A new understanding of raindrop shape
A new understanding of raindrop shape
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DOI：
10.1016/j.atmosres.2010.02.001
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发表时间：
2010-09-01
影响因子：
5.5
通讯作者：
Thurai, M.
中科院分区：
文献类型：
--
作者：
Beard, Kenneth V.;Bringi, V. N.;Thurai, M.
本文对实验室、野外和模型试验中的雨滴形状进行了研究，以得到一致的雨滴轴比随尺寸变化的图像。第一个真实的雨滴形状测量是在加州大学洛杉矶分校的垂直风洞中进行的，该风洞是在汉斯·R。随后的研究通过实验室研究、实地测量和模拟对雨滴形状进行了更详细的研究。对夜间降雨的后向散射光测量表明，直径大于1毫米的雨滴在风和碰撞太弱而不能激发振荡的条件下振荡。这些振荡的一个明显来源是与涡流脱落的共振。随后的实验室研究表明，这种振荡增加了小雨滴的平均轴比。过去的理论研究的细节表明，雨滴的轴比随着尺寸的增加而减小，就像固着雨滴一样，这是因为雨滴的重量增加，但也是因为前后气动压力差增加。现代的分析和数值模型预测类似的轴比下降的水滴在终端速度和这些结果通常是一致的实验室和现场measurement.Although轴比首次测量在加州大学洛杉矶分校的风洞和最近的实验室测量导致了更广泛的理解雨滴轴比，新的信息雨滴形状是从最近的现场测量使用改进的技术。本文还报道了用二维视频多普勒仪（2DVD）观测的雨滴轴比、方向和倾斜角。这些结果包括在静风条件下进行的人工降雨实验的测量结果，其中允许水滴（超过115，000）下降80米。轴比和雨滴形状与美因茨风洞的数据和图像显示出很好的一致性，倾斜角分布关于0度对称，所有>2 mm的雨滴的标准差为7-8度。从冲绳、苏门答腊、多伦多、亨茨维尔和布里斯班收集了2DVD雨滴形状的测量结果。在D = 1.6-1.8 mm区域观察到异常宽的轴比分布，这可能是由涡流脱落引起的液滴振荡的证据。对于较大的液滴（例如D = 4 mm），也可以看到一些细微的差异，垂直弦略大于水平弦，导致与80 m坠落数据相比轴比略大。此外，考虑使用来自2DVD测量的单个液滴信息的示例，这增强了与C波段极化雷达测量的一致性。在这种特殊的情况下，一个相当大的轴比分布的变化为3.5毫米的下降期间观察到的事件的初始强对流阶段。进一步正在进行的工作将结合联合收割机的2DVD测量与极化雷达观测在协同的方式，以确定气象或微物理条件下，滴形状可能会发生显着的变化，在自然rain.Improved极化雷达估计倾斜角的S波段系统与高交叉极化性能天线的报告。从伴有嵌入对流的层状小雨天气雷达资料得到的标准差直方图显示出约7度的倾斜角模式，具有显著的正偏度。从雷达测量中还推断出稳定性随液滴直径的增加，这与2DVD的直接估计一致。美因茨风洞正在进行的实验正在评估雨滴形状如何受到雨井内毛毛雨水滴碰撞的影响。由爱思唯尔公司出版
In this paper raindrop shapes from laboratory, field and model investigations are examined in order to distill a consistent picture of raindrop axis ratios as a function of size. The first realistic measurements of raindrop shape were made in the UCLA vertical wind tunnel that was built under the supervision of Hans R. Pruppacher at UCLA in 1968.Subsequent research has investigated raindrop shape in more detail by laboratory studies, field measurements and modeling. Measurements of rain at night in backscattered light have revealed that raindrops larger than 1 mm diameter oscillate in conditions where winds and collisions are too weak to excite oscillations. One apparent source of these oscillations is a resonance with eddy shedding. Subsequent laboratory studies have demonstrated that such oscillations increase the average axis ratio of small raindrops.Details of past theoretical research show that raindrop axis ratios decrease with size in the manner of a sessile drop because of its increasing weight, but also because of an increasing differential in the fore-aft aerodynamic pressure. Modern analytical and numerical models predicted similar axis ratios for water drops falling at terminal velocity and these results are generally consistent with laboratory and field measurements.Although the axis ratios first measured in the UCLA wind tunnel and the more recent laboratory measurements have led to a broader understanding of raindrop axis ratios, new information about raindrop shape is available from recent field measurements using improved techniques. Raindrop axis ratios, orientation and canting angles using 2-D video disdrometer (2DVD) are also reported. The results include measurements from an artificial rain experiment conducted under calm wind conditions where the drops (over 115,000) were allowed to fall a distance of 80 m. Axis ratios and drop shapes show good agreement with the Mainz wind-tunnel data and images, as well as canting angle distributions symmetric about zero degrees with a standard deviations of 7-8 degrees for all drops >2 mm.Measurements of 2DVD raindrop shape has been collected from Okinawa, Sumatra, Toronto, Huntsville and Brisbane. An unusually wide axis ratio distribution was observed in the D = 1.6-1.8 mm region, which may be evidence for drop oscillations caused by eddy-shedding. Some subtle differences can also be seen for larger drops (e.g. D = 4 mm), the vertical chord being slightly larger than the horizontal chord leading to slightly larger axis ratios compared with the 80 m fall data.Additionally, an example using individual drop information from 2DVD measurements is considered, which enhances agreement with C-band polarimetric radar measurements. In this particular case, a considerable shift in the axis ratio distribution for 3.5 mm drops is observed during the initial strong convective phase of the event. Further ongoing work will combine the 2DVD measurements with polarimetric radar observations in a synergistic manner in order to identify the meteorological or microphysical conditions under which significant changes in drop shapes may occur in natural rain.Improved polarimetric radar estimates of canting angles are reported from an S-band system with high cross-polar performance antenna. The histogram of the standard deviation derived from the radar data obtained in light stratiform rain event with embedded convection shows a canting angle mode of about 7 degrees, with a significant positive skewness. An increase in stability with drop diameter is also inferred from the radar measurements, which is in agreement with direct estimates from the 2DVD. Ongoing experiments in the Mainz wind tunnel are evaluating how the raindrop shapes are affected by collisions with drizzle drops within rain shafts. Published by Elsevier B.V.