湍流相干结构在中蒙中区一次沙尘暴起沙中的作用

起沙机制是沙尘暴天气研究中一个重要问题.本文基于中蒙中区一次沙尘暴天气过程，通过提取沙尘源地涡动相关数据中的湍流相干结构（Coherent structure，简称为CS），研究了CS在起沙中的作用.结果发现：（1）起沙期间存在CS，其典型特征表现为上扬—下扫循环，即暖空气的辐合上升与冷空气的辐散下沉相伴；（2）起沙期间，CS具有频次高、持续时间短、间歇性适中、连发频繁、速度切变大、水平尺度远大于垂直尺度和尺度变化比较大的特点；（3）CS是重要的起沙机制，它可起动粒径在0.1~156 μm之间的各种沙粒，起动最多的是粒径低于8 μm的粉粒与粘粒，但CS不是唯一的起沙机制；（4）CS的上扬与下扫两过程均可引起起沙，区别在于前者通过暖空气的上升将沙粒向上空输送，后者则是将上空高速冷空气拖带下来引起地表沙粒的起动；（5）CS起沙分为单起式和连发配合式两种形式.单起式是单发CS产生的起沙形式，连发配合式是连发的CS产生的跃移—上扬的配合起沙形式.其中，连发配合式为主要形式；（6）下扫过程对起沙的贡献是上扬过程的1.8~15倍，上扬过程可将下扫过程中起动沙粒的1/3左右向上输送到空中；（7）一般情况下，CS对起沙具有稳定的贡献，其贡献率为51%，当临界起沙风速大于13 m·s^-1时，其贡献急剧降低.

The role of turbulent coherent structure in sand-dust emissions in a sanddust storm of the middle China-Mongolia regime

The mechanism of sand-dust emission is an outstanding problem in the study of dust weather. The role of turbulence coherent structure (CS) in the sand-dust emissions is studied by extracting CSs from observed data of Eddy Covariance System over sand-dust source based on a sandstorm weather process occurring in the middle China-Mongolia regime in this paper. The mainly results show that: (1) CS exists in the process of the sand-dust emissions, which presents a typical ejection-sweep cycle, i.e. the coupling of warm air convergent rising accompanied with cold air divergent sinking; (2) CS is characterized as high frequency, short duration, moderate intermittency, frequent consecutive occurrence, large velocity shear, horizontal length scale much greater than vertical length scale, and great changes in both length scales; (3) CS is an important sand-dust emission mechanism, which can emit sand-dust particles with a particle size of 0.1~156 μm. The powder and clay particles with particle size less than 8μm are the most emitted. However, CS is not the unique sand-dust emission mechanism; (4) Both the ejection process and sweep process of a CS can cause the sand-dust emissions. The difference is that the sweep process due to the sinking of cold air mainly causes the sand-dust particles on the surface to move while the ejection process due to the rise of warmer air mainly transports the particles upward to the sky. Both processes constitute the core of the CS sand-dust emission mechanism; (5) There are two forms of sand-dust emissions of CS: single emission type and matching consecutive emission type. The single emission type is caused by a single CS, and the matching consecutive emission type refers to the sand-dust emission type which the saltation process matches the ejection process in consecutively occurring CSs and the matching consecutive emission type is more prevailing; (6) The contribution of the sweep process to sand-dust emission is 1.8~15 times that of ejection process. The ejection process can transport upward to the sky about 1/3 of the sand-dust particles emitted by the sweep process;(7) In general, CS's contribution to sand-dust emissions has a stable value which is 51%. Its contribution will decrease sharply when the threshold wind velocity is greater than 13 m·s^-1.