Knowledge Resource Center for Ecological Environment in Arid Area
| 大气边界层风的阵性和相干结构——条带和准流向涡对 | |
| 其他题名 | Gustiness and Coherent Structure in Atmospheric Boundary Layer:Streaks and Quasi-streamwise Vortex pairs |
| 李奇龙 | |
| 出版年 | 2016 |
| 学位类型 | 博士 |
| 导师 | 李正强 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 研究指出，我国北方春季冷锋过境后，在平均风上常叠加有较有规律的周期3-6min的阵风，且有明显的相干结构：风速峰期有下沉运动，谷期有上升运动，这种相干性有利于动量下传。为了更全面地认识阵风扰动谱区的特性，本文对北京325米气象塔，甘肃民勤巴丹吉林沙漠观测塔和湛江市徐闻县观测塔的超声风速资料，采用傅立叶展开变换，按频率分成平均流（周期10分钟以上），阵风扰动（周期1到10分钟），湍流脉动（周期小于1分钟）三部分。分析发现：在小风情况下，阵风扰动是最强的脉动，贡献了约60%的涡动能和约80%的动量通量。在小风情况下，阵风扰动为各向异性且仍然存在相干结构。在小风情况下，阵风扰动和湍流脉动的摩擦速度随高度变化不大。机械作用主导的大气边界层，阵风区就会出现相干结构或者说阵风相干结构出现在静力中性、不稳定甚至略微稳定的条件下。阵风相干结构的物理图像为高低速条带和准流向涡对，准流向涡的轴向几乎与平均风的方向平行，有一个小夹角。准流向涡对将低层的低速空气卷入高空，形成低速条带，将高空高速空气卷至低空，形成高速条带。这种条带随着平均风移过观测点就会引起阵风；准流向涡对诱导的上升低速气流和下沉高速气流就是阵风的相干结构。可见风的阵性和相干结构就对应高低速条带和准流向涡对。准流向涡将高空高动量（能量）的空气卷下，将低动量（能量）的空气卷入高空，从而形成有效的动量（能量）输运。如果位温、水汽、物质浓度（CO2，污染物，沙尘，海盐等）有垂直分布也能形成有效的热量、水汽、物质输运。在寒潮大风天气下，准流向涡对的垂直尺度约为240m，条带的流向尺度至少为1-2公里，准流向涡的轴向与流向的夹角约为13°。阵风相干结构是机械湍流的现象。阵风的相干结构和壁湍流条带结构，朗缪尔环流在结构上是相似的，都是类似条带和流向涡结构，其中，阵风的相干结构和壁湍流条带结构都出现在切变最大的边界层底层，阵风相干结构中的条带和壁湍流的条带的长度都约为一个边界层厚度。阵风的相干结构和对流云街有相同的结构（准流向涡对），但是阵风的相干结构并不是对流云街。阵风的相干结构也不是小尺度的对流云街，相反，笔者认为阵风的相干结构是对流云街的成因，阵风的相干结构像对流云街的“种子”，出现在广泛的稳定度下，在合适的稳定度下就形成或者“长大”形成边界层大小的对流云街，不稳定不能太弱——“长”不到边界层那么大，也不能太强——阵风的相干结构会被热力湍流打乱，只能是“中等”不稳定的条件下。事实上，观测结果正是如此，对流云街出现在“中等”不稳定的条件下。台风中小尺度的滚轴涡就是阵风的相干结构，笔者认为台风中大尺度的滚轴涡是对流云街。本文推导了平均流、阵风扰动、湍流脉动的发展方程，水汽、热量、CO2，污染物等被动标量的发展方程，平均流、阵风扰动和湍流脉动的能量发展方程，指出了平均流、阵风扰动和湍流脉动之间的能量传输项。并结合北京325m气象塔小风资料计算了平均流与阵风扰动，湍流脉动之间能量传输。结果表明由平均流直接向湍流脉动输送的能量只有很小一部分，主要的能量传输路径为平均流经阵风扰动传输给湍流脉动，或者说由平均流直接生成的湍流脉动很少，平均流首先生成周期3分钟左右的阵风扰动，然后经阵风扰动（破碎）生成高频的湍流脉动。 |
| 英文摘要 | After the passage of a cold front, spring in northern China, there are rather regular gust wave packets superimposed on the basic strong wind flow. The gusty wind wave packets have a period equal to around 3–6 min and possess coherent structure: horizontal and vertical components are negatively correlated and make downward transport of momentum more effectively. To comprehensively understand the characteristics of gusty disturbance spectral region, we analyses the data from the multilevel ultrasonic anemometer-thermometers on Beijing 325m meteorological tower, Badan Jilin Desert monitoring tower (in Minqin County, Gansu Province) and monitoring tower in Xuwen County, Guangdong Province. The time series of 3D atmospheric velocity were analyzed by using conventional Fourier spectral analysis and decompose into three parts: basic mean flow (period > 10 min), gusty disturbances (1 min < period < 10 min) and turbulence fluctuations (period < 1 min). The results show:The gusty disturbances are the strongest fluctuations, contribute about 60% eddy kinetic energy and 80% downward flux of momentum under weak mean wind condition.Even under weak mean wind condition, the gusty wind disturbances are anisotropic with moderate coherency.The friction velocities relate to turbulence fluctuations and gusty wind disturbances are approximately constant with height in the surface layer under weak mean wind condition.The coherent structure of the gusty wind disturbances occurs on the condition that turbulence is mainly produced by mechanical action. In other words, coherent structure occurs in neutral, unstable and even in slightly stable boundary layer.The physical picture of the coherent structure of the gusty wind is high speed streaks, low speed streaks and quasi-streamwise vortex pairs, which are nearly aligned with the mean wind. The quasi-streamwise vortexes drag aloft air moving at high speed downward to the surface, generating high-speed streaks, while drag low speed air near the ground upward, generating low-speed streaks. The streaks move over the observation point, causing gusty disturbance. The upward air at low speed and