Horizontal structure of marine boundary layer clouds from centimeter to kilometer scales
Horizontal structure of marine boundary layer clouds from centimeter to kilometer scales
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厘米至千米尺度海洋边界层云的水平结构
DOI：
10.1029/1998jd200078
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发表时间：
1999
影响因子：
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通讯作者：
W. Wiscombe
中科院分区：
文献类型：
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作者：
A. Davis;A. Marshak;H. Gerber;W. Wiscombe
云液态水含量（LWC）在前所未有的4厘米分辨率测量的水平断面进行统计分析，按比例尺。1993年7月26日，在冬季南大洋云实验（SOCEX）期间，在破碎层积云/塔状积云复合体中用粒子体积监测器（PVM）探测器收集了数据。两个标度区被发现，在这个意义上，两个不同的幂律，k−β需要代表波数谱E（k）在整个范围内的尺度r <$1/k。详细的数值模拟结果表明，在2-5米的尺度突变是不可追溯的正常变化的LWC在PVM的瞬时采样体积（1.25立方厘米）驱动的泊松波动的液滴的数量和大小。因此，这两种制度在本质上是不同的。小尺度LWC变异性的非泊松特征与Baker [1992]从前向散射光谱仪探测器（FSSP）数据获得的液滴数浓度的类似发现一致：在几厘米的尺度上，空间液滴分布并不总是遵循统一的泊松定律。当β = 0.9 ± 0.1时，小尺度（8-12 cm × 2-5 m）区域是稳定的：LWC中的跳跃在大小上变化很大，并迅速相互抵消，导致短程相关。相比之下，β = 1.6 ± 0.1的大尺度（5 m × r × 2 km）变率是非平稳的：跳跃通常很小，传达了一定程度的像素间连续性，从而在低通滤波信号中建立了长程相关性。复杂的SOCEX云系的大尺度结构被证明是多重分形的，这意味着大的跳跃确实间歇性地发生，也就是说，在空间的稀疏分形子集上。低阶，因此更强大，多重分形特性的SOCEX云是非常相似的第一ISCCP区域实验（FIRE）和大西洋层积云过渡实验（ASTEX）的同行，也是被动标量在充分发展的湍流。这是一个显着的相似性的微观物理和宏观物理过程，确定云结构的海洋边界层在非常偏远的地方，特别是因为这里调查的特定SOCEX云系统是相当不典型的。有趣的差异也被发现：一方面，在标度范围内，另一方面，在高阶矩。最后，我们讨论了云辐射效应的大尺度和小尺度的变化。
Horizontal transects of cloud liquid water content (LWC) measured at unprecedented 4-cm resolution are statistically analyzed scale-by-scale. The data were collected with a Particulate Volume Monitor (PVM) probe during the winter Southern Ocean Cloud Experiment (SOCEX) on July 26, 1993, in a broken-stratocumulus/towering-cumulus cloud complex. Two scaling regimes are found in the sense that two distinct power laws, k−β are needed to represent the wavenumber spectrum E(k) over the full range of scales r ≈ 1/k. Detailed numerical simulations show that the scale break at 2–5 m is not traceable to the normal variability of LWC in the PVM's instantaneous sampling volume (1.25 cm3) driven by Poissonian fluctuations of droplet number and size. The two regimes therefore differ physically. The non-Poissonian character of the small-scale LWC variability is consistent with a similar finding by Baker [1992] for droplet number concentration obtained from Forward Scattering Spectrometer Probe (FSSP) data: at scales of a few centimeters, spatial droplet distributions do not always follow a uniform Poisson law. With β = 0.9 ± 0.1, the small-scale (8–12 cm ≲ r ≲ 2–5 m) regime is stationary: jumps in LWC are highly variable in size and rapidly cancel each other, leading to short-range correlations. By contrast, the large-scale (5 m ≲ r ≲ 2 km) variability with β = 1.6 ± 0.1 is nonstationary: jumps are generally quite small, conveying a degree of pixel-to-pixel continuity and thus building up long-range correlations in the low-pass filtered signal. The large-scale structure of the complex SOCEX cloud system proves to be multifractal, meaning that large jumps do occur on an intermittent basis, that is, on a sparse fractal subset of space. Low-order, hence more robust, multifractal properties of the SOCEX clouds are remarkably similar to those of their First ISCCP Regional Experiment (FIRE) and Atlantic Stratocumulus Transition EXperiment (ASTEX) counterparts, and also to those of passive scalars in fully developed turbulence. This is indicative of a remarkable similarity in the micro-physical and macrophysical processes that determine cloud structure in the marine boundary layer at very remote locales, especially since the particular SOCEX cloud system investigated here was rather atypical. Interesting differences are also found: in the scaling ranges on the one hand, and in higher-order moments on the other hand. Finally, we discuss cloud-radiative effects of the large- and small-scale variabilities.