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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 155 Figure 11.4 Variation of the electric field with height (electrode effect) found over the tropical ocean illustrating the observed large fluctuations of the instantaneous field from the average field caused by atmospheric turbulence. The curves are numerical solutions for the governing equations using gradient diffusion model for ion transport (from Hoppel and Gathman, 1972). PHENOMENOLOGY OF ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER Although typical average values are often cited for atmospheric-electrical observables, the greatest interest and significance by far has been attached to variations with time over a broad range of time scales. There have been studies of atmospheric-electrical variations with annual and even 11-yr periodicities; short-period fluctuations have also received some attention in the past, and they are currently a subject of renewed interest, as will be discussed later. By far the greatest effort has been focused on diurnal variations with respect to either universal or local time and, to a lesser extent, on seasonal changes in diurnal patterns. One of the first, and certainly the most famous, demonstrations of a reproducible variation pattern with universal time is the hourly average potential gradient curves obtained over the world ocean on the Carnegie cruises. These curves have long served as de facto standards with which to evaluate the viability of attempts to measure universal variations. The result of the 1928-1929 Carnegie cruise (Torreson et al., 1946) is plotted in Figure 11.5 along with several of the more successful subsequent attempts to measure this average variation. These are integrated (and extrapolated) ionospheric potential from 125 balloon ascents (MÃ¼hleisen, 1977), 20 aircraft profiles in the Bahamas (Markson, 1977), and 6 aircraft profiles at a selected Arctic location (Clark, 1958). Also shown are quasi-continuous measurements of current density made above the PBL during the Arctic winter, which, when allowance is made for the small (6 percent) measured change in columnar resistance during the measurement period, should accurately mirror variations in ionospheric potential (Anderson, 1967). Consideration of these curves strongly indicates that there is indeed a well-defined average global diurnal variation but that there are equally important real local deviations with time. This is demonstrated in Figure 11.6 (IsraÃ«l, 1973) in which the effect of turbulent processes on warmer (March to October) afternoons is seen to dominate the average diurnal curve. For this reason a better knowledge of ionospheric potential variations would play a key role in the identification of effects attributable to PBL processes. However, subtraction of Figure 11.5 Universal diurnal variations in atmospheric electricity from different measurement techniques.
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 156 the average diurnal variation does not necessarily isolate a particular local phenomenon. Figure 11.6 Average diurnal variations in potential gradient showing effect of afternoon turbulence mixing (from IsraÃ«l, 1958). Electric field and vertical current density are driven by the global circuit and, as shown in Figure 11.5, can exhibit the characteristic variation pattern even within the PBL in certain cases if suitably averaged. There are other observables such as the ionic conductivity, which are only weakly influenced by global processes while depending strongly on local effects. Conductivity has been seen to be a result of ionization, ionic mobility, recombination, and attachment of ions to particulates, all of which can be influenced by local conditions and variable trace constituents. It is reasonable therefore to expect that large variations in conductivity will be primarily determined within the PBL and have a marked dependence on local time. The columnar resistance is a parameter usually defined as the resistance of a vertical column of unit cross section from ground level to the base of the ionosphere. It is commonly derived from a measured vertical profile of atmospheric conductivity, and it is observed that most of this resistance lies within the PBL. The validity of this is seen in Figure 11.7, Figure 11.7 Average diurnal variation in columnar resistance as a function of local time (from Sagalyn and Faucher, 1956).