the downward air at high speed induced by quasi-streamwise vortexes is the coherent structure of the gusty wind. Thus, gustiness and coherent structure correspond to streaks and quasi-streamwise vortex pairs.The quasi-streamwise vortexes drag aloft air with high momentum(kinetic energy) downward to the surface and drag low momentum(kinetic energy) air near the ground upward, make downward transport of momentum(kinetic energy) effectively. They can also make vertical transport of heat, moisture and air pollutants(CO2, dust, sea salt) if potential temperature, moisture and air pollutants are different with heights.After the passage of a cold front, the vertical extent of quasi-streamwise vortex pairs is about 240m. The scales of the streaks are at least 1-2 km, and the angle between the streamwise vortexes and the mean wind is about 13°.The coherent structure of the gusty wind is produced by dynamical process.The structures of gusty disturbance, streak structures of wall turbulence and Langmuir circulation are similar, they are streak and streamwise vortex. Both gusty disturbance and streak structures of wall turbulence occur at the bottom of the boundary layer where the shear rate is largest. Both the length scales of the streak structures of the gusty wind and the wall turbulence are about 1 time the thickness of the boundary layer.The structures of gusty disturbance and cloud streets(horizontal convective rolls) are the same, but gusty disturbance is different from cloud streets. The gusty disturbance should not be treated as smaller-scale cloud streets, instead, it should be the cause of the cloud streets. The coherent structure of the gusty wind like “seed” of cloud streets, occurs in various atmospheric stratification, “grows up” in appropriate atmospheric stratification and becomes cloud streets. It cannot become as large as cloud streets under weak unstable stratification, and thermal turbulent will disturb the coherent structure of the gusty wind under strong unstable stratification. Thus, the appropriate atmospheric stratification is moderate unstable stratification. And the observations show that cloud streets occur in moderate unstable stratification.Smaller-scale rolls in tropical cyclone are the coherent structure of the gusty wind. Larger-scale rolls in tropical cyclone should be cloud streets.This research deduced evolution equations of momentum, moisture, heat and scalar quantity for gusty disturbances, mean wind and turbulence fluctuations, and evolution equations of gusty disturbances kinetic energy, mean kinetic energy and turbulence fluctuations kinetic energy. Pointed out the terms describing the interactions between the mean flows, gusty disturbances and turbulence fluctuations. And calculated the energy transfer between mean wind, gusty disturbances and turbulence fluctuations with weak wind data of the Beijing 325m meteorological tower. The results show that: the mean winds transfer little energy to the turbulence fluctuations directly, while the mean winds transfer much energy to the turbulence fluctuations through the gusty disturbances. In other words, less turbulence is generated by the mean wind directly, as an alternative, the mean winds generate the gusty disturbances, then the gusty disturbances breakdown and generate high-frequency turbulence fluctuations. |
| 中文关键词 | 大气边界层 ; 湍流 ; 阵风 ; 相干结构 ; 条带 ; 准流向涡对 ; 动量下传 |
| 英文关键词 | atmospheric boundary layer turbulence gusty wind coherent structure streak quasi-streamwise vortex pair downward flux of momentum |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 大气物理学与大气环境 |
| 来源机构 | 中国科学院大气物理研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/287639 |
| 推荐引用方式 GB/T 7714 | 李奇龙. 大气边界层风的阵性和相干结构——条带和准流向涡对[D]. 中国科学院大学,2016. |
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