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OCR for page 149
1 1
INTRODUCTION
Atmospheric Electricity in the
Planetary Boundary Layer
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
Naval Research Laboratory
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 evapo-
ration (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, ulti-
mately controlling the mean profiles of wind speed,
temperature, and water vapor in the PBL. Since the
height of penetration depends on the direction, magni-
tude, 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 kilome-
ters.
Because of its position next to the Earth's surface, the
PBL has been the site of the vast majority of atmo-
spheric-electrical measurements. If the history of atmo-
spheric electricity begins with Franklin, Lemonnier,
and Coulomb, as is customary, there have been more
than two centuries of effort in this region. It was ob-
served early that measured quantities such as electric
field respond strongly to meteorological processes. This
led Lord Kelvin to suggest that the day would come
149
when weather forecasting would be done with an elec-
trometer (Dolezalek, 1978~. Unfortunately, this opti-
mistic prediction has not materialized, largely because
of the complexity of the dependence of electrical vari-
ables on a bewildering variety of other phenomena.
Most atmospheric processes are interrelated and can-
not 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 dif-
ficult. 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 electri-
cal signatures in their own right. In each type of investi-
gation it has been found necessary to minimize the
effects of unwanted processes on the data. This filtering
OCR for page 150
150
is never entirely successful, however, and investigators
must always be aware of the whole range of influences
and alert for contamination.
While atmospheric-electrical variables respond to
many processes, they usually have little influence on the
phenomena to which they respond. Thus the electrical
state of the PBL is irrelevant to the fields of environmen-
tal radioactivity, air pollution, boundary-layer turbu-
lence, and global meteorology, for example. The inverse
is not true, however. Atmospheric electricity in the PBL
is a truly interdisciplinary study requiring a knowledge
of all these areas in addition to ionic conduction in gases,
aerosol physics, and electrostatics.
Recent advances in the disciplines cited above make it
a propitious time to re-examine the relationships be-
tween atmospheric electricity and PBL processes. Such
a re-examination should ultimately lead to a better un-
derstanding not only of atmospheric-electrical phenom-
ena but of the related disciplines as well. With this in
mind, the remainder of this chapter presents a brief
overview of atmospheric electricity in the PBL. First the
primary physical mechanisms influencing the electrical
phenomena are discussed. The gross phenomenology is
described next, including spatial and temporal variabil-
ity of the important electrical parameters. Then the
most important aspects of modeling and theory are sum-
marized in an effort to relate the physical causes and
their electrical effects. Finally, the chapter is closed
with a discussion of principal applications and areas of
needed research.
PHYSICAL MECHANISMS THAT AFFECT
ATMOSPHERIC ELECTRICITY IN THE
PLANETARY BOUNDARY LAYER
Ionization
The electrical conductivity of the air is due to ions
produced primarily by ionizing radiation. The early in-
vestigations of the sources of atmospheric ions led to the
discovery of cosmic rays. Cosmic radiation is the pri-
mary source of ions over the oceans and above a couple
of kilometers over land. In the PBL the cosmic-ray con-
tribution to the ionization rate is about 1 to 2 ion pairs
per cubic centimeter per second. It is quite constant in
time, and the latitudinal dependence caused by the
Earth's magnetic field is well understood.
The primary source of ions in the PBL over land is
natural radioactivity originating from the ground. This
ionization source can be divided into two parts: (i) oes,
Us, and lays radiating directly from the Earth's surface
and (ii) radiation from radioactive gases and their radio-
active daughter products exhaled from the ground. The
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
gases originate in both the uranium and the thorium de-
cay series where one of the daughters is the noble gas,
radon. In the uranium decay series, the daughter is
222Rn with a half-life of 3.8 days; and in the thorium
series, 220Rn (thoron) with a half-life of 54 sec. During
the radon part of the decay the radioactivity can diffuse
from the ground into the atmosphere and contribute to
the volume ionization. The amount of radon that es-
capes depends on the amounts of 226Ra and 232Th in the
ground; the type of ground cover; and porosity, damp-
ness, and temperature of the soil. The height distribu-
tion in the atmosphere depends on atmospheric mixing
in the boundary layer and the half-life of the isotope.
It is obvious that radiation directly from the ground
will vary greatly depending on the geographical varia-
tions in ground radioactivity. Ground radiation inten-
sity also decreases with height; ax ionization is confined
to the first few centimeters, ~ to the first few meters, and
to the first few hundred meters. The amount of ioniza-
tion in the first few centimeters resulting from offs is
largely unknown. Values of ionization due to As in the
first few meters are typically in the range of 0.1 to 10,
and those due to days in the lowest hundred meters are in
the range of 1 to 6 ion pairs cm ~ 3 see ~ i.
Ionization due to radioactive gases in the air is even
more variable and depends not only on the amount ex-
haled from ground but also on atmospheric dispersion.
Direct measurements of ionization due to radioactive
gases in the atmosphere are difficult and have not gener-
ally been satisfactory. Estimation of the ionization rate
is therefore based on measurements of radioactive prod-
ucts in the air. The height distribution of Rn in the at-
mosphere as a function of turbulent diffusion has been
the subject of a number of investigations and is often
used to determine the turbulent diffusion coefficient.
On cool nights with nocturnal temperature inversions
the radioactive gases can be trapped in a concentrated
layer close to the ground, whereas during unstable con-
vective periods, the gases can be dispersed over an alti-
tude of several kilometers. Ionization at 1 to 2 m due to
radioactive gases and their short-lived daughter prod-
ucts is typically in the range of 1 to 20 ion pairs cm~3
sec~ ~ and is predominantly caused by or particles.
Figure 11.1 illustrates the vertical variation of ioniza-
tion in the PBL. Ionization due to cosmic rays is nearly
constant in the first kilometers. The ionization from
ground ~ and By radiation will vary geographically de-
pending on the abundance of radioactivity in the local
soil. The curves shown are typical, with ~ radiation be-
ing predominant below 1 m and the effect of By radiation
extending to several hundred meters. The curves labeled
Qmax and Qmin represent the sum of ionization due to
cosmic rays, lays and As from the ground, and the decay of
OCR for page 151
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
100
1n
1.0
0.1
TOTAL
'me
FROM EARTH \ POW///:
\ ~ TYPICAL RANGE ~
\ ~ DUE TO RADIOACTIVE ~-
\ ~ EMANATIONS FROM EARTH
.01
1.0
10
IONIZATION RATE (pairs/cm~s)
100
FIGURE 11.1 Ionization profiles showing range of values that might
be expected over land. Qmax would represent high values; Qmin low
values.
