Abstract. Clouds in the fair weather return path of the global electric circuit (GEC) reduce conductivity because of the limited mobility of charge due to attachment to cloud water droplets, effectively leading to a loss of ions. A highresolution GEC model, which numerically solves the current continuity equation in combination with Ohm's law, is used to show that return currents partially flow around clouds, with current divergence above the cloud and convergence below the cloud. An analysis of this effect is presented for various types of clouds, i.e., for different altitude extents and for different horizontal dimensions, finding that the effect is most pronounced for high clouds with a diameter below 100 km. Based on these results, a method to calculate column and global resistance is developed that can account for all cloud sizes and altitudes. The CESM1(WACCM) (Community Earth System Model -Whole Atmosphere Community Climate Model) as well as ISCCP (International Satellite Cloud Climatology Project) cloud data are used to calculate the effect of this phenomenon on global resistance. From CESM1(WACCM), it is found that when including clouds in the estimate of resistance the global resistance increases by up to 73 %, depending on the parameters used. Using ISCCP cloud cover leads to an even larger increase, which is likely to be overestimated because of time averaging of cloud cover. Neglecting current divergence/convergence around small clouds overestimates global resistance by up to 20 % whereas the method introduced by previous studies underestimates global resistance by up to 40 %. For global GEC models, a conductivity parameterization is developed to account for the current divergence/convergence phenomenon around clouds. Conductivity simulations from CESM1(WACCM) using this parameterization are presented.
[1] This study examines the current that is driven to the ionosphere and to the ground before, during and after single negative cloud-to-ground (CG) and intracloud (IC) lightning discharges. A numerical model has been developed, that calculates the quasi-electrostatic field before the lightning, due to the slow accumulation of the charge in the thundercloud, and after the lightning by taking into account the Maxwellian relaxation of charges in conducting atmosphere and accounting for the dissipation stage of the thunderstorm development. From these results, the charges that are transferred to the ionosphere and to the ground are calculated. We demonstrate the significance of considering the pre-lightning and the dissipation stages and accounting for realistic distribution of the conductivity inside of the thundercloud for the accurate calculation of the charge flow to the ionosphere and to the ground. We show that the charge transfer to the ionosphere depends mainly on the altitudes of the charges inside of the thundercloud and on their spatial separation. The amount of charge that is transferred to the ground, due to currents flowing in the vicinity of the thundercloud during a transient time period following a lightning discharge, is significantly affected by the conductivity distribution in the thundercloud and can be several times smaller than the amount of charge that is transferred to the ionosphere during the same time period.Citation: Mallios, S. A., and V. P. Pasko (2012), Charge transfer to the ionosphere and to the ground during thunderstorms,
[1] Terrestrial gamma-ray flashes (TGFs) have been correlated with an early development stage of high altitude positive intracloud (+IC) flashes in which the negative leader propagates up toward the upper positive charge region, while the positive leader propagates down toward the lower negative charge region. The resultant bidirectional leaders develop electrical potential differences in the vicinity of their heads with respect to the ambient potential distribution created by the thundercloud charges. These potential differences are believed to be of essential importance for the generation of TGFs. Using electrostatic calculations and a three-dimensional Cartesian fractal model, we quantify these potential differences produced in a developing +IC lightning discharge for given thunderstorm electric configurations. We present a case of a +IC lightning discharge in a realistic thunderstorm configuration that leads to a very high ($300 MV) potential difference and show how a delay in the development of the negative leader with respect to the positive one in a bidirectional leader system can facilitate a high potential difference in the negative leader head region.Citation: Mallios, S. A., S. Celestin, and V. P. Pasko (2013), Production of very high potential differences by intracloud lightning discharges in connection with terrestrial gamma ray flashes,
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