23 research outputs found
Electrical charging of ash in Icelandic volcanic plumes
The existence of volcanic lightning and alteration of the atmospheric
potential gradient in the vicinity of near-vent volcanic plumes provides strong
evidence for the charging of volcanic ash. More subtle electrical effects are
also visible in balloon soundings of distal volcanic plumes. Near the vent,
some proposed charging mechanisms are fractoemission, triboelectrification, and
the so-called "dirty thunderstorm" mechanism, which is where ash and convective
clouds interact electrically to enhance charging. Distant from the vent, a
self-charging mechanism, probably triboelectrification, has been suggested to
explain the sustained low levels of charge observed on a distal plume. Recent
research by Houghton et al. (2013) linked the self-charging of volcanic ash to
the properties of the particle size distribution, observing that a highly
polydisperse ash distribution would charge more effectively than a monodisperse
one. Natural radioactivity in some volcanic ash could also contribute to
self-charging of volcanic plumes. Here we present laboratory measurements of
particle size distributions, triboelectrification and radioactivity in ash
samples from the Gr\'{i}msv\"{o}tn and Eyjafjallaj\"{o}kull volcanic eruptions
in 2011 and 2010 respectively, and discuss the implications of our findings.Comment: XV Conference on Atmospheric Electricity, 15-20 June 2014, Norman,
Oklahoma, US
Droplet charging in stratiform clouds
The role of droplet charge in stratiform clouds is one of the least well
understood areas in cloud microphysics and is thought to affect cloud radiative
and precipitation processes. Layer clouds cover a large proportion of the
Earth's surface and are important in regulating the planetary radiation budget.
Using a new remote sensing method developed at our University Observatory, we
demonstrate that charge in the base of stratiform clouds is typically of
negative polarity, as expected from theory considering the vertical current
flow into and out of the cloud. More detailed vertical charge structure of
layer clouds can be found using balloon-carried instruments. Our previous
research using in situ balloon observations has demonstrated that, on average,
the bulk charge polarity and location agrees with theoretical predictions of
positive charge at the upper edge and negative charge at the lower edge. Here
we present optical and charge measurements of droplets from a variety of
stratiform clouds, demonstrating the typical variability which is observed.Comment: XVI International Conference on Atmospheric Electricity, 17-22 June
2018, Nara city, Nara, Japa
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Atmospheric point discharge current measurements using a temperature-compensated logarithmic current amplifier
Measurements of atmospheric corona currents have been made for over 100 years to indicate the atmospheric electric field. Corona currents vary substantially, in polarity and in magnitude. The instrument described here uses a sharp point sensor connected to a temperature compensated bi-polar
logarithmic current amplifier. Calibrations over a range of currents from Ā±10 fA to Ā±3 Ī¼A and across Ā±20 ā¦C show it has an excellent logarithmic response over six orders of magnitude from 1 pA to 1 Ī¼A in both polarities for the range of atmospheric temperatures likely to be encountered in the southern UK. Comparison with atmospheric electric field measurements during disturbed weather confirms
that bipolar electric fields induce corona currents of corresponding sign, with magnitudes ā¼0.5 Ī¼A
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Evaluating stratiform cloud base charge remotely
Stratiform clouds acquire charge at their upper and lower horizontal boundaries due to vertical current flow in the global electric circuit. Cloud charge is expected to influence microphysical processes, but understanding is restricted by the infrequent in-situ measurements available. For stratiform cloud bases below 1 km in altitude, the cloud base charge modifies the surface electric field beneath, allowing a new method of remote determination. Combining continuous cloud height data during 2015-2016 from a laser ceilometer with electric field mill data, cloud base charge is derived using a horizontal charged disk model. The median daily cloud base charge density found was -0.86 nCm-2 from 43 daysā data. This is consistent with a uniformly charged region ~ 40 m thick at the cloud base, now confirming that negative cloud base charge is a common feature of terrestrial layer clouds. This technique can also be applied to planetary atmospheres and volcanic plumes
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Recent advances in global electric circuit coupling between the space environment and the troposphere
The global atmospheric electric circuit is driven by thunderstorms and electrified rain/shower clouds and is also influenced by energetic charged particles from space. The global circuit maintains the ionosphere as an equipotential atā¼+250 kV with respect to the good conducting Earth (both land and oceans). Its āloadā is the fair weather atmosphere and semi-fair weather atmosphere at large distances from the disturbed weather āgeneratorā regions. The main solar-terrestrial (or space weather) influence on the global circuit arises from spatially and temporally varying fluxes of galactic cosmic rays (GCRs) and energetic electrons precipitating from the magnetosphere. All components of the circuit exhibit much variability in both space and time. Global circuit variations between solar maximum and solar minimum are considered together with Forbush decrease and solar flare effects. The variability in ion concentration and vertical current flow are considered in terms of radiative effects in the troposphere, through infra-red absorption, and cloud effects, in particular possible cloud microphysical effects from charging at layer cloud edges. The paper identifies future research areas in relation to Task Group 4 of the Climate and Weather of the Sun-Earth System (CAWSES-II) programme