220Rn and 222Rn gases and their daughter products in the
atmosphere. The area between the two curves illustrates
the variability expected depending on atmospheric and
soil conditions. The high ionization rate below 1 m indi-
cated by Qmax is due to an accumulation of 220Rn in the
lowest meter and occurs only under strong surface-tem-
perature inversions and low winds. During convective
(daytime) periods with significant wind shear the ion-
ization profile below 10 m would be expected to follow
Qmin values closely. The data used to construct Figure
11.1 were taken largely from Ikebe (1970), Ikebe and
Shimo (1972), Crozier and Bites (1966), and Moses et al.
(1960~. There is a limited amount of data on the vertical
distribution of 220Rn (thoron), and therefore Qmax is the
expected maximum ionization based on measurements
at only three sites. Geographical areas with high ground
radioactivity may exhibit even larger ionization rates
than shown in Figure 11. 1.
Ionization within the first few centimeters of the
Earth's surface due to ax particles has never been ade-
quately investigated. There is the possibility that o`-
emitting daughter products of radon that attach to aero-
sol particles are deposited on the Earth's surface,
enhancing surface activity. This activity is usually ne-
glected in the study of environmental radiation because
it represents a small fraction of the total environmental
151
activity. Yet this source of ionization is likely to be im-
portant in determining the ion concentration at the sur-
face. Surface ion concentration is an important bound-
ary condition influencing the atmospheric-electrical
structure in the interior of the PBL.
Our intention here is not to review the field of envi-
ronmental radioactivity but rather to point out that over
land the ionization in the PBL depends primarily on
ground radiation and radioactive gases and to empha-
size the necessity of studying the geographical variation
of ground radioactivity and the dynamics of the disper-
sion of radioactive gases if we are to understand atmo-
spheric electricity in the PBL.
Over the oceans far from land the ionization rate is
determined solely by cosmic rays. The 3.8-day half-life
of 222Rn permits it to advect over oceans for hundreds of
kilometers before its ionization is negligible compared
with that of the cosmic-ray background.
Compared with the ionization due to natural sources,
ionization from nuclear power plants and weapons is
negligible on a global scale. This was true even during
the active period of nuclear weapons testing in the 1950s
and 1960s (Israel, 1973~. There can, of course, be locally
significant effects (Huzita, 1969~.
In addition to ionizing radiation, electrical discharges
can also form ions in the atmosphere. This requires high
electric fields that generally occur only in disturbed
weather near thunderstorms and in regions of blowing
dust or snow. The field is greatly augmented at points on
electrically grounded, elevated objects such as vegeta-
tion and antennas. As the electric field increases, the
field in very small regions near such points reachs break-
down values and a small ionic current is discharged into
the atmosphere. A large number of unipolar ions is in-
jected locally into the atmosphere, and the ionic space
charge thus formed tends to reduce the high electric
field.
Ions can also be produced by the bursting of water
films. In nature this occurs in waterfalls, falling rain,
and breaking waves. Ions generated by this mechanism
are not formed in pairs, and a net charge is introduced
into the atmosphere. In most cases the residue remain-
ing after evaporation of the water is much larger than a
small ion and is more appropriately identified as a
charged aerosol particle.
Properties of Ions
The radioactive ionization process separates an elec-
tron from a molecule of nitrogen or oxygen. The elec-
tron attaches rapidly to a neutral molecule to form a
negative ion. During the next few milliseconds both pos-
itive and negative ions undergo a series of chemical,
OCR for page 152
152
charge-exchange, and clustering reactions with the mo-
lecular species present in the air. Much progress has re-
cently been achieved in understanding this chain of
ionic reactions in well-defined laboratory gases (Huer-
tas et al., 1978) and in the upper atmosphere (Arnold
and Ferguson, 1983~. In the troposphere, where trace
gases are numerous and variable, the ion chemistry is
complicated, and the composition of the terminal ion
must be regarded as uncertain at best. If mass spectro-
scopic measurements can be extended to identify ambi-
ent ions, it is possible that this identification can be used
as a measure of certain elusive trace gases in the tropos-
phere.
Fortunately, the ion nature enters the equations that
govern atmospheric electricity in the PBL only as it af-
fects the ionic mobility and recombination coefficient.
The mobility is defined as the mean drift velocity of the
ion per unit electric field. The mobilities of ambient ions
are more easily determined than their mass or chemical
composition. (There is no unique relationship between
mobility and mass, and a rather large uncertainty in
mass translates into a much smaller uncertainty in mo-
bility.) Values for aged positive and negative ion mobili-
ties at STP are about 1.15 and 1.25 cm2 V ~ ~ sec~ i, re-
spectively (Mohnen, 1977), are inversely proportional
to air density, and are independent of electric-field
strength.
If ions are formed in pairs by ionizing radiation and
annihilated in pairs by ion-ion recombination, then the
positive and negative ion concentrations are equal and,
in the absence of particulates, are given by
n+ = (q/cx)~/2 (11.1)
where q is the ionization rate and is the recombination
coefficient. If there are no aerosol particles in the atmo-
sphere the ion lifetime is given by
T = n+Iq = (llqor) ~/2 (11.2)
Recombination of ions in the troposphere is a three-
body process. The present theoretical treatment of
three-body recombination is inadequate for calculation
of absolute values of the recombination coefficient. This
is due in part to our inability to identify the ion chemis-
try. Measurements of the recombination coefficient
yield a value of about 1.4 X 10 - 6 cm3/sec for aged atmo-
spheric ions at STP (Nolan, 1943~.
The movement of ions in an electric field gives rise to
the electrical conductivity of the atmosphere. The con-
ductivity is defined as
A= e~n+k+ + n k ), (11.3)
where k+ is the mobility and e is the elementary charge.
