0
400
-250
V1 (0.5m)
-500
V2 (1.0m)
300
-750
V3 (1.5m)
-1000
V4 (2.0m)
-1250
Sg (W/m2)
-1500
200
-1750
-2000
100
-2250
-2500
Results
Figure 1a) shows the time series of potential and solar radiation for a day
with fair weather conditions (average windspeed at 3m was 1.9ms-1,
aerosol mass concentration M = 38 μgm-3, few clouds). During the day
the potential increases with height and the mean value at 1m is about
180V between 15:00 and 20:00 (British Summer Time). Fluctuations are
correlated at different heights and increase in magnitude with height.
1
600
0.75
0.5
0.25
0
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00
-0.25
potential (volts)
-0.5
-0.75
-1
-1.25
-1.5
-1.75
-2
time (BST)
Figure 1b) gradient Richardson number for 7th August 1998. Values
of -Ri < 1 indicate moderately unstable conditions.
600
1000
750
500
500
250
0
-250
400
-500
V1 (0.5m)
-750
300
V2 (1.0m)
-1000
V3 (1.5m)
-1250
-1500
V4
(2.0m)
200
-1750
Sg (W/m2)
-2000
100
-2250
-2500
-2750
0
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00-3000
-3250
-100
-3500
time (BST)
Figure 2a): Solar radiation and time series of potential profile for
day 11th August 1998, fair weather conditions.
Figure 1b) shows the gradient Richardson number stability parameter
1.00
g T
where T is temperature and U is wind speed. It represents the relative
importance of buoyancy and wind shear in producing turbulence. Onset
of turbulence in unstable conditions occurs when Ri<0.25. Turbulence is
supressed in stable conditions when Ri>1. It can be seen from figures 1a)
and 1b) that the potential starts to increase within an hour of the onset of
turbulence. Values of potential reduce dramatically shortly before 21:00
BST when the atmosphere has returned to stable conditions (Ri >1). This
suggests a strong link between the diurnal variation of potential in the
surface layer and local atmospheric stability.
0.75
gradient Richardson number
z
Ri  T
2
 U 


 z 


1000
750
500
250
0
-250
400
-500
-750
V1 (0.5m)
300
-1000
V2 (1.0m)
-1250
V3 (1.5m)
-1500
200
V4 (2.0m)
-1750
Sg (W/m2)
-2000
100
-2250
-2500
-2750
0
-3000
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00
-3250
-100
-3500
500
0.50
0.25
0.00
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00
-0.25
-0.50
time (BST)
Figure 3 a) Solar radiation and potential profile for 20th August
1998.
It has been observed that clouds have an effect on surface electrical
parameters (Whitlock and Chalmers, 1956) assumed to be due to their
charge. Figure 3a) shows solar radiation and potential profile for 12 th
August 1998. The sharp minimum in solar radiation was due to a band of
altocumulus cloud that passed over the Sonning measurement site. It can
be seen that the potential at all heights was considerably reduced in the
presence of the cloud.
400
1000
750
500
250
300
0
-250
250
-500
-750
V1 (0.5m)
200
-1000
V2 (1.0m)
-1250
V3 (1.5m)
150
-1500
-1750
Sg (W/m2)
100
-2000
-2250
50
-2500
-2750
0
-3000
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00-3250
-50
-3500
350
time (BST)
Figure 3a) solar radiation and potential profile time series for 12th
August 1998. Altocumulus cloud causes large reduction in potential.
100
95
90
23
potential gradient
22
temperature
85
21
80
75
20
70
1.00
19
65
0.75
gradient Richardson number
An electrostatic fieldmill (Chubb Instruments, UK) was used to make
fast response measurements of potential gradient at 2.85m. It was
mounted adjacent to a sonic anemometer and platinum resistance
thermometer. All three instruments were logged at 4Hz and the data was
used to perform eddy correlation and calculate turbulence statistics.
Figure 1a) Solar radiation and potential profile time series for 7 th
August 1998. Fair weather conditions prevailed.
solar radiation (W/m2)
Instrumentation
Passive wire antennas of length 20m were used to sense atmospheric
potential. Electrometers (Harrison, 1997) were used to measure the
potential on each wire. Four antennas were mounted in a vertical array at
heights 0.5, 1, 1.5 and 2m above the surface. Vertical profiles of
temperature and wind were measured using fine wire platinum resistance
thermometers and cup anemometers. Aerosol mass concentration in the
size range 0.1 to 15 μm was monitored using an optical counter. Solar
radiation was measured using a solarimeter and wind speed and direction
at 3m were measured using a cup anemometer and a wind vane. All
instruments were logged at 1Hz and data was averaged and stored every
5 minutes.
-3500
time (BST)
gradient Richardson number
Field Campaign
Measurements were made in the U.K.at Sonning Farm, a semi-rural site
near to Reading, during July and August 1998. Surrounding terrain
consisted of gently sloping fields divided by shrubs and low trees. Traffic
on a busy road to the south of the site presented a considerable source of
particulate pollution with semi-diurnal fluctuation. Anti-cyclonic
conditions prevailed for most of the measurement period.
