On the role of clouds in the fair weather part of the global electric circuit

Abstract

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.

A 3-D RBF-FD solver for modeling the atmospheric global electric circuit with topography (GEC-RBFFD v1.0)

“…11, causing a jump in the fields which would be expected. In the angular directions, one would expect the fields to bend around the cloud layers, increasing the divergence of those fields between the cloud layers (Baumgaertner et al, 2014). This can in fact be seen by the increased bulge between the cloud layers in the plots of the latitudinal and longitudinal components of the electric field and current density, shown by the middle and bottom rows of Fig.…”

“…In lieu of an analytical exponential conductivity profile, we now consider a conductivity profile from model data computed with WACCM (https://www2.cesm.ucar.edu/ working-groups/wawg), as described in Baumgaertner et al (2014). This discrete conductivity profile includes aerosols and fair-weather clouds as well as topography, varying not only in the radial direction but also in latitude and longitude.…”

“…It can be an analytic function or a discrete data set output from a different numerical model, such as the Whole Atmosphere Community Climate Model (WACCM; for further details about how the conductivity is computed see Baumgaertner et al, 2013Baumgaertner et al, , 2014. In the latter case, the conductivity is interpolated to the node distribution used in this paper.…”

Effects of conductivity perturbations in time‐dependent global electric circuit model

“…This is motivated by the need to establish a quantitative value when the two models agree or disagree. Baumgaertner et al [] already described the role of clouds, and conductivity perturbations of horizontal scales from several to hundreds of kilometers, in the fair‐weather part of GEC. They modify the global resistance of GEC.…”

Parameterizing total storm conduction currents in the Community Earth System Model

“…The resulting parameterization can be used as part of an evolving physics‐based GEC global modeling framework that includes conductivity, source currents, and external influences from the magnetosphere to the global electric circuit, such as that in Baumgaertner et al . [] and Lucas et al . [].…”