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
The conductivity is non-ohmic (field dependent) if the
ion densities depend on electric field, as is the case near a
boundary (electrode effect).
Equations (11.1~-~11.3) predict concentrations of
about 3000 ions/cm3, lifetimes of about 5 min. and a
conductivity of about 1 X 10- i3 mho/m for an ioniza-
tion rate of 10 ion pair cm ~ 3 see ~ i. In reality the values
of these variables are considerably smaller because ions
are destroyed not only by recombination but also by at-
tachment to aerosol particles, as described in the next
section.
Attachment of ions to Aerosol Particles
Suspended in the atmosphere are many small parti-
cles mostly in the size range between 0.01 to 0.5 ,um ra-
dius. The concentration of aerosol particles varies from
a few hundred per cubic centimeter over remote regions
of the oceans to a hundred thousand per cubic centi-
meter in polluted urban environments. Ions diffuse to
these particles and, on contact, transfer their charge to
them; thus the particles act as centers of recombination.
In most continental areas the loss of ions by attachment
to aerosol particles is greater than the loss by ion-ion
recombination. The attachment process also establishes
a size-dependent statistical charge distribution on the
aerosol particles. (Some are negatively, some positively,
and some neutrally charged.)
To treat the problem of ion-aerosol attachment in its
totality requires the solution of a system of balance
equations for ions and particles with various numbers of
elementary charges. Since we are interested here only in
the effect of aerosols on ion depletion, we can write a
simplified ion balance equation as
<3' = q - orn2 - hnZ, (11.4)
where Z is the total particle concentration irrespective
of the charge state of the individual particles and ,6 is the
effective ion-aerosol attachment coefficient. For steady-
state conditions and q = 10 ion pairs cm~3 sec~i, the
dependence of ion density on the particle concentration
is shown in Figure 11.2 for an effective attachment coef-
ficient of ,B = 2 X 10 - 6 cm3 sec~ i. It is readily seen that
when the particle concentration is greater than about
1000 cm ~ 3, the concentration of ions is more dependent
on the aerosol attachment than on ion-ion recombina-
tion. Only in remote oceanic and Arctic regions can the
effect of particles on the ion density sometimes be ig-
nored.
Atmospheric aerosols are hydroscopic, and their equi-
librium radius increases as the relative humidity in
OCR for page 153
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
10,000
-
o
-
o
1,000
An
He
o
o
_ '
. ~
-
100 1 · ,'.,lel1 , 11111111 1 ·',,,,'1 ,W,B,,
10 100 1,000 10,000 100,000
PARTICLE CONCENTRATION (NO. PER cc)
FIGURE 11.2 Ion depletion as a function of increasing aerosol con-
centration for an ionization rate of 10 ion pairs cm ~ 3 sec~ ~ and a typi-
cal value of ion-aerosol attachment coefficient.
creases. This growth is especially pronounced at relative
humidities above about 90 percent. An increase in the
size of aerosol particles makes them more efficient scav-
engers of ions, resulting in a lower conductivity. When
the relative humidity exceeds 100 percent, some of the
particles are activated and grow rapidly (by about 2 or-
ders of magnitude) to radii larger than 1 ,um, forming
fog or cloud droplets. These droplets are effective scav-
engers of ions and are responsible for the low conductivi-
ties found in fogs and clouds. The atmospheric-electric
fog effect (see section below on Phenomenology of At-
mospheric Electricity in the Planetary Boundary
Layer), where conductivity decreases and the electric
field increases before the formation of fog, is probably
related to aerosol growth with increasing humidity.
Recent advances in aerosol measurements have re-
sulted in accurate aerosol size distributions that could
now be used to evaluate more thoroughly the ion-aero-
sol equilibrium. An accurate treatment would include
not only a realistic aerosol size distribution but also the
statistics of diffusional charging of particles by ions.
Effect of the Global Circuit
Electric fields and currents in the PBL arise primarily
from the voltage impressed across it by the global circuit
discussed in Chapter 15 (this volume). The weak con-
ductivity of the PBL causes it to act as a resistive element
in the global circuit, conducting a fair-weather current
density of about 1 to 3 pA/m2 to ground where the fair
153
weather electric field is about 100-200 V/m. Because the
PBL is the region of greatest electrical resistance, it
largely controls the discharge rate of the global circuit.
If the conductivity of the atmosphere were uniform,
it would be a passive ohmic medium with no accumula-
tion of space charge to alter the electric field. Space
charge is generated internally in the unperturbed atmo-
sphere in two ways: (1) by conduction down a conduc-
tivity gradient and (2) by the imbalance of ion flow near
a boundary. The first mechanism can be understood in
terms of Ohm's law, 7 = XE, where..T is the conduction-
current density. For steady-state conditions and in the
absence of any convective charge transport, .! is uni-
form, and, therefore, the electric field is inversely pro-
portional to the conductivity. Since conductivity
changes with altitude, there must be an inverse altitude
dependence of the electric field. Poisson's equation re-
quires this change in field to be accompanied by a space
charge given by
~ = rev · ~ = 6° v · ~ - 6> E. v\,
~ - flu ~ - ~
(11.5)
where en is the dielectric permeability of air. By the
steady-state assumption, the first term on the right-
hand side vanishes and the space charge is proportional
to the electric field and conductivity gradient and in-
versely proportional to the conductivity. This process
can be thought of as a pileup of space charge due to con-
duction down a conductivity gradient.
The second mechanism for producing space charge
operates only near a boundary. Across any horizontal
area in the atmosphere stressed by a vertical electric
field, positive and negative ions will flow in opposite
directions. However, at a boundary, ions of one sign can
flow to that boundary, but there will be no compensat-
ing flow of the opposite sign away from it. This im-
balance in ion flow gives rise to a space charge in the
vicinity of the boundary. This second mechanism for
generating space charge is aptly referred to as the elec-
trode effect. For the case of uniform volume ionization
in laminar airflow with no aerosols and bounded on the
bottom by a conducting surface, the ion-balance equa-
tions can be solved together with Poisson's equation to
give the solution shown in Figure 11.3. The ionization
rate was taken to be 10 ion pairs cm-3 sec~i, and the
solution illustrates that the effect of the electrode, in this
idealized case, would extend to about 3 m (Hopper,
1967). In the turbulent atmosphere, the electrode effect
extends to much higher altitudes and the space charge
formed by these two mechanisms is dispersed by turbu-
lent mixing, causing a convective flux of charge in addi-
tion to the conduction current.
