This session will deal with current knowledge on the microphysics and electrification in severe storms.
A detailed understanding of the microphysics of thunderstorms is necessary for the understanding of how severe storms are electrified.
It is now generally accepted that most storms develop a tripolar charge structure with the main negative charge centre sandwiched between the main upper and lower positive charge centres. A large majority of lightning strokes to ground bring down a negative charge. However, there are several recorded cases where unusually high occurrences of positively charged lightning have been observed.
The polarity, intensity and multiplicity of lightning ground flashes all depend on many parameters such as cloud base height, convective available potential energy and mixed phase depth. In addition, thunderstorms ingesting smoke from forest fires have been shown to exhibit inverted dipoles, suggesting that aerosols may play an important role in the generation of electricity, perhaps through their effect on cloud dynamics or by altering the chemical content of cloudwater.
There is a huge gap in our knowledge of how these severe storms are electrified. Although, the macroscopic conditions that give rise to a particular sign of charging are fairly well known, the microphysics of the interactions between cloud particles is still not well understood at all. In the non-inductive theory of cloud electrification, the relative state of the surface of the two particles, determined primarily by growth, evaporation, droplet freezing, defect structure and surface chemistry are all of potential importance. In the inductive mechanism, there are many difficult questions pertaining to the probability of impact, coalescence and quantity of charge transferred between cloud particles. Other mechanisms, such as the convective hypothesis, continue to be propounded in the current literature. However, in all these theories, there is much controversy between the laboratory, field and modelling results. Thunderstorm electrification is no doubt one of the open questions in cloud physics and requires substantial further investigation and interpretation, both in terms of experiments and modelling, before an acceptable solution may be achieved.
We invite submissions on all the above aspects for inclusion in this session.
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The electrical development in thunderclouds depends on both the dynamics and microphysical processes in the clouds. While the updraft and the liquid water content determine the vigor of the clouds, the electrical buildup depends on the rate at which charge can be separated locally due to collisions of ice crystals with graupel particles in the presence of water drops and the subsequent spatial separation of these charges. This implies that the efficiency of charge separation depends to a great extent on the concentrations of ice crystals and graupel particles and on the amount of liquid content. Since the electrical development starts early in the lifetime of the clouds, it is important to understand the rate and mechanism of ice crystals formation.
Although the primary mechanisms of ice formation are theoretically understood, it is still not clear how ice crystals are formed in clouds. Measurements of ice nuclei were found to be in much smaller concentrations than ice crystals in clouds, suggesting that other mechanisms may be responsible for the formation of most of the ice crystals we measure. Furthermore, there are some recent measurements suggesting that on the average, ice crystals concentrations in stratus clouds are independent from temperatures between about -5C to -30C. Many of these ice crystals are below the resolution of most optical spectrometers presently being used.
Moreover, the effects of aerosol pollution on clouds (more cloud condensation nuclei) have been shown to modify cloud drop size, which could lead to changes in the processes leading to ice formation. This is because the same amount of liquid water has to be divided among many more drops, leading to smaller drops, to an increase in latent heat release and to longer lived clouds. The ice in such clouds could form at higher levels, thus affecting the rate and magnitude of the electrical development by modifying the height (temperature) and rate of the charge separation.
Submissions of papers on all the above aspects for inclusion in this session are encouraged.