A modeling study of thunderstorm electrification and lightning flash rate
Solomon, Robert, 1968-
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Observing the range of spatial scales important in thunderstorm electrification and subsequent lightning development is huge and would be an impossible task. The charge transfer processes within clouds takes place during collisions of particles with diameters on the order of 10-1000 $\mu$m, whereas the distance between the centers of accumulated charge and the length of lightning channels are on the order of kilometers.Numerical models have proved to be an effective means of gaining insight into the processes leading to thunderstorm electrification and lightning production. A one-and-a-half-dimensional cloud model is developed that retains those processes thought to be most important in the small and large scale generation and separation of charge. This model is combined with a lightning parameterization that treats the lightning channel as a conductor whose final length is determined by a crude charge criterion. Charge induced on the channel surface modifies the surrounding electric field and is redistributed onto hydrometeors prior to the next lightning flash.Observations and results from the numerical thunderstorm model are used together to examine thunderstorm development and lightning flash rates in a variety of environments. The dependence of lightning occurrence, flash rate and type (cloud-to-ground and intracloud) on variations in cloud condensation nucleus concentrations, primary and secondary glaciation mechanisms, liquid water flux and updraft velocities is studied.Observations and model results suggest that in order for a cloud to become electrified, threshold values of both liquid water flux and updraft velocity near -5$\sp\circ$C must be exceeded.Once clarified, ice crystal concentrations at temperatures between -10 and -20$\sp\circ$C greatly influence the lightning flash rate.Initially, lightning channels initiated within the simulated thunderclouds are intracloud. With further development of the cloud, long, intracloud channels deposit positive charge below the main, lower region of negative charge which leads to the initiation of cloud-to-ground channels. In general, lightning channels initiated between the two main bodies of positive and negative charge result in intracloud lightning, whereas channels initiated below the maximum negative charge region result in cloud-to-ground lightning. Lightning continues until the separation of charge is insufficient to maintain the electric fields needed to initiate lightning.
- Atmospheric sciences