OCR for page 154
54
4.0:
3.5
_ 3.0 _
an
~ 2.5 _
-
2.0
1.5
1.0
_
0.5
O
0 20
5.0( ) Oj4
4.5 _
no x log (IONS PER M3)
0.8 1.2 1.6
2.0 2.4
1 1
l
l
l
l
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
Cal integral scale comparable to the height above the
. . surface. Both heating and evaporation from the surface
I I _ tend to produce an unstable density gradient. The struc
ture of the turbulence produced by buoyant instability
can be quite different from that of shear-generated tur
bulence because warm, moist parcels starting near the
surface accelerate as they rise through an unstable envi
ronment. The resulting eddies tend to be elongated ver
tically and to have a size scale determined by the thick
ness of the entire PBL.
The turbulent kinetic energy injected into the flow by
these two mechanisms is not dissipated at the scales
/ where it is produced. Instead, energy cascades down to
n / I ever smaller eddy sizes and is dissipated primarily at
t-F ~In+ scales smaller than a centimeter and results in a wide
l range of eddy sizes. These eddies efficiently mix passive
contaminants like ions, aerosols, space charge, and ra
dioactive gases. The turbulent transport of space charge
is equivalent to an electrical convection current through
the atmosphere. It is evident from Figure 11.3 that tur
bulent diffusion would disperse the space charge result
I ing from the imbalance of positive and negative ions in
the lower layers, producing a net upward flow of charge
(opposing the downward conduction current).
Electric field fluctuations are also caused by turbu
lent movement of this space charge. To specify the aver
age electric field, the instantaneous field must therefore
be averaged over an interval of time longer than the pe
riod of the largest eddy. Figure 11.4 shows an electric
field profile over the ocean in the tradewinds off the
coast of Barbados and illustrates the increase in vertical
extent caused by turbulence. Each circle represents an
average of 50 to 300 measurements of 10-see duration,
and the bars are the standard deviations. The large devi
ations illustrate the variability in the instantaneous field
and the necessity of averaging to obtain a meaningful
profile in the PBL. The variations are greatest where the
gradient is steepest and are much less at a height of 160
m above the surface. The solid and dotted lines are the
result of numerical modeling where x is a parameter re
lated to the strength of turbulent mixing. It is obvious
from this discussion that any treatment of atmospheric
electricity in the PBL must include the effects of turbu
lence.
40 60 80 100 120
-E (VOLTS PER M)
FIGURE 11.3 Simple electrode effect in nonturbulent air with con-
stant volume ionization rate of 10 ion pair cm~3 sec~i over a plain
surface (from Hoppel, 1967~.
Turbulent Transport of Electrical Properties
Most aspects of the structure of the PBL are domi-
nated by the effects of turbulence (Haugen, 1973; Wyn-
gaard, 1980), and electrical processes are no exception.
Turbulent mixing prevents the buildup of radioactive
emanations in shallow layers near the ground except un-
der very stable conditions, disperses aerosols over a
greater depth increasing the columnar resistance, and
redistributes space charge, producing convection cur-
rents. The almost continual state of turbulent motion in
the atmosphere is caused by the combined influences of
drag, heating, and evaporation from the underlying
surface. It is only in cases of extremely low wind speed
and strong surface cooling that laminar flow may be
found, and even then only for short periods and over
limited areas.
Drag generates turbulence through shear instability,
which transfers kinetic energy from the mean flow to
the turbulence. The energy goes into eddies on the scale
of the mean velocity gradient, which is strongest near
the surface, and tends to produce turbulence with a lo
This variability with time and position at low alti-
tudes demonstrates the danger inherent in balloon
soundings used to obtain integrated ionospheric poten-
tial. Neither spatial nor temporal averaging is possible
during the rapid ascent and only a coarse vertical resolu-
tion is available where fields are largest and most vari-
able.
OCR for page 155
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
180 _
160 ~ ,
140
`~: 120
~100 L
80 ~ .
I 60
40 _
O 11 1 ~1 1
0.3 0.40.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
NORMALI ZED ELECTRIC F I E LO
- NUMERICAL SOLUTION
~ BARBADOS MEASUREMENTS
- - --NUMERICAL SOLUTION WITH
x=6 Z= 500 cm~3
X=6\\ \X=12
' \\\'
'' \~= '
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
Hi
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 2
annual and even 11-yr periodicities; short-period fluc
tuations 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 uni
versal or local time and, to a lesser extent, on seasonal
changes in diurnal patterns.
One of the first, and certainly the most famous, dem
onstrations of a reproducible variation pattern with uni
versal time is the hourly average potential gradient
curves obtained over the world ocean on the Carnegie
cruises. These curves have long served as he facto stan
dards with which to evaluate the viability of attempts to
measure universal variations. The result of the l928
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.
155
These are integrated (and extrapolated) ionospheric po-
tential from 125 balloon ascents (Muhleisen, 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 Arc-
tic winter, which, when allowance is made for the small
(6 percent) measured change in columnar resistance
during the measurement period, should accurately mir-
ror 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 (Israel, 1973) in which the effect of turbulent pro-
cesses 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 at-
tributable to PBL processes. However, subtraction of
- 2t
o
_ 1
N
N
A
-
IN
Cal
\ ~
~ .
en,
O _.'
o o O '/
_ >~- .; ,~
W.' ,.r
~,
`` ,_,
a
- Current Density (Anderson)
· Potential Gradient (Carnegie)
--- Total Potential (Muhleisen)
- Total Potential (Markson)
0 0 Total Potential (Clark)
0 12
TIME - GMT
. .
o
400
J
200~
C)
It
~2
¢
FIGURE 11.5 Universal diurnal variations in atmospheric electric-
ity from different measurement techniques.
OCR for page 156
156
FIGURE 11.6 Average diurnal variations
in potential gradient showing effect of after-
noon turbulence mixing (from Israel, 1958~.
