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中国物理学会期刊

基于探空数据分析低云对大气折射率结构常数的影响

CSTR: 32037.14.aps.71.20211792

Influence of low clouds on atmospheric refractive index structure constant based on radiosonde data

CSTR: 32037.14.aps.71.20211792
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  • 本文基于实测的热力湍流探空数据, 使用WR95方法识别低云的垂直结构, 对比分析了低云与晴空天气下大气折射率结构参数 C_n^2 、气象条件和大气稳定度的平均统计结果. 结果表明, 低层薄云对 C_n^2 起伏变化的影响微乎甚微, 仅仅表现出轻微增大的趋势, 云底 C_n^2 相对于晴空天气平均增大1.6倍, 云顶之上最大程度增大2.5倍. 低层中厚云在云顶处 C_n^2 相对于晴空天气增大了3.80—6.61倍, 且云顶区域 C_n^2 增大的幅度大于云底区域. 云底区域大气湍流特性受到地面热力驱动与低云冷却的联合作用, 沉降气流与地面向上气流发生了耦合, 增强了风切变, C_n^2 在这一高度附近也出现了增强. 综合对比晴空和有云天气 C_n^2 大小可知, 云对 C_n^2 的增强效应大致在10–16量级. 一方面, 风切变在云顶处或者云顶之上达到最大值; 另一方面, 因为云顶短波辐射增温和长波辐射冷却的共同作用, 云顶之上会形成不同厚度的逆温层, 致使云顶处位温变化率急剧增大, Brunt-Vaisala频率 N^2 值较晴空天气下增大了0.5—3.0倍; 而云底区域 N^2 均小于晴空天气. 由于云层多尺度活动引发的湍流效应, 势必会引起对激光传输大气效应评估和订正的偏差. 正确掌握不同相态云层及边界处湍流的变化规律, 也可以为进一步建立云层周围大气湍流的变化规律模型奠定基础.

     

    Based on the measured thermal radiosondes, the WR95 method is used to identify the vertical structure of low clouds. The atmospheric refractive index structure constantC_n^2, meteorological conditions and atmospheric stability are contrastively analyzed under cloudy and clear sky weather. The results show that the influence of low-level thin clouds on the fluctuation of C_n^2 is negligible, showing only a slight increase trend. The C_n^2 at low-level thin clouds base and top is about 1.6 and 2.5 times that under clear sky weather to a greatest extent, respectively. The C_n^2 at the low-level medium-thick clouds top is 3.8–6.61 times the amplitude of that under clear sky weather, and enhanced amplitude of C_n^2 near the cloud top is greater than that near the cloud base. Atmospheric turbulence near the cloud base is driven by the combined effect of ground heat and low clouds cooling. The sinking airflow from clouds is coupled with the upward airflow from ground, which motivates wind shear, resulting C_n^2 increases near this height. A comprehensive comparison of the C_n^2 between clear sky and cloudy weather shows that the enhancement effect of clouds on C_n^2 is roughly on the order of 10–16. Wind shear reaches its maximum value at or above the cloud top. Because of the combined effect of short-wave radiation warming and long-wave radiation cooling near the cloud top, temperature inversion layers with different thickness will be formed obove the cloud top, resulting in a sharp increase in the potential temperature lapse rate at the cloud top, and the Brunt-Vaisala frequency N^2 is increased by 0.5–3.0 times. And N^2 near the cloud base is less than that under the clear sky weather. Owing to the turbulent effect caused by cloud multi-scale activities, it is inevitable to cause assessment and correction deviations in the laser transmission. A deep understanding of how turbulence behave within different phase clouds or around cloud boundaries can also lay the foundation for further modeling the atmospheric turbulence around clouds.

     

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