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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 149 11 Atmospheric Electricity in the Planetary Boundary Layer William A. Hoppel, R. V. Anderson, and John C. Willett Naval Research Laboratory INTRODUCTION The planetary boundary layer (PBL) is that region of the lower atmosphere in which the influences of the Earth's surface are directly felt. The primary influences of the surface are drag, heating (or cooling), and evaporation (or condensation). These processes cause vertical fluxes of momentum, sensible heat, and moisture, which penetrate into the lower atmosphere to a finite height. These fluxes, in turn, generate turbulence, ultimately controlling the mean profiles of wind speed, temperature, and water vapor in the PBL. Since the height of penetration depends on the direction, magnitude, and persistence of the surface fluxes and on the large-scale meteorological conditions, the PBL can range in thickness from tens of meters to a few kilometers. Because of its position next to the Earth's surface, the PBL has been the site of the vast majority of atmospheric- electrical measurements. If the history of atmospheric electricity begins with Franklin, Lemonnier, and Coulomb, as is customary, there have been more than two centuries of effort in this region. It was observed early that measured quantities such as electric field respond strongly to meteorological processes. This led Lord Kelvin to suggest that the day would come when weather forecasting would be done with an electrometer (Dolezalek, 1978). Unfortunately, this optimistic prediction has not materialized, largely because of the complexity of the dependence of electrical variables on a bewildering variety of other phenomena. Most atmospheric processes are interrelated and cannot be studied in isolation, but it is usually possible to identify one or two dominant influences. In the case of atmospheric electricity in the PBL, however, separating the various causes and their effects can be extremely difficult. In fact, this field may be unique with respect to its sensitivity to many disparate phenomena spanning a tremendous range of scales in both space and time. For example, locally produced turbulent fluctuations in space-charge density have an effect roughly comparable in magnitude to that of changes in global thunderstorm activity on electric-field variations within the PBL. Over the years this responsiveness of atmospheric electricity has led to its exploitation for many different purposes. Electrical measurements have been made in the PBL to observe large-scale processes such as the global circuit, to study local phenomena like boundary-layer turbulence, or simply to examine unusual electrical signatures in their own right. In each type of investigation it has been found necessary to minimize the effects of unwanted processes on the data. This filtering