The Earth's Electrical Environment (1986)

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 223 forests, deserts, glaciers), aerosol and smoke generation and dispersion, and anthropogenic processes (Anderson, 1977). A global-scale representation of these lower-atmospheric processes is necessary to evaluate their impact on the global circuit. One means of obtaining information on such global processes is to use output from the various general circulation models (GCMs) that have been developed to study the dynamic meteorology of the Earth's atmosphere. The GCM-calculated winds, temperature, humidity, cloudiness, turbulence, and other meteorological phenomena could be used to develop the electrical parameterizations for use in global models of atmospheric electricity. A coupled interactive electrical-dynamic GCM may greatly improve our understanding of various electrical processes within the global circuit. Regional Modeling Global models of atmospheric electricity are generally constrained by computer size to a grid that is on the order of 5° in latitude and longitude (about 500 km). The electrical processes need to be parameterized on that scale for insertion into the global models. There is a clear need for the development of regional and local electrical models that use appropriate boundary conditions provided by a global model to resolve subgrid-scale phenomena and to investigate electrical phenomena in a more limited area. Considerably more physics can be incorporated into such models, and the results in turn can then be used to provide appropriate parameterizations of these processes for inclusion into global models. The calculated electrostatic potential contours and vectors of current flow over a mountain plateau and mountain peak (Tzur and Roble, 1985a) are shown in Figures 15.14(a) and 15.14(b), respectively. The regional model employed for these calculations used a detailed representation of electrical conductivity throughout the atmosphere, with boundary conditions that allow free current exchange between the global and regional models. It is assumed that the plateau and mountain perturb electrical quantities locally but are small enough with respect to the globe that their feedback into the global circuit is small. Over the plateau the vertical electric current flow is about three times larger than over sea level, primarily because of the reduced columnar resistance over the elevated surface. The results also show considerable horizontal current flow in the upper atmosphere, because of the presence of the plateau, indicating a local readjustment of the current system. A similar calculation for a mountain peak is shown in Figure 15.14(a). The mountain is seen to distort significantly the potential pattern in such a manner as to cause Figure 15.14 Regional calculation of the potential distribution and current density flow over a mountain peak (a) and a mountain plateau (b). The length of the maximum arrow indicates a current density of 2 × 10–12 A m–2 in (a) and 1.8 × 10–11 A/m2 in (b). (Tzur and Roble, 1985b.) an enhanced current to flow into the peak. These calculations are numerical extensions of the analytic mathematical procedure that Kasemir (1977) used to calculate the electric current and field distributions around mountains. Such calculations are important to interpret various measurements of the fair-weather electric field and current in the vicinity of mountaintops (Cobb et al., 1967; Cobb, 1968) and provide a quantitative framework to evaluate the extent of the electrical disturbance source by mountains of various shapes and also to determine the important characteristics that need to be incorporated into global models. Another important regional problem is to investigate the electrical interaction of a thunderstorm with its immediate environment. Thunderstorms are considered point current sources in the global model, and regional calculations on a much smaller scale are needed to examine such problems as the magnitude of the current output from thunderstorm models and its relationship to the characteristics of its electrical environment. For example, the effect of the ionospheric magnetic-fieldline configuration on the vertical current output from a thunderstorm that is represented as a dipolar current source (Tzur and Roble, 1985b) is shown in Figures 15.15(a) and 15.15(b). When the geomagnetic- field lines are assumed to be vertical [Figure 15.15(a)], the upward current flow in the middle atmosphere is confined to the immediate vicinity of the storm, whereas when the geomagnetic-field lines are assumed to be horizontal there is considerably more horizontal current flow in the middle atmosphere. The calculations suggest differences between the current output from thunderstorms in equatorial regions and in high latitudes. These calculations illustrate the types of problems that need to be addressed with regional models, not only