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 157 which shows the resistance R of a 1-cm2 column from the surface to 4.57 km over land as a function of local time. The strong effect of the midday upward dispersion of aerosols on R is readily apparent, and the shape of the curve and the 40 percent variation are in reasonable accord with our knowledge of the daily variation of turbulent activity. Figure 11.8 Normalized potential gradient showing rush-hour effect at polluted urban sites (from Anderson and Trent, 1969). The strong dependence of the columnar resistance on PBL conditions provides a mechanism for the modification of electrical observables with local time. Such mechanisms can embrace the entire PBL as does fully developed turbulent convection, or they can be confined to a shallow region near the surface. One example of a shallow effect is seen in the observed response to the typical urban morning rush hour (Anderson and Trent, 1969). In the morning there is still little turbulent activity, since solar heating of the surface has just begun, and there is an abrupt injection of combustion products into the stratified atmosphere. Conductivity is reduced by particulates, but this reduction is confined to a shallow layer; so the total columnar resistance and thus the vertical current are largely unaffected. Consequently, the local surface field increases as required by Ohm's law. This increase is seen in Figure 11.8 between 1100 and 1600 GMT (0600 and 1100 EST) at three sites located in urban areas. Because of these local diurnal variations, single-station potential gradient recordings, even when heavily averaged, rarely exhibit a classical Carnegie type diurnal variation pattern (IsraÃ«l, 1961). These diurnal variations with both universal and local time are not the only fluctuations observed in atmospheric- electricity recordings. Shorter-period variations are always observed and usually dismissed as noise. Observed fluctuations on atmospheric-electricity recordings made within the PBL are comparable in magnitude to the mean values of those recordings (Takagi and Toriyama, 1978). A typical recording is seen in Figure 11.9. Much of the fluctuation content in the range from roughly 0.1 sec to tens of minutes is produced by local turbulence. There are two obvious consequences of this coupling. First, all the methodology of turbulence analysis, such as eddy-correlation, profile, and dissipation analyses, can be applied to the electrical observations. The second consequence is the possibility of utilizing electrical observations as a tool in the study of turbulent processes. The relationship between atmospheric electricity and turbulence will be considered in more detail in the section below on Modeling and Theory. There is also a suggestion that fluctuations in the total Maxwell current density in the range of 10 to 1000 sec can correlate at intercontinental distances (Ruhnke et al., 1983). It is clear that short-period fluctuations in atmospheric-electrical recordings strongly influence attempts to observe global scale phenomena, are relatively underexploited, and constitute a fertile area for possible applications. Although horizontal gradients of atmospheric-electrical variables within the PBL are much smaller than vertical ones (largely a consequence of the geometric scales involved), significant horizontal variability is observed. Significant instances include the effect of organ Figure 11.9 Records of the potential gradient (F), air-earth current density (I), and brightness (H) at Aachen on July 31, 1954 (from IsraÃ«l, 1958).
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 158 ized convection activity on a large scale as seen in Figure 11.10 (Markson, 1975), terrain effects produced by mountains and coastlines, and the sunrise effect caused by differences in turbulent mixing between the heated and dark regions. To a first approximation such effects are seen to be the result of imposition of a local perturbation on an otherwise uniform situation and are, hence, essentially comparable with local phenomena such as the rush-hour effect previously described. The spatial variation on which attention has been focused is in the vertical dimension. The interest in global-scale phenomena has led to the use of a vertical profile as a convenient observational unit. Measurements are made of one or more atmospheric-electrical variables, typically field, conductivity, and/or current density, at a variety of altitudes in a relatively short time span (from a few to tens of minutes). The sensors are carried aloft with aircraft, balloons, or rockets, and data are presented both as profiles and as numerically integrated totals. Profiles have been made over land because of convenience and to study specific terrain effects and over water in attempts to eliminate land effects. Profile data have been responsible for the detection of convection currents in the PBL comparable in magnitude with the total current, the classical electrode effect over water under stable conditions, the response of columnar resistance to pollutant buildup, and the classical diurnal variation in ionospheric potential. Typical vertical profiles of atmospheric potential through the PBL are seen in Figure 11.11. The Greenland profiles are characterized by extremely low levels of particulate contamination, and the vertical variation of conductivity closely approximates that predicted theoretically for an aerosol-free atmosphere. The addition of particulate burdens, whether in a shallow layer as in curve C or in a thick layer as in D, markedly affects the observations within the PBL. It is apparent that, in the presence of atmospheric contaminants, the voltage drop across the PBL is significantly greater than in the Greenland observations. Figure 11.10 Horizontal variations in potential gradient showing effect of organized convection (from Markson, 1975). Figure 11.11 Typical vertical distributions of atmospheric potential (from Clark, 1958). The conduction-current density, defined as the product of electric field and conductivity, can easily be computed from airborne measurements. Two typical profiles of conduction-current density are shown in Figure 11.12. Above the PBL the current density is seen to be essentially constant with altitude. This is a direct result of the small space-charge density above the PBL and the greatly reduced turbulent mixing found there. This vertical constancy led to the aforementioned use of current-density measurements above the PBL to follow universal variations. The increases seen at low altitudes were the first unambiguous evidence of the existence and significant magnitude of convective charge transport within the PBL, as discussed in the next section. In addition to variations that depend on time or height are variations that are associated with a specific phenomenon. The most well known such case is the atmospheric-electric fog effect. It has been observed that the conductivity decreases markedly in fog and that the start of the decrease may precede the actual fog onset.