持续性浓雾爆发性增长的高分辨率观测和大涡模拟

The aim of this study was to lay a foundation for improving the accuracy of continuous fog forecasting. The phenomenon of fog droplet spectrum and water content increasing rapidly in a short period of time, which led to a sharp decline in visibility, was observed many times in Nanjing. It is known as explosive growth of fog. The causes of persistent heavy fog’s formation and maintain were analyzed. However, the main problems about the micro-physical mechanism and the role of turbulence in explosive growth process were not well understood, due to the lack of high-resolution observation and large-eddy simulation. Firstly, aerosol, fog microstructure (sampling frequency of fog spectrum will be improve from the original 1 Hz to 10 Hz, increase fog droplet spectrum observational site from one to three), boundary layer (including radiation, turbulence), meteorological factors and so on will be directly measured and also detected via remote sensing. Secondly, continuous fog processes will be simulated using a large-eddy model instead of previous MM5/WRF model. Based on the fact that results of simulations and measurements are basically the same, a comprehensive study will carry out to reveal the group structure of continuous fog, uniformity of the horizontal distribution of fog droplet spectrum, and micro-physical mechanism about the effects of factors such as activation of coagulation nucleus on the explosive growth of droplets under turbulent and high humidity conditions. The quantitative effect of the vertical distribution and time evolution of factors such as fog temperature, humidity, wind, radiation rate, and turbulence kinetic energy on the explosive growth of fog will be revealed.

南京雾观测研究中多次出现短时间内雾滴谱、含水量增长极快从而导致能见度急剧下降的现象，称为爆发性增长。对持续性浓雾的天气成因已做了分析，但对其爆发性增长的微物理机制、湍流的作用等核心问题，还缺乏高分辨率观测和大涡精细模拟。首先，对气溶胶、雾细微结构（雾滴谱采样频率由原来1Hz提高到10Hz、由单点观测增至3点观测）、边界层（含辐射、湍流）和气象要素等直接测量和遥感探测；第二，由以往MM5/WRF模拟改为大涡模式WRF-LES对持续性浓雾过程进行高分辨率模拟；在两者基本一致的基础上进行综合研究，揭示持续性浓雾团状结构、雾滴谱水平分布均匀性及湍流、高湿条件下凝结核活化等因子对雾滴谱爆发性增长作用及微物理机制；揭示雾温、湿、风、辐射降温率、湍流动能等要素垂直分布及其时间演变对持续性浓雾爆发性增长中的定量作用；为提高持续性浓雾预报准确率奠定基础。

为深入探究不同地区辐射雾的理化性质、雾发展过程中爆发性增长的微物理机制、气溶胶以及湍流对雾影响等核心问题，从而进一步提高对辐射雾的预报预警能力并减少雾害损失，项目组团队在南京北郊、西双版纳热带雨林及东海县新型城镇化地区开展了5期辐射雾理化特性的综合观测试验。观测结果发现：西双版纳热带雨林地区雾凉季辐射雾中过饱和度较高但雾滴数浓度较低，雾水呈弱碱性，电导率低，且化学组分在小滴富集；在南京北郊观测到的平流辐射雾过程中存在爆发性增长的特征，表现为17 min内含水量的量级由10-4 g/m3增加至10-1 g/m3，其中雾滴数浓度的增加对其贡献率可达67%，意味着持续性活化过程主导此次雾爆发性增长；庐山清洁地区的雾水酸度比南京污染地区更强，电导率更低，总离子浓度低一个量级；浓雾过程对爱根核模态粒子的湿清除作用最明显，减少28.3%，粒径范围在10 nm至10000 nm气溶胶的总数浓度下降了14.6%；提出订正CCN测量结果中未活化颗粒的新方法，并评估对气溶胶间接辐射效应的高估；在国内率先使用两台不同采样频率(5hz和1hz)的雾滴谱仪同时开展精细化雾观测试验，5Hz和1Hz均能反映此次雾过程中不同阶段微物理之间的关系，主要差别出现在雾的生成阶段且5Hz更易观测到极值。除开展雾精细化观测实验之外，项目组团队还使用大涡模式WRF-LES对持续性浓雾过程及其边界层结构进行高分辨率模拟，模拟结果表明：在分辨率高于100米时大涡模式对涡粘系数模拟结果的可信度较大；NBA方案不太适用于较粗分辨率（如300米分辨率）的垂直动量通量模拟；三种大涡方案(Smagorinsky方案、TKE方案和NBA方案)模拟得到的风廓线基本互相吻合。项目研究成果为提高持续性浓雾预报准确率奠定基础。