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塔里木盆地一次典型沙尘暴过程沙尘气溶胶时空变化及区域传输特征的模拟研究

Modeling Study on Temporal and Spatial Variation and Regional Transport of Dust Aerosols during A Typical Dust Storm Over the Tarim Basin

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【Author in Chinese】 孟露

【Supervisor】 赵天良杨兴华

【Author's Information】 南京信息工程大学, 大气物理学与大气环境, 2018, 硕士

【Abstract in Chinese】 塔克拉玛干沙漠位于我国西北干旱荒漠区塔里木盆地,是世界上最大的流动性沙漠之一,沙尘暴天气常年频繁发生,其沙尘气溶胶起沙、沉降、传输以及沙尘时空变化均受到沙漠地区大气边界层条件和盆地大地形的制约,形成了塔里木盆地独特的大气沙尘气溶胶时空变化及区域传输特征,是一系列依然有待深入研究的环境问题。因此,本文针对这一科学问题,利用中尺度数值模式WRF/WRF-Chem模拟塔克拉玛干沙漠沙尘暴过程,选择合理边界层参数化方案,分析塔里木盆地一次典型沙尘暴沙尘气溶胶分布变化、区域输送以及辐射强迫。主要研究内容和结论如下:1、塔克拉玛干沙漠腹地大气边界层参数化方案的模拟评估沙尘起沙、沉降、传输均受到沙漠地区大气边界层条件的制约。沙漠地区观测资料匮乏,限制大气边界层模拟效果的检验和评估。利用中尺度数值模式中五种边界层参数化方案(ACM2、BL、MYJ、MYNN2.5、YSU),模拟2014年4月塔克拉玛干沙漠大气边界层特征,并与中国气象局塔克拉玛干沙漠气象野外科学试验基地塔中80米塔及风廓线雷达晴朗天气下的观测资料对比分析。结果表明:5种方案均能模拟出近地面气温、地表温度、边界层高度、感热、潜热及地表热通量的变化趋势,但未能模拟出边界层风速的日变化趋势,温风湿廓线能较好的反映晴天沙漠地区边界层结构的变化特征,但未模拟出风速随高度变化趋势。沙漠地区下垫面干燥,热容量低,晴天极易形成对流边界层,非局地湍流参数化ACM2方案是沙漠地区大气边界层模拟较为合理的选择。2、塔克拉玛干沙漠一次典型春季沙尘暴过程沙尘气溶胶时空分布变化利用WRF-Chem(v3.8.1)模式,结合GOCART起沙方案、ACM2边界层方案,模拟了 2015年4月27至5月1日塔里木盆地一次典型的春季沙尘暴过程,并与地面和CALIPSO卫星资料对比评估,讨论了盆地沙尘气溶胶排放特征及输送路径,描绘沙尘气溶胶三维分布情况。地面冷锋活动是引发此次沙尘暴的天气系统。冷锋活动受盆地独特地形的影响,冷空气从三条路径进入盆地:1)西路的冷空气翻过帕米尔高原从西侧进入盆地(西进)、2)北路冷空气翻越天山(翻山)进入盆地,3)东路的强冷空气从盆地东北口灌入(东灌)。较强的东灌沙尘带驱动两股弱沙尘带向盆地西南部输送,地表沙尘气溶胶分布受青藏高原地形影响。观测和模拟结果发现,沙尘气溶胶主要集中在塔里木盆地独特的3-5km较厚边界层内,由于盆地南部青藏高原北坡地形阻挡,沙尘气溶胶可抬升至自由大气7km以上。塔里木盆地边界层上部(3-5km)沙尘气溶胶空间分布受盆地近地面冷空气辐合作用及周边高原和山脉地形阻挡的影响,进一步抬升形成盆地大气浮尘。这一模拟刻画了塔克拉玛干沙漠一次典型的春季沙尘暴过程大气沙尘气溶胶三维分布变化特征。3、塔里木盆地沙尘气溶胶的收支关系及区域传输结构塔里木盆地沙尘排放源主要集中在地势高度2000m以下沙漠区域,其沙尘排放通量达24μg/m2/s,干沉降作为沙尘气溶胶的重要移除过程,其分布状况与沙尘排放通量类似。沙尘柱浓度最大值分布在盆地东南部沙尘源区附近,强东灌风驱动沙尘气溶胶向盆地西南部输送,受盆地地形及边界层高度的限制,进一步抬升形成盆地大气浮尘,且沙尘气溶胶粒子主要累积在3km以下。此次沙尘暴过程中,塔克拉玛干沙漠内沙尘气溶胶多被抬升到4km以上的高空,沿41°N向东输送,最大输送通量可达3000μg/m2/s。与此同时,大量沙尘在昆仑山脚下累积,向青藏高原北坡输送。对比分析盆地东、西、南、北四个面沙尘输送通量及模拟区域沙尘气溶胶的收支情况发现:盆地东面和北面为沙尘气溶胶的主要输送方向,强冷空气从东北部进入盆地,抬升大量沙尘,一旦沙尘被卷夹到3.5km以上,则易在气流影响下输出盆地。此次典型沙尘暴过程中,塔克拉玛干沙漠释放的沙尘气溶胶,52.01%通过气流传输至盆地外,部分沙尘气溶胶从南面输出到达青藏高原,高度达7km,16.78%的沙尘气溶胶沉降到塔里木盆地,剩余31.21%的沙尘气溶胶悬浮于盆地形成浮尘。4、塔里木盆地沙尘气溶胶辐射强迫特征为了探索沙尘气溶胶的辐射效应对大气边界层结构的反馈作用,基于WRF-Chem模拟,我们进一步分析这次塔里木盆地沙尘暴过程沙尘气溶胶辐射强迫特征。白天,塔里木盆地沙尘气溶胶通过短波辐射加热大气,冷却地表,沙尘长波辐射冷却大气,加热地表。低层沙尘长波辐射强迫作用大于短波,高层沙尘长波辐射作用小于短波;夜间,地表、大气、大气顶辐射强迫的数值均小于白天的长波辐射强迫,其中地表辐射强迫最为显著;与白天不同,夜间沙尘气溶胶长波辐射加热近地层大气,100米以上至沙尘层顶的沙尘冷却大气;整层大气中沙尘层顶受气溶胶长波、短波辐射强迫影响最显著。此次沙尘暴期间,大气顶接收到的沙尘区域平均辐射强迫为0.53W/m2,地表接收辐射强.迫为-5.90W/m2,大气为6.43W/m2。沙尘短波辐射强迫对地表和大气的影响远大于长波,TOA受沙尘气溶胶辐射强迫作用远小于地表。

【Abstract】 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 24μg/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 μg/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.

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