On the behaviour of raindrop fall speed due to wind
On the behaviour of raindrop fall speed due to wind
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风对雨滴下落速度的影响
DOI：
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
F. García
中科院分区：
文献类型：
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作者：
G. Montero‐Martínez;F. García
本文介绍了几年来在地面收集的雨滴下落速度的测量结果，并根据取样时的水平风强度进行了分析。据证实，超终端下降只发生在直径小于约0.7 mm的情况下，并且其百分比数量在多风期间增加。此外，通过统计和事后数据分析，它表明，风影响单个雨滴的下降速度。一个显著的特点是检测到亚终端雨滴，即那些具有比其计算的终端速度更慢的下降速度的雨滴，小于2毫米。风影响雨滴行为的证据包括观察，显示在有风的条件下：（i）超终端雨滴显示出比在平静的风期间更大的下降速度偏离预期的终端速度;和（ii）直径在1和2 mm之间的亚终端液滴的百分比数量增加。这两个发现意味着相应的速度分布的普遍扩大。根据湍流（小雨滴对空气条件变化的反应更快）、雨滴振荡（通过仪器采样体积产生更长的通过时间）以及横向漂移和相对于垂直方向的轨迹偏差的增强，给出了发生亚终端雨滴的可能解释。然而，对于具有最大下落速度异常（大于仪器不确定性的两倍）的液滴，还应解决其他可能性及其对微物理过程的影响。
Measurements of raindrop fall speed gathered at the ground over several years are presented and analysed in view of the horizontal wind intensity during sampling. It is confirmed that super‐terminal drops occur only for diameters less than about 0.7 mm and that their percentage numbers increase during windy periods. Furthermore, through statistical and post hoc data analyses, it is shown that wind affects the fall speed of individual raindrops. A remarkable feature is the detection of sub‐terminal raindrops, i.e. those having slower fall speeds than their calculated terminal velocities, smaller than 2 mm. Evidence that wind affects raindrop behaviour includes observations showing that under windy conditions: (i) super‐terminal drops exhibit larger fall speed departures from the expected terminal speeds than in calm wind periods; and (ii) the percentage number of sub‐terminal drops with diameters between 1 and 2 mm increases. These two findings imply a general broadening of the corresponding velocity distributions. Potential explanations for the occurrence of sub‐terminal drops are given based on turbulence (small raindrops respond faster to changes in air conditions), drop oscillations (which produce larger transit times through the instrument sampling volume) and the enhancement of transverse drifts and trajectory deviations with respect to the vertical. However, other possibilities should be also addressed for drops with the largest fall speed anomalies (greater than two times the instrument uncertainty) as well as their implications for microphysical processes.