%
120
278 V/m= 100
~0
t20
2tO K/m = 100
80
120
166///m= 100
~0
o
the average diurnal variation does not necessarily isolate
a particular local phenomenon.
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 de-
pencling strongly on local effects. Conductivity has been
seen to be a result of ionization, ionic mobility, recombi-
nation, and attachment of ions to particulates, all of
14.0
° 12.0
to
x
LL, 1 0.(3
cr
LU
~ 8.0
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
Pled am J an.
Feb.
~ l
hiarch _
Am,
. _ _
l ~
6 12 18 24
Local time
%
120
ADO =270
~0
Ah
120
15 7 K/m-100
~0
120
t69~/m- 100
BE
\
April 120
_ in 100 = 1861//m
slay
7: 80 120
June
/ \ 120 80
100 = 151/m
726V//n= 100
a
_ ~
, ~
, ~
, ~
Porsd am | July |
_ ~
6 12 10
Local time
F'f
. i.
OWL
1 ~
l
51%0
~100 - 154///tn
V Sept.
~1
120
1 ~I -~^ a^^ .,,
1120
:100 = t90 Jam
Nov. I
.~\1 SO
T Dec. |
Em'
1
120
100 s276K/m
~0
~4 lIr
which can be influenced by local conditions and vari-
able trace constituents. It is reasonable therefore to ex-
- pect that large variations in conductivity will be primar-
ily 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,
~ :
A:
1 1 1
1 1 1
~K · ~:
4 8 12 16 20 24 4 8
EDT HOURS
FIGURE 11.7 Average diurnal variation in columnar resistance as a function of local time (from Sagalyn and Faucher, 1956~.
OCR for page 157
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
a 150
'= I30
J
- 110
o
J
o
90 ~,/
it_
N
UNPOLLUTED (2 STATIONS)
- POLLUTED (3STATIONS)
501 l I t , , , , , ,
0 2 4 6 8 10 12 14 16 18 20 22 24
GMT
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 ac
cord with our knowledge of the daily variation of turbu
lent activity.
The strong dependence of the columnar resistance on
PBL conditions provides a mechanism for the modifica
tion of electrical observables with local time. Such
mechanisms can embrace the entire PBL as does fully
developed turbulent convection, or they can be con
fined 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 activ
ity, 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 ver
tical 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 (Israel, 1961~.
These diurnal variations with both universal and lo
cal time are not the only fluctuations observed in atmo
spheric-electricity recordings. Shorter-period variations
are always observed and usually dismissed as noise. Ob
served fluctuations on atmospheric-electricity record
ings made within the PBL are comparable in magnitude
to the mean values of those recordings (Takagi and To
riyama, 1978~. A typical recording is seen in Figure
11.9. Much of the fluctuation content in the range from
157
FIGURE 11.8 Normalized potential gradi-
ent showing rush-hour effect at polluted ur-
ban sites (from Anderson and Trent, 1969).
roughly 0.1 see 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 ob-
servations as a tool in the study of turbulent processes.
The relationship between atmospheric electricity and
turbulence will be considered in more detail in the sec-
tion below on Modeling and Theory. There is also a sug-
gestion that fluctuations in the total Maxwell current
density in the range of 10 to 1000 see can correlate at
intercontinental distances (Ruhnke et al., 1983~. It is
clear that short-period fluctuations in atmospheric-elec-
trical 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-elec-
trical variables within the PBL are much smaller than
vertical ones (largely a consequence of the geometric
scales involved), significant horizontal variability is ob-
served. Significant instances include the effect of organ
tL
FIGURE 11.9 Records of the potential gradient (F), air-earth cur-
rent density (1), and brightness (H) at Aachen on July 31, 1954 (from
Israel, 1958).
OCR for page 158
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 perturba-
tion 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 fo-
cused 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 convec-
tion currents in the PBL comparable in magnitude with
the total current, the classical electrode effect over wa-
ter 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 Green-
land profiles are characterized by extremely low levels
of particulate contamination, and the vertical variation
of conductivity closely approximates that predicted the-
oretically 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 Green-
land observations.
70/ / ~ / / ~ / /
~\ i\ ~
o
0 10 20 30 40 50 60
DISTANCE (km)
FIGURE 11.10 Horizontal variations in potential gradient showing
effect of organized convection (from Markson, 1975).
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
7.S
O 100 200 300 400
ATMOSPHERIC POTENTIAL ( kVl
A- GREENLAND
2200 GMT 30 AUG 1955
° TYPE B-GREENLAND
1700 GMT 28 AUG 1955
o TYPE C
6 0 _ CALIFORNIA COAST
. 1900 GMT
8 DEC 1955
x TYPE D
VIRGINIA
1945 6MT
-23 M" 1955 .
4.S _
E 30
-
1_
1.5
o
A
// ~
// ~J / .
_
FIGURE 11.11 Typical vertical distributions of atmospheric poten-
tial (from Clark, 1958~.
The conduction-current density, defined as the prod-
uct of electric field and conductivity, can easily be com-
puted from airborne measurements. Two typical pro-
files 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 ver-
tical constancy led to the aforementioned use of current-
density measurements above the PBL to follow univer-
sal variations. The increases seen at low altitudes were
the first unambiguous evidence of the existence and sig-
nificant 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 at-
mospheric-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.
OCR for page 159
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
7.5
1 1 1 1
~ CALIFORNIA COAST
j 2330 GMT
~ · as_
!
AVE RAG E VA LUE
I ABOVE E X C MANGE
\! LAYER
1 1.85 t 0.19
. 1
. 1 1
1 1
| TOP OF
EXCHANGE
f LAYER
~ 1
VIRGINIA COAST
1530 GMT
MAY 17, 1956
AVERAGE VALUE
I ABOVE EXCHANGE
LAYER
1.48 t 0.13
1
TOP O F
EXCHANGE
LAYER
U . , . W~
0 2 4 0 2 4 6
CONDUCTION CURRENT DENSITY MAGNITUDE (10~~2omp/m2)
FIGURE 11.12 Two profiles of convection current density com-
puted from aircraft measurements of field and conductivity (from
Kraakevik, 1958~.