-100
potential (volts)
These results underline the need to treat electrical and
micrometeorological parameters as a coupled system in the atmospheric
surface layer. This poster presents some results from a field campaign
where the effects of turbulence and aerosol concentration on
measurements of potential were investigated.
-2750
0
-3000
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00
-3250
Effect of clouds on surface layer parameters
60
18
0.50
55
0.25
0.00
00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00
-0.25
solar radiation (W/m2)
250
Figure 2a) shows solar radiation and potential profile time series for the
11th August 1998. There were few clouds, mean 3m windspeed was
1.5 ms-1 and aerosol mass concentration was 8 μgm-3. The average
potential at 1m is approximately 100 Vm-1 between 14:00 and 22:00
which is lower than for the 7th August 1998. As there is less aerosol this
may be due to increased conductivity. Figure 2b) shows large negative
values of Ri during the day. This indicates strong convection and thermal
plumes can occur for values of Ri < -1 (Lumley and Panofsky, 1964).
Quasi-periodic fluctuations in potential occur at each height between
approximately 13:00 and 17:00. Similar fluctuations were observed in
temperature and wind at the same time. They coincide with a period of
strong convective activity. The fluctuations in potential may be due to
thermal plumes transporting space charge upwards. If this is the case
then true fair weather conditions are not occuring as the plumes
represent a local charge generator. The definition of fair weather should
therefore be extended to exclude strongly convective periods.
50
11:30
11:40
11:50
12:00
12:10
12:20
temperature (degrees Celsius)
500
potential (volts)
750
potential gradient (V/m)
1000
500
solar radiation (W/m2)
600
potential (volts)
Introduction
Turbulent transport of space charge in the surface layer is established as
important in determining profiles of electrical parameters. Any realistic
model of the electrode effect (Hoppel,1967) has to include the effects of
turbulence and aerosol. Law (1963) deduced that there was a
considerable convection current when examining measured profiles of
ions. In addition, the nature of the turbulent transport of space charge is
dependent on its composition (Anderson,1977), whether it is particulate
or ionic.
solar radiation (W/m2)
Turbulent transfer of space charge in the atmospheric surface layer
J.F.Barlow and R.G.Harrison
Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading, RG6 6BB, U.K.
17
12:30
time (BST)
-0.50
-0.75
-1.00
time (BST)
Figure 3 b) gradient Richardson number for 20th August 1998. Low
values show near neutral stability.
Figure 3a) shows potential and solar radiation for a non fair weather day.
From the solar trace it can be seen there was considerable cloud, the
mean windspeed at 3m was 3.9 ms-1 and aerosol mass concentration was
moderate (M = 35μgm-3). Figure 3 b) shows that the gradient
Richardson number was small (|Ri|0.03) from 07:00 onwards. This
indicates that atmospheric conditions were near neutral meaning that
turbulence was predominantly mechanical in origin.
Figure 3b) 4 Hz measurements of potential gradient and
temperature between 11:30 and 12:30 on 12th August 1998. As the
cloud passes both temperature and potential gradient are markedly
reduced.
Figure 3b shows fast response measurements of temperature and
potential gradient made at 2.85m. The minimum in temperature occurs
roughly 4 minutes before the minimum in potential gradient. The
reduction in "noise" in the temperature trace shows that turbulence
strength was reduced considerably. The gradient Richardson number was
also observed to decrease during this time, indicating a change from
unstable to near neutral conditions. Although the cloud could have been
electrified, the reduction in the turbulent flux of surface charge
associated with the thermal effects of the cloud is considered much more
likely to have caused the potential gradient changes.
-0.75
-1.00
-1.25
-1.50
-1.75
-2.00
time (BST)
Figure 2b): gradient Richardson number for 11th August 1998, fair
weather conditions. Large negative values indicate strong
convection.
The potential decreases at all heights between 19:00 and 20:00 but later
increases. This behaviour was only observed when conditions at night
were not stable (Ri < 1). Under such conditions mechanical production
of turbulence is not supressed by buoyancy. Therefore the overnight
increase in potential is probably due to the turbulent mixing of space
charge.
References
Anderson, R.V. (1977) Atmospheric electricity in the real world in "Electrical
processes in Atmospheres",ed. H. Dolezalek and R. Reiter, Steinkopf Verlag,
Darmstadt
Harrison, R.G.(1997) An antenna electrometer system for atmospheric electrical
measurements, Rev. Sci. Ins., 68 (3), 1599-1603
Hoppel, W.A.(1967) Theory of the electrode effect, J.A.T.P., 29, 709-721
Law, J (1963) The ionisation of the atmosphere near the ground in fair weather,
Q.J.R.M.S., 89, 107-121
Lumley, J.L. and Panofsky, H.A. (1964) The structure of atmospheric turbulence,
John Wiley, New York.
Whitlock, W.S. and Chalmers, J.A.(1956) Short-period variations in the atmospheric
potential gradient, Q.J.R.M.S., 82, 325-336
Turbulent transfer of space charge in the atmospheric surface layer
J.F.Barlow and R.G.Harrison
Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading, RG6 6BB, U.K.