PDF《wind》

1 A new approach for rain gush formation associated with ionic wind S.

L. Chin Center for Optics, Photonics and Laser (COPL) Laval University Quebec City, QC G1V 0A6 Canada. Xueliang Guo3 , Huanbin Xu Key Laboratory for Cloud Physics, Chinese Academy of Meteorological Sciences Zhong-guan-cun South Avenue No.46, Haidian District, Beijing 100081, China Fanao Kong, Andong Xia, Hongmei Zhao and Di Song Institute of Chemistry Chinese Academy of Sciences Beijing, China Tie-Jun Wang1 , Gengyu Li, Sheng-zhe Du, Jingjing Ju, Haiyi Sun, Jiansheng Liu, Ruxin Li2 , Zhizhan Xu State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, China. Corresponding authors: [email protected] ;

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[email protected] Abstract Based upon experimental observation in the laboratory, we propose that ionic wind from corona discharge inside a thundercloud would play an important role in producing a rain gush. A cyclic chain of events inside a super-saturated environment in a thundercloud is proposed, each event enhancing the successive ones until lightning occurs. These successive events are collision between snowflakes and rimers, charge separation, corona discharge, avalanche ionization, ionic wind originating from the positively and negatively charged masses of cloud, vortex motion and turbulence when mixed with the updraft, more collision, more charge separation, stronger corona discharge, and so on. Meanwhile, avalanche ionization would produce more CCN (cloud condensation nuclei) resulting in more precipitation and hence rimers formation in the super- saturated environment. More collision in the buoyant turbulence would lead to more fusion of droplets and the formation of larger rimers. The cyclic processes would repeat themselves until the electric field between the two oppositely charged masses of cloud was strong enough to induce a breakdown. The latter would create a sudden short circuit between the two charged masses of cloud neutralizing the charges. There would be no more ionic wind, hence, much less buoyant turbulence.

2 The updraft alone would not be sufficiently strong to support larger rimers which would fall down '

suddenly'

to the earth surface as a rain gush. 1. Introduction It is well-known that after a strong lightning in a thundercloud with a strong updraft of moist air (convective cloud), a gush of rain will fall down with a good probability [1-6]. Several mechanisms have been proposed to explain how lightning can trigger the rain gush. Early scientists proposed that the charged particles were levitated in the strong thunderstorm electric fields until a lightning flash destroyed the field [1]. Then a numerical model [7] that couples the growth of the cloud particles with electrical development was developed, but failed to agree with the observations on variations of electric field and precipitation rate following lightning strokes [8,9]. Rain gush was also explained through the collisions and coalescence of cloud droplets under lightning electric field firstly proposed by Moore at al. [3]. Thunderstorm electrification could play an important role in the development of precipitation in the theory [10]. The above hypothesis is based on the fact that lightning occurs first and then triggers precipitation. In this hypothesis, the particles through electrification process must move some distance through the cloud before they can grow and become large enough to fall down to the ground as rain. This may take several minutes and longer. Aayaratne and Saunders [11] suggested an alternative hypothesis where the lightning flash is caused by the falling precipitation. They assumed the flash is initiated by the positively charged graupel pellets comprising the commonly observed lower positive charge center. The enhanced local electric field around this positive charge pocket may trigger a ground flash from the main negative charge center above. Then the following precipitation may be relatively close to the ground resulting in a rain gush within the observed short time intervals. This theory does not require a long time internal between the lightning flash and rain on the ground, which seems to agree with the observation of rain gush in 2-6 minutes after overhead lightning [5]. Another theory based on radial wind was also proposed [12,13]. The proposed radial wind could be generated by acoustic wave (explosions or lightning), leading to an increase on the rate of coalescence of water droplets and hence triggering precipitation. However, these theories are still under debate [5,10]. In a thundercloud, there are strong updraft and downdraft [14, 15]. Rain gush is believed to be due to a much stronger downward draft that pushes the existing raindrops and ice and snow to fall more rapidly [14]. However, such a downward draft was not identified physically. In such a thundercloud, there exist static strong electric fields (high voltage) and hence, the........