Analogously, the increase in conductivity at the termi-
nation of the fog event may also precede actual dissipa-
tion. A typical data recording through a fog event is seen
in Figure 11.13. This phenomenon has been reported by
many observers (Dolezalek, 1963), but it has not as yet
received an adequate physical analysis.
Similarly, there are effects associated with surf and
waterfalls wherein some charge separation is produced
400
z 300
'-ad E
Z C ~ 200
~ ~ -
o ~ 1 00
O
> _ 5
o 4
3
o ~
c) o 2
~ o ~1 1
o
~ in 4
, an
LOCAL TIME
159
by mechanical breakup of the water surfaces (Blan-
chard, 1963~. For example, the positive space charge
produced by breaking surf can lead to an appreciably
larger electric field on shore than outside the surf zone
during onshore winds. In contrast to the positive charge
produced along ocean coasts, sufficient negative charge
has been observed along the shore of a freshwater body
(Lake Superior) during heavy surf to reverse the fair-
weather electric field (Gathman and Hoppel, 1970~.
Negative charge is also observed from waterfalls. In
some cases there are also strong local effects associated
with smoke plumes and with volcanic eruptions. Again
there is a separation of charge, which then diffuses away
from the source.
MODELING AND THEORY
The complicated dependencies of the local electrical
variables on the ionization profile, aerosol concentra-
tions, turbulent structure of the PBL, and temporal var-
iations in the global electrical circuit make it dangerous
to trust intuitive notions when interpreting measure-
ments made in the PBL. Increased insight into the
meaning of the observations is obtained by modeling
various physical mechanisms mathematically. Some of
the more important results of these theoretical efforts
follow.
The electric field tends to be nearly vertical in fair
weather, and the meteorological structure of the PBL
usually changes slowly in comparison to the electrical
relaxation time. This has led naturally to the assumption
of a quasi-steady, horizontally homogeneous mean state
and to one-dimensional, time-independent models of
the mean electrical structure. These assumptions imply
that the conduction- and convection-current densities
FIGURE 11.13 Atmospheric-electric fog
effect-a typical example of successful fore-
casts of both on set and dissipation (from
Serbu and Trent, 1958~.
OCR for page 160
160
are vertically directed and that their sum, the total cur-
rent density, is height independent, leading to great
simplifications from a modeling point of view. About
this mean state, of course, are fluctuations caused by
turbulent eddies. (A parallel may be drawn here to the
prevalence of one-dimensional models of the mean me-
teorological structure of the PBL, in which small-scale
phenomena such as turbulence enter only in terms of
their horizontally averaged effects.)
The primary goal of electrical modeling of the bound-
ary layer to date has been to understand the mean pro-
files of the electrical variables resulting from currents
driven by the global circuit. Thus, theoretical develop-
ments have focused on the sources of charge within the
PBL and the phenomena produced by the vertical tur-
bulent transport of that charge. After an introduction to
the modeling of turbulent mixing, we shall examine the
electrode layer and the convection currents resulting
from electrode-effect space charge.
Turbulence Modeling
The simplest and most pervasive mathematical de-
scription of turbulent mixing is the gradient-diffusion
model. Applied to the vertical transport of space-charge
density by vertical velocity fluctuations w ', it states that
the mean turbulent flux (or convection current) is pro-
portional to the local mean gradient:
he'd'= -K(z) aaP. (11.6)
Here K(z) is the eddy-diffusion coefficient, usually
taken to be proportional to height above the surface.
Values at 1 m generally lie between 0.01 and 1.0 m2/sec.
This model is a form of first-order closure of the con-
servation equation for space charge. It is generally be-
lieved to provide an acceptable description of mixing
near the surface, where shear production predominates
over buoyant production of turbulent kinetic energy.
Gradient diffusion is often applied throughout the PBL,
sometimes with a stability-dependent coefficient and a
decrease at the top to represent an inversion, although it
is known to provide poor results in the interior of an
unstable mixed layer. In atmospheric electricity its use is
best restricted to modeling of the electrode effect.
There are many more sophisticated (and more com-
plex) approaches to turbulent transport modeling. A
popular one is second-order closure, in which conserva-
tion equations are derived for evaluating the second mo-
ments, such as w 'p ' and p '8 v (A is the virtual potential
temperature), in terms of each other and of mean vari-
ables like reply. Such models allow the mean charge flux
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
at a given height to depend on the dynamics of the entire
PBL, not just on local properties. This capability is es-
sential for the correct modeling of unstable mixed lay-
ers, where local flux-gradient relationships are known
to break down.
Before leaving this discussion of turbulent transport
modeling, some cautionary mention should be made of
secondary flows. There are many circumstances when
the largest scale of motion in the boundary layer is or-
ganized into a nonrandom structure. The most familiar
example is the stationary roll vortices, which sometimes
form over the tropical ocean. Such organized motions
can carry a large fraction of the vertical transports, and
since they cannot be adequately represented by turbu-
lence models of the types discussed above, these trans-
ports must be described explicitly with a dynamical
model of two or more dimensions.
The Electrode Effect
The electrode effect has already been defined, and its
simplest manifestation has been described for laminar
flow and uniform ionization in aerosol-free air (Figure
11.3~. In this case the electrode layer has a thickness of
only a few meters, over which the electric-field magni-
tude decreases by about a factor of 2. The importance of
the phenomenon lies in its ability to separate substantial
amounts of charge near the Earth's surface. In condi-
tions of low turbulence this leads to high space-charge
densities in shallow layers, which can produce high elec-
trical noise levels as intermittent eddies move this charge
around. In strongly unstable conditions, on the other
hand, it provides a source of charge to be carried deep
into the interior of the PBL by convection currents.
The theory of the nonturbulent electrode effect is
fully developed and has been verified over water and, at
least qualitatively, over land. The simplest case, the so-
called "classical" electrode effect, is illustrated schemat-
ically in the first frame of Figure 11.14. The thickness of
the layer is determined by the lifetime of the small ions
and their drift velocity in the ambient electric field.
