southern TB driven by the northern slope of the Tibetan Plateau.The distribution of dust aerosols in the upper boundary layer at the height of 3-5km was controlled by the convergences of near-surface air flows over the TB and the obstruction of plateaus and mountains surrounding the TB,building the floatling dust layer over the basin.This modeling study depicted the three-dimensional distribution of dust aerosols in a typical dust storm over the TB,Northwest China.3.Modeling dust aerosol budget,regional transport structures within the TBThe sources of dust emissions are located over the desert areas below 2000 metre height above sea levels with the dust emission flux up to 24?g/m2/s.Dry deposition is the dominant removal process of dust aerosols.The spatial distribution of dry deposition in the TB was similar to the dust emission pattern.With impacts of the TB terrain and BL structures,the high column loading of dust aerosols are mostly distributed in the southeast TB.In addition,dust aerosols mainly accumulated in the lower levels,especially below 3km.During this dust storm,dust aerosol transport flux(>3000 ?g/m2/s)area peaked at near 4km along 41°N above the TD.Meanwhile a large amount of dust accumulated at the foot of the Mountain Kunlun caused by strong winds,transporting to the northern slope of the Tibetan Plateau(TP).The strong east cold air from the northeast area invaded into the basin,blew up the dust to 3.5km.Once dust aerosols were entrained to a high elevation(>3.5km),dust aerosols exported from the basin under the influence of the westerly jets.Based on the aerosol budgets over the TB,the eastern and northern borders were found to be the larger contributor to dust transport from the TB.It was estimated that among dust aerosols emitted from the dust source regions,52.01%was exported for regional transport with a small part of dust outflow to TP from the southern border at 7 km height,about 16.78%was deposited onto the TB.The remaining 31.21%was suspended in the atmosphere.4.The characteristic of dust radiative forcing over the TBTo understand the feedback effect of dust aerosols' radiative forcing on BL structures,we further analyse the dust radiative forcing during this dust storm over the TB based on the WRF-Chem simulations.Dust aerosol solar shortwave(SW)effects heat the atmosphere(ATM)but cool the surface(SUR),while the longwave(LW)effects cool the ATM and heat the SUR.At the same time,dust LW exerted more evident impact on the air temperature in lower dust layer,dust SW had important impacts on air temperature in higher dust layer.At nighttime,the average radiative forcing of dust aerosols over the TD at the top of the atmosphere(TOA),SUR,and in the ATM were less than the average radiative forcing during daytime,especially at TOA.The LW radiative forcing was generally positive in lower atmosphere.The cooling effect induced by dust aerosols within 100 meters to the top of the dust layer.Compared to the heating rate in the ATM,dust radiative forcing had the most significant effects on the top of the dust layer.During this dust storm,the average direct radiative forcings induced by dust particles over the TD at all-sky were 0.53,-5.90 and 6.43 W m-2 at the top of the atmosphere,the surface,and in the atmosphere,respectively.The SW radiative forcing induced by dust aerosols was much larger than the LW radiative forcing at the SUR and in the ATM.Moreover,the radiative forcing at the SUR was significantly larger than that at the TOA.