东亚地区不同下垫面人为沙尘起沙参数化的数值模拟研究

Dust emission is a key process of dust cycle which determines dust long-term transport, dry/wet deposition, radiation forcing and other dust-related processes at both regional and global scales. Scientists have devoted to constructing and developing dust schemes that can be used to simulate dust emission at regional and global scales since the 1980s. Although the development of dust emission schemes has greatly improved the comprehension of dust-related processes and the understanding of the effects of dust on the environment and climate, yet present studies on dust emission mostly focus on natural dust and thus there is still a great gap in the research on anthropogenic dust emission from human activity via modifying or disturbing soil. . Research showed that anthropogenic dust flux is stronger than natural dust flux in some congested areas such as eastern China, India, North America and Africa, especially in the areas with freshly exposed soil containing more fine material for emission and disrupted surface which is easier for dust deflation, which emphasizes the importance of anthropogenic dust. Therefore, quantitative estimation of anthropogenic dust emission is crucial to reinforce the understanding of global dust cycle and its climate effects. . We will develop the anthropogenic dust emission based on the WRF-Chem (Weather Research and Forecasting model with chemistry) model in this project. To account for the vegetation effects on the threshold friction velocity, the Moderate Resolution Imaging Spectroradiometer (MODIS) monthly normalized difference vegetation index combined with the wind tunnel test will be used to derive the surface roughness parameters and the threshold friction velocity. Further, the spatial distribution of the threshold friction velocity and anthropogenic dust source regions over East Asia will be estimated. The project could reflect the ‘direct’ anthropogenic dust emission induced by human activities and the ‘indirect’ dust emission induced by land use based on dust emission schemes. The implement of the project will have greatly improved the understanding of dust uplifting mechanism.

沙尘起沙是指土壤颗粒在风力作用下的空间位移，是沙尘循环的关键环节，决定了沙尘事件强度、时空分布特征及沙尘粒子的输送和沉降等过程。定量和准确地描述起沙过程是实现沙尘模拟和预报的基础。然而，在目前的天气和气候模式中，绝大部分的起沙参数化方案只考虑了自然因素下强风导致的沙尘暴，忽视了直接人为活动或通过人为活动改造土地利用/覆盖所产生的人为起沙过程，使得起沙通量的模拟存在很大的不确定性。针对以上亟待解决的关键科学问题，本项目将观测试验和卫星遥感数据相结合，以区域模式为基础，定量估计东亚地区临界起沙摩擦速度的时空分布，构建描述人为沙源动态变化的高分辨率数据集，建立人为起沙通量、摩擦速度和人为活动强度的经验关系，进而发展一套适应于东亚地区的人为起沙参数化方案，在数值模式中合理地反映人为活动造成的“直接”起沙和通过人为活动改造的地表所产生的“间接”起沙过程。

沙尘气溶胶的气候效应是研究全球变化中影响能量和水的重要纽带和关键环节，沙尘物理过程是沙尘模拟和预报中的核心科学问题。然而，目前大部分研究单纯将沙尘粒子归类为自然气溶胶，忽视人为活动对沙尘释放的贡献，从而在数值模式中对起沙通量的模拟存在很大不确定性。基于此，该项目明确了人为沙尘的定义，在数值模式中将人为起沙分为间接人为起沙和直接人为起沙两大类。针对农田、稀疏草地和城市等下垫面开展了一系列起沙及人为沙源土壤理化性质的野外观测试验，获取人为起沙的关键参数，建立了高时空分辨率的地表风蚀度动态数据集，发展了局地-区域-全球不同尺度的人为沙尘排放清单，构建了国际上起沙种类最为完备的沙尘模块，系统揭示了人为沙尘的时空分布特征，以及源区和沉降区的质量收支情况，有效提高了数值模式对东亚沙尘的模拟精度。此外，将人为沙尘排放清单耦合到全球模式中，进一步揭示了人为沙尘的气候环境效应。研究表明，发展中国家印度和中国人为沙尘排放通量最大，分别为5.58±0.03和4.55±0.10 μg m2 s-1，直接由人为沙尘造成的过早死亡超过4000人/1000平方公里。在气候效应影响方面，人为沙尘辐射反馈有效的冷却地表，抵消了温室气体的增温效应，并减少了边界层高度，减弱了感热和潜热，从而加剧非沙尘型人为气溶胶排放，对地球系统产生双重气候惩罚。在项目资助下在JC、ES&T、ERL、JGR等大气科学领域权威期刊发表学术论文14篇，一篇文章入选ES&T封面论文。该项目已完成成果集成，初步搭建起《我国西北地区沙尘天气预报与预警系统》，目前正投入到甘肃、新疆等地气象和环保部门的业务工作中，对春季89%的沙尘过程预报有改进效果，有效改善了西北地区的沙尘模拟和预报水平，服务于国家防灾减灾和西部生态安全建设需求；此外，人为沙尘模块目前已被耦合到清华大学CIESM地球系统模式，有效改进了人口密集区沙尘气溶胶模拟精度（提高10-12%）。