Since aerosol particles act as recombination sites for
small ions, they reduce the ion lifetimes and, hence, the
thickness of the nonturbulent electrode layer, as illus-
trated in the second frame of Figure 11.14. A shallow
layer of enhanced ionization, which can arise from sur-
face radioactivity or trapping of radioactive emanations
from the soil, can cause a reversed electrode effect, as
illustrated in the third frame of the figure. Here a stra-
tum of negative space charge is developed owing to the
sweeping of negative ions upward out of the highly io-
nized layer, causing the electric-field magnitude to in
OCR for page 161
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
ELECTRIC FIELD PROFILES IN THE ELECTRODE LAYER:
EFFECT OF AEROSOL, TRAPPED RADIOACTIVITY, TURBULENCE
-
~ 2
-
E~
:e
C)
3
1
o
l
\
. \
CLASSICAL AEROSOL
/
/
.~ 1 1
RADIOAC TIVITY TURBULENC E
crease with height. Unfortunately, these relatively sim-
ple cases rarely occur in the real atmosphere because of
pervasive turbulent mixing.
The most obvious effect of turbulence on the elec-
trode 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 be-
cause trapping of radioactive emanations is eliminated
and because the thicker layer appears to be less sensitive
to surface radioactivity. The presence of aerosol parti-
cles thickens the layer further, in contrast to their effect
in the nonturbulent case, by increasing the electrical re-
laxation time. These processes may increase the height
scale of the electrode layer so much as to make it virtu-
ally 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 signifi-
cant 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 sur-
face (Willett, 1983~. Hoppel and Gathman (1972) ob-
tained reasonable agreement between experimental ob-
servations 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.
161
FIGURE ~ I. 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 ioniza-
tion, as in Figure 11.3; the effect of aerosol
attachment on the nonturbulent case; the ef-
fect of a shallow layer of high ionization rate
on the nonturbulent case; and the effect of
strong turbulence.
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 homo-
geneous, 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-con-
servation equation
_ - 6\
p - E
ap
=
at co LIZ
,`8 (w'p') (11.7)
and Poisson's equation. This assumes that the mean con-
ductivity 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
OCR for page 162
162
ers, where the turbulent transport time across the entire
PBL can be comparable with the electrical relaxation
time. The controlling meteorological variables are the
surface fluxes of momentum and buoyancy and the
mean conductivity and thickness of the mixed layer. It
appears that the convection of electrode-effect space
charge can have a major impact on the electrical struc-
ture of the boundary layer. It can even reduce the mag-
nitude of the total downward current density locally on
the order of 50 percent owing to a mechanically gener-
ated electromotive force (EMF) of more than 100 kV in
extreme cases. Although these theoretical predictions
are not inconsistent with existing data, they remain to
be tested thoroughly in the field.
Effect of the Planetary Boundary Layer on the
Fair-Weather Electrical Circuit
Two boundary-layer processes can have a substantial
impact on the fields and currents appearing throughout
the entire atmospheric column from the Earth to the
ionosphere. These are variations in the columnar resis-
tance and convection currents. To appreciate the im-
portance of these effects, consider a steady convection-
current density JC(Z) below an inversion at height H
when a steady ionospheric potential VOO is applied from
above. Since the total current density JO must be inde-
pendent of height, it is easy to show that
VOO = - Jt;Roo +
H
Jc dz (11.8)
o
where Roo is the total columnar resistance. The second
term on the right may be considered an EMF generated
by boundary-layer convection. This steady-state analy-
sis is valid as long as X, Jc, and VOO change slowly com-
pared to the electrical relaxation time near the ground.
If we assume VOO to be constant, Eq. (11.8) shows that
the magnitude of J' is inversely proportional to Roo and
decreases linearly as the boundary-layer EMF increases.
An aerosol-related increase in columnar resistance of 40
percent can therefore produce a similar decrease in the
total current density. A simultaneous 100-kV increase in
the PBL EMF can cause a further 30 percent decrease,
for a total reduction in JO of 52 percent. This makes J'
alone a relatively poor indicator of global processes.
NEEDED RESEARCH AND POTENTIAL
APPLICATIONS
Measurement of Global-Scale Phenomena
As discussed in detail in other chapters of this volume,
there are ample reasons for interest in global-scale atmo
WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT
spheric-electrical phenomena. For example, valuable
information about the distribution and temporal vari-
ability of horizontal potential differences in the iono-
sphere could be provided by monitoring the ionospheric
potential simultaneously in different locations. Further-
more, the widely accepted relationship between global
thunderstorm activity and ionospheric potential has yet
to be verified on any but the crudest statistical basis.
From the present perspective, finally, a detailed knowl-
edge of the forcing from the global circuit would be use-
ful in evaluating the electrical response of the PBL.
Unfortunately, the measurement of global-circuit pa-
rameters is complicated by the action of boundary-layer
processes. Although local PBL structure cannot appre-
ciably affect the total current in the global circuit, or
even the local ionospheric potential, it can cause a redis-
tribution of that current and alter the vertical profile of
electric field. Therefore, the proper interpretation of lo-
cal measurements in terms of global parameters requires
a thorough understanding of noise sources in the PBL.
The electrostatic potential of the upper atmosphere
with respect to Earth is the single parameter most indic-
ative of the electrical state of the global circuit. Yet the
temporal variability of this ionospheric potential is
largely unknown outside of its average diurnal varia-
tion. Methods of measuring the ionospheric potential
(such as aircraft and balloon soundings) have for the
most part systematically excluded the detection of any
shorter-term variations. Fluctuations in electric-field
and current-density measurements in the PBL with pe-
riods shorter than a few hours are usually attributed en-
tirely to local sources, primarily turbulence and pollu-
tion.
One way to separate local and global sources is to cor-
relate measurements at widely separated stations or to
make instantaneous measurements averaged over large
horizontal areas. A preliminary attempt (Ruhnke et al.,
1983) to detect global variations with periods of seconds
to minutes in the total Maxwell current (conduction,
convection, and displacement) measured simultane-
ously in the United States and the Soviet Union revealed
an apparent correlation that is difficult to attribute
solely to chance. This approach deserves further atten-
tion as a relatively simple prospective method of moni-
toring short-period variations in the global circuit.
