Stratiform cloud electrification: comparison of theory with multiple in-cloud measurements
Stratiform cloud electrification: comparison of theory with multiple in-cloud measurements
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DOI：
10.1002/qj.2858
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发表时间：
2016-10-01
影响因子：
8.9
通讯作者：
Harrison, R. C.
中科院分区：
文献类型：
--
作者：
Nicoll, K. A.;Harrison, R. C.
层状云约占全球云量的40%，在确定行星辐射收支方面起着关键作用。带电仍然是对它们的微物理过程了解最少的影响之一。在层状云的顶部和底部边缘处的液滴充电来自于由全球大气电路驱动的穿过云的垂直电流。云中电荷数据是评估电荷在液滴生长过程中的作用的核心，液滴生长过程影响液滴尺寸分布和相关的云辐射特性和降水。这项研究首次提出了在多层云中进行的高垂直分辨率电测量。在22个采样的云中，所有云的边缘都是带电的，这明确表明，所有层状云都可以在不同程度上在其上下边界包含电荷。云底和云顶的电荷是不对称的，平均云顶空间电荷为+32 pC m（-3），云底空间电荷为-24 pC m（-3）。较大的云顶电荷与较强的逆温和较高的垂直电导率梯度有关。在低海拔（< 2 km）云层（20.2 pC m（-3））中观察到更多的充电，而在高海拔（> 2 km）云层（7.0 pC m（-3））中观察到更多的充电，这与较低海拔处与宇宙射线电离减少相关的较小的空气传导率一致。总之，这些测量结果表明，最大的云滴电荷在广泛的层状云发生在云顶低海拔（< 2公里）的云，当垂直混合抑制明显的逆温，证实了理论的预期。云动力学的影响层云边缘充电报告这里应该通知云微物理学的云滴充电效应的模拟研究。
Stratiform clouds constitute similar to 40% of global cloud cover and play a key role in determining the planetary radiation budget. Electrification remains one of the least understood effects on their microphysical processes. Droplet charging at the top and bottom edges of stratiform clouds arises from vertical current flow through clouds driven by the Global atmospheric Electric Circuit. In-cloud charge data are central in assessing the role of charge in droplet growth processes, which influence droplet size distributions and associated cloud radiative properties and precipitation. This study presents the first high vertical resolution electrical measurements made in multiple layer clouds. Of the 22 clouds sampled, all were charged at their edges, demonstrating unequivocally that all stratiform clouds can be expected to contain charge at their upper and lower boundaries to varying extent. Cloud base and cloud top are shown to charge asymmetrically, with mean cloud-top space charge +32 pC m(-3) and base space charge -24 pC m(-3). The larger cloud-top charges are associated with strong temperature inversions and large vertical electrical conductivity gradients at the upper cloud boundary. Greater charging was observed in low altitude (< 2 km) clouds (20.2 pC m(-3)), compared to higher altitude (> 2 km) cloud layers (7.0 pC m(-3)), consistent with the smaller air conductivity at lower altitudes associated with reduced cosmic ray ionisation. Taken together, these measurements show that the greatest cloud droplet charges in extensive stratiform clouds occur at cloud tops for low altitude (< 2 km) clouds, when vertical mixing is suppressed by appreciable temperature inversions, confirming theoretical expectations. The influence of cloud dynamics on layer cloud edge charging reported here should inform modelling studies of cloud droplet charging effects on cloud microphysics.