沙尘气溶胶对半干旱区地表能量平衡及边界层结构的影响研究

Dust aerosols have caused significant impacts on climate change and environment by (1) adjusting shortwave/longwave radiative process, (2) changing the microphysical properties and life of clouds, and (3) altering the cloud evaporation, which were also called as direct, indirect and semi-direct radiative effect of dust aerosols. Due to the limited observational evidence of physical/chemical/optical properties of dust aerosols and a lack of thoroughly understanding of the complex nonlinear interaction between dust aerosol and climate, the aerosol radiation forcing (ARF) has become one of the most uncertain factors for the climate change in arid and semiarid regions. Because of the inhomogeneous spatiotemporal distribution of dust aerosols and absorption property, it is not enough to focus only on the Top of Atmospheric (TOA) aerosol direct radiation forcing. Both observation and models could not provide enough evidence for the vertical distribution of aerosols, temperature and water content, so it is worthy to study the interaction between dust aerosols and planetary boundary layer (PBL). Semi-arid region was the source region and influence region of dust aerosols. While because of lacking continuous observation for dust properties, energy budget factors and planetary boundary layer in semi-arid region of China, the study on dust aerosols-radiation interaction and the feedbacks between meteorological elements and dust aerosols in models was far from satisfactory. .In this project, continuous observations of particulate matter (PM10 concentration), meteorology elements, energy fluxes at land surface, vertical distribution of dust aerosols and temperature/relative humidity were collected at Semiarid Climate Observatory and Laboratory (SACOL) in northwestern China from 2007 to 2016. Furthermore, the aerosol data from CALIPSO and NCEP FNL data were also used. Typical dusty/nondusty events were selected based on statistics of PM10 concentration, horizontal visibility, Air Pollution Index (API) and Aerosol Optical Depth (AOD) from SACOL. Firstly the difference of diurnal variations of meteorological elements under different synoptic processes is analyzed to study the interactions between dust aerosols, meteorological elements, energy factors, and heat fluxes. Secondly the difference between observed temperature profile and NCEP FNL temperature profile, which means Observation minus reanalysis, is conducted to analyze the interaction between the vertical distribution of dust and the planetary boundary layer. Lastly, long-term measurements and our new findings will be used to verify the simulation ability for WRF-Chem in semi-arid region of China, in order to find out the optimal parameterization scheme. Then WRF-Chem will be used to quantitatively understand the interaction of dust aerosols and PBL in semi-arid region of China.

沙尘气溶胶可以通过散射/吸收长短波辐射、参与云-降水过程及影响云滴蒸发等直接/间接/半直接效应对气候/环境产生影响。沙尘辐射效应仍存在较大不确定性，这主要由于缺少沙尘气溶胶时空分布的观测证据，以及对其与气候相互作用机理的认识不足。半干旱区是沙尘的主要源区/影响区，由于缺乏在该地区沙尘特征、地表能量收支和边界层特征的长期观测，导致用模型开展沙尘气溶胶-大气边界层双向反馈作用的研究缺乏数据支撑，造成其辐射效应成为该地区气候变化研究中最不确定的因子之一。本项目拟利用兰州大学半干旱气候与环境观测站的观测资料，结合CALIPSO和NCEP FNL再分析资料，合成分析沙尘气溶胶对地表微气象学要素、能量平衡和大气边界层结构的影响。利用观测数据验证并改进双向耦合模型WRF-Chem在半干旱地区的模拟性能，并进一步认识该地区沙尘气溶胶的辐射特性、地表能量平衡及大气垂直加热等过程的反馈作用及气候/环境效应。

气溶胶可以通过散射或吸收长、短波辐射产生不同效应的辐射强迫，同时作为凝结核参与云-降水过程及影响云滴蒸发等过程从而对气候/环境产生影响。以黄土高原为代表的半干旱区是沙尘的主要源区/影响区，由于缺乏在该地区沙尘特征、地表能量收支和边界层特征的长期观测，导致用模型开展沙尘气溶胶-大气边界层双向反馈作用的研究缺乏数据支撑，造成其辐射效应成为该地区气候变化研究中最不确定的因子之一。. 本项目在3年的执行过程中，利用兰州大学半干旱气候与环境观测站（SACOL）长期连续观测资料，结合CALIPSO和NCEP FNL再分析资料，通过聚类分析、统计分析、模型模拟验证等方法，合成分析了沙尘气溶胶对地表微气象学要素、能量平衡和大气边界层垂直结构的影响；完成了既定的研究任务，达到了预期目标。. 具体结论如下：. （1）偏北、偏西气流是造成半干旱地区沙尘气溶胶浓度升高的主要来源方向，高空天气槽与低空强对流天气是主要的背景环流形势。沙尘天气的相对湿度和地表气压较晴天分别低15％和70％，地表气温则呈现白天偏低夜间偏高的变化特征。. （2）沙尘天气的地表净辐射及感热、潜热在日间偏低夜间偏高，且能量平衡闭合度更低。具体而言，沙尘天日间地表向下/向上的太阳辐射比晴天小，最大的差异分别为206.7 W/m2和33.25 W/m2。沙尘天与晴天的向下长波辐射的差值为35 W/m2，分别占晴天和沙尘天日平均值的11.7％和14％，而感热/潜热通量的最大差异则分别可以达到最大净辐射的41.9％和12.1％。. （3）气溶胶浓度升高的边界层响应特征表现为低层大气降温、高层大气升温，同时伴随着相对湿度随高度逐渐增加。气溶胶粒子通过加热所在大气层和减少地表气温，造成大气稳定度的进一步增加，从而导致污染的加重。. 该研究为进一步认识半干旱地区沙尘气溶胶的辐射特性、地表能量平衡及大气垂直加热等过程的反馈作用及气候/环境效应提供了数据证据和理论支持。. 截至目前项目资助发表第一资助SCI论文1篇，培养优青1名，培养研究生2名；还有部分成果整理成稿中，预计后续发表论文1-2篇。