If it can be demonstrated that short-period and day-
to-day global variations do indeed exist, then not only is
the source of these variations of importance but also the
usual interpretation of local variations in terms of tur
bulence must be re-evaluated. In light of the importance
of the ionospheric potential as an indicator of the electri-
cal state of the global circuit and the need to separate its
variation from local fluctuations in the PBL, the iono
OCR for page 163
ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER
spheric potential should be measured continuously and
simultaneously at two or more locations for a period of
days and with a time resolution of seconds, preferably in
conjunction with observations of global thunderstorm
activity and of upper-atmospheric disturbances. The di-
rect measurement of potential in the lower atmosphere
using a tethered balloon has been attempted (Willett
and Rust, 1981; Holzworth et al., 1981~. Extension of
these techniques to higher altitudes and faster time reso-
lution should be encouraged.
Potential Toolfor Study of PZanetary-Boundary
Layer Turbulence
We have seen how strongly the electrical structure of
the PBL is influenced by turbulent mixing. Space charge
is unique among natural scalar contaminants in having
a lifetime (the electrical relaxation time) comparable in
magnitude with the time scales of the largest eddies in
the PBL. Charge density, used in conjunction with a
conservative tracer like water vapor, therefore offers the
possibility of useful information about the structure of
these energy-containing motions, which the natural ra-
dioactive tracers, thoron (54-see half-life) and radon
(3.8-day half-life), cannot provide. If the sources of
space charge and moisture are understood, comparison
of their relative distribution through the boundary layer
might be useful in determining the Lagrangian time
scale of the transport process.
Another important consequence of the finite lifetime
of charge density is that the convergence of its turbulent
flux can be deduced from its mean distribution and the
convergence of conduction-current density under
steady-state conditions. Since these functions depend
only on the mean profiles of electric field and conduc-
tivity, they are readily measured. Thus, convection-
current density is one of the few turbulent fluxes the pro-
file of which can be observed without recourse to the
complex and technology-intensive eddy-correlation
method.
Because the lifetime of space charge is comparable
with that of the largest eddies, it resists becoming well
mixed in an unstable PBL, where conservative scalars
tend to be uniformly distributed. This fact and the ease
of measuring the turbulent transport have recently been
exploited to obtain profiles of the eddy-diffusion coeffi-
cient for space charge through the boundary layer from
individual aircraft soundings (Markson et al., 1981~.
To realize this potential, it will be necessary to de-
velop a more thorough understanding of the sources and
turbulent transport of electrical charge within the PBL.
The most urgent need is for a field program to gather
data on the dependence of these phenomena on the me
163
teorological structure of the boundary layer. Further
theoretical modeling will then be required to integrate
these data into a coherent understanding of the pro-
cesses involved. Areas of particular ignorance at present
are the ionization rate within the plant canopy and the
disposition of space charge accumulating at an inversion
because of the discontinuity of conductivity usually
found there. Further research into these areas may even-
tually lead to the use of atmospheric-electrical measure-
ments to observe, perhaps remotely, the meteorological
structure of the PBL.
Ion Physics and Balance in the Planetary Boundary
Layer
Small atmospheric ions, existing by virtue of a bal-
ance between ionization of the neutral gas and recombi-
nation and attachment to aerosol particles, cause the
conductivity of the air. Yet, many facets of the nature
and behavior of these particles are still poorly under-
stood. More research is needed in the area of ion physics,
especially (1) identification of the terminal positive and
negative species in the PBL, (2) determination of the de-
pendence of ion chemistry on trace gases, (3) measure-
ment of the attachment coefficients of ions to charged
and uncharged aerosol particles of various sizes, and
(4) evaluation of the resulting charge distribution on the
aerosols. (1) and (2) show promise of becoming sensitive
methods for detecting certain trace gases. Further iden-
tification of exact ion chemistry is required by physiolo-
gists before they can evaluate claims of physiological ef-
fects of air ions (MEQB, 1982~.
Values of ion-aerosol attachment coefficients as a
function of particle radius and charge are necessary to
determine accurately the loss of ions (or conductivity) as
a function of aerosol load. Few measurements of abso-
lute values of attachment coefficients have ever been at-
tempted. Usually only ratios of attachment coefficients
are measured and compared with theoretically pre-
dicted values of the ratios. Better measurement of the
absolute values of the coefficients are necessary to pre-
dict ion loss and validate theory.
The use of conductivity or columnar resistance as a
pollution monitor depends on the inverse relationship
between conductivity and aerosol burden. The conduc-
tivity is sensitive to the aerosol concentration, and mea-
surements spanning several decades have been used to
evaluate changes in global particulate pollution (Cobb
and Wells, 1970~. However, the quantitative reliability
of these indicators of particulate burden should be more
fully investigated. The use of conductivity measure-
ments for deducing aerosol burden is complicated by
their sensitivity to the ionization rate. This latter sensi
OCR for page 164
164
tivity has even led to the suggestion that conductivity be
used as a monitor of nuclear operations and accidents.
The electrical state of the atmosphere depends criti-
cally on the ionization profile. Over the ocean ioniza-
tion is due only to cosmic rays, and oceanic measure-
ment of the electrode effect are in satisfactory
agreement with numerical solutions of the governing
equations. Over land, the ionization profile is compli-
cated owing to ionization from ground radioactivity
and radioactive gases as discussed earlier. Simultaneous
measurement of all contributions to the ionization pro-
file has never been accomplished. It is imperative that
future studies of atmospheric-electrical profiles in the
PBL over land include such measurements.
A theoretical explanation is needed for the atmo-
spheric-electric fog effect. This is most likely to be found
in the dependence of the conductivity on changes in the
aerosol size distribution with changes in relative humid-
ity. Measurements of all pertinent parameters are
needed during a fog event to formulate a physical theory
that adequately accounts for the observations. The pos-
sible usefulness of the observed precursor phenomenon
could then be evaluated.
Finally it should be mentioned that there are some
experimental techniques that do not yet exist in satisfac-
tory form for proper studies of PBL electrical processes.
Briefly, these include a continuous measurement of
ionospheric potential with fine time resolution, ion-
sampling techniques with spatial resolutions suitable for
profile determinations in the lowest few centimeters of
the PBL (and within the plant canopy), and an ade-
quately instrumented platform for making accurate at-
mospheric-electrical and micrometeorological profiles
throughout the PBL.
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Representative terms from entire chapter:
space charge
