Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 161 crease with height. Unfortunately, these relatively simple cases rarely occur in the real atmosphere because of pervasive turbulent mixing. Figure 11.14 Heuristic profiles of electric field in the electrode layer illustrate the effects of various physical processes. From left to right are the "classical" electrode effect in clean, nonturbulent air with uniform ionization, as in Figure 11.3; the effect of aerosol attachment on the nonturbulent case; the effect of a shallow layer of high ionization rate on the nonturbulent case; and the effect of strong turbulence. The most obvious effect of turbulence on the electrode layer is to increase its thickness by mixing the space charge upward. The impact of nonuniform ionization is reduced as the turbulence intensity increases, both because trapping of radioactive emanations is eliminated and because the thicker layer appears to be less sensitive to surface radioactivity. The presence of aerosol particles thickens the layer further, in contrast to their effect in the nonturbulent case, by increasing the electrical relaxation time. These processes may increase the height scale of the electrode layer so much as to make it virtually undetectable with surface-based measurements, as illustrated in the fourth frame of Figure 11.14. For this reason turbulence blurs the distinction between the electrode effect proper and convective currents in the interior of the PBL. Turbulence can also cause significant loss of ions and space charge by diffusion to the surface. The theory of the turbulent electrode effect is not so fully developed as that of the nonturbulent case, owing primarily to the difficulty of parameterizing the lower boundary conditions at an aerodynamically rough surface (Willett, 1983). Hoppel and Gathman (1972) obtained reasonable agreement between experimental observations and a numerical model of the turbulent electrode layer in clean maritime air over the tropical ocean (see Figure 11.4). At present, however, there is no satisfactorily verified model that applies over land. In view of the importance of the electrode effect as a charge source for convection currents throughout the PBL, the development and testing of such a model should be a high priority for future research. Convection Currents in the Planetary Boundary Layer The downward conduction-current density is often observed to vary with altitude in the PBL. Based on the reasonable assumption of a steady, horizontally homogeneous, mean charge-density distribution, Kraakevik (1958) concluded that these deviations from vertical uniformity imply the existence of a height-dependent "convection-current density" such that the total current density is constant with altitude. He speculated that this convection current is produced by the upward turbulent transport of space charge produced near the surface. Convection currents can be modeled relatively easily in many circumstances using only the mean charge- conservation equation and Poisson's equation. This assumes that the mean conductivity profile is not influenced by the electric field, which appears to be the case under conditions of strong turbulent mixing. The first term on the right-hand side of Eq. (11.7) represents local electrical relaxation due to the mean conductivity. The effect of mean conduction down the conductivity gradient causes "piling up" of space charge and is represented by the second term. The third term is usually negligible compared with one of the first two. The convergence of convection current is, of course, represented by the final term. Recent modeling of convection currents has shown that they only become important in unstable mixed lay