Dust Aerosol Radiative Property Over Arid And Semi-arid Region Of Northwest China

Aerosol climate effect is one of the hot spot and difficult problems in climate change. The vital foundation of sdutying aerosol climate effect is to understand the spatial and temporal distribution of aerosol and radiative properties. Meanwhile, due to the Arid and semi-arid zones of Northwest China is always affected by dust weather and is more sensitive to climate change, to study the spatial and temporal distribution of aerosol and radiative properties over this area has important theoretical and practical value. The data of 2006²012 AERONET（Aerosol Robotic Network） level 2.0 and CM₂1（solar shortwave radiometer） in SACOL（Semi-Arid Climate and Environment Observatory of Lanzhou University） was used to analyze aerosols radiative properties over Northwest China and examine the reason to cause the positive TOA（Top of Atmosphere） radiative forcing. WRF-chem coupling with dust emission scheme raised by Shao（2004） was used to simulate two sand-dust processes in spring in 2007 to quantify the continuous spatial and temporal distribution of dust emission sources, deposition and dust load. Counting the frequency of sever dust events from 1954 to 2012 in North China based on the observation of wind and visibility from stations record, and analyzed the correlation between climatic variables and sever dust frequency.（1） The coarse mode aerosols dominate in the northwest of china. The annual average of BOA（Bottom of Atmosphere）, TOA and Atmosphere radiative forcing is-59.43,-17.03 and 42.40 W/m2, respectively. The annual average of aerosol optical depth（AOD 550 nm）, single scattering albedo（SSA 675nm）, asymmetry factor（g 675nm） and the real part of complex refractive index（675nm） is 0.37, 0.93, 0.68 and 1.48 respectively. The maximum of atmospheric heating rate appeares at 0²km and the heating rate of winter half year and summer half year is 2.6K/day and 0.6K/day on surface. Only taking account of the surface albedo and SSA when explain the reason to cause the positive radiative forcing at TOA, the contribution of surface albedo is 22.5%, and SSA is 77.5%.（2） Temperature and radiation can be simulated accurately by WRF-chem and the correlation coefficient with observed value is 0.9 and 0.95. The dust emission scheme（Shao04） can simulate the time evolution of dust better. Sand sources distribute in the desert of arid and semi-arid area during the first sand-dust process, including Badain Jaran, Mu Us, Tengri, Gobi desert in Zhangye. The max emission and deposition is 2.8×107μg/m2 and 154μg/m2/s in sand area. The northwest wind entrains the dust delivering to SACOL causing the max dust load and the concentration of PM10 is 9.4×106μg/m2 and 6541μg/m3, respectively. Strong downdraft related to divergence causes the wind increasing near SACOL, soon after the dust concentration increase in SACOL secondly because of the blown local sand with the max dust emission and deposition is 1.0×107μg/m2 and 141μg/m2/s. The max dust load and PM10 concentration is 1.3×106μg/m2 and 775μg/m3, respectively.（3） Analysising the sever sandstorm frequency of 1954²012, total sever sandstorm frequency is 241, of which 190（78.8%） occur in the spring. Since 1954, the sever sandstorm frequency decreases drastically in northern China, the interannual variability of the sever sandstorm frequency is wavy developpment. Compositing the atmospheric circulation, precipitation and soil moisture in high-frequency and low-frequency year indicates that the main difference in atmospheric circulation is the strength and position of the polar vortex and East Asian trough. The polar vortex weakens in high frequency and East Asian trough strengthens. In low frequency year, the polar vortex enhances, and East Asia trough weakens and moves eastward. Current spring precipitation affectes dust frequency significantly with correlation coefficient more than-0.5. The correlation coefficient between early autumn precipitation and dust frequency is more than-0.4, therefor analyzing the precipitation in autumn is conductive to predict the dust frequency in follow spring.