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Note: A balloon-borne accelerometer technique for measuring atmospheric turbulence
A weather balloon and its suspended instrument package behave like a pendulum with a moving pivot. This dynamical system is exploited here for the detection of atmospheric turbulence. By adding an accelerometer to the instrument package, the size of the swings induced by atmospheric turbulence can be measured. In test flights, strong turbulence has induced accelerations greater than 5g, where g = 9.81 m sā2. Calibration of the accelerometer data with a vertically orientated lidar has allowed eddy dissipation rate values of between 10ā3 and 10ā2 m2 sā3 to be derived from the accelerometer data. The novel use of a whole weather balloon and its adapted instrument package can be used as a new instrument to make standardized in situ measurements of turbulence
Shear-induced electrical changes in the base of thin layer cloud
Charging of upper and lower horizontal boundaries of extensive layer clouds results from current flow in the global electric circuit. Layer-cloud charge accumulation has previously been considered a solely electrostatic phenomenon, but it does not occur in isolation from meteorological processes, which can transport charge. Thin layer clouds provide special circumstances for investigating this dynamical charge transport, as disruption at the cloud-top may reach the cloud base, observable from the surface. Here, a thin (~300 m) persistent layer-cloud with base at 300 m and strong wind shear at cloud-top was observed to generate strongly correlated fluctuations in cloud base height, optical thickness and surface electric Potential Gradient (PG) beneath. PG changes are identified to precede the cloud base fluctuations by 2 minutes, consistent with shear-induced cloud-top electrical changes followed by cloud base changes. These observations demonstrate, for the first time, dynamically driven modification of charge within a layer-cloud. Even in weakly charged layer-clouds, redistribution of charge will modify local electric fields within the cloud and the collisional behaviour of interacting charged cloud droplets. Local field intensification may also explain previously observed electrostatic discharges in warm clouds
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Precipitation modification by ionization
Rainfall is hypothesised to be influenced by droplet charge, which is related to the global circuit current flowing through clouds. This is tested through examining a major global circuit current increase following release of artificial radioactivity. Significant changes occurred in daily rainfall distribution in the Shetland Islands, away from pollution. Daily rainfall changed by 24%, and local cloud optically thickened, within the nuclear weapons test period. This supports expectations of electrically induced microphysical changes in liquid water clouds from additional ionisation
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Saharan dust plume charging observed over the UK
A plume of Saharan dust and Iberian smoke was carried across the southern UK on 16th October 2017, entrained into an Atlantic cyclone which had originated as Hurricane Ophelia. The dust plume aloft was widely noticed as it was sufficiently dense to redden the visual appearance of the sun. Time series of backscatter from ceilometers at Reading and Chilbolton show two plumes: one carried upwards to 2.5 km, and another below 800m into the boundary layer, with a clear slot between.
Steady descent of particles at about 50 cmsā1 continued throughout the morning, and coarse mode particles reached the surface. Plumes containing dust are frequently observed to be strongly charged,often through frictional effects. This plume passed over atmospheric electric field sensors at Bristol, Chilbolton and Reading. Consistent measurements at these three sites indicated negative plume charge. The lower edge plume charge density was (ā8.0Ā±3.3) nCmā2, which is several times greater than that typical for stratiform water clouds, implying an active in situ charge generation mechanism
such as turbulent triboelectrification. A meteorological radiosonde measuring temperature and humidity was launched into the plume at 1412 UTC, specially instrumented with charge and turbulence sensors. This detected charge in the boundary layer and in the upper plume region, and strong turbulent mixing was observed throughout the atmosphereās lowest 4 km. The clear slot
region, through which particles sedimented, was anomalously dry compared with modelled values, with water clouds forming intermittently in the air beneath. Electrical aspects of dust should be included in numerical models, particularly the charge-related effects on cloud microphysical properties, to accurately represent particle behaviour and transport
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Measuring global signals in the potential gradient at high latitude sites
Previous research has shown that the study of the global electrical circuit can be relevant to climate change studies, and this can be done through measurements of the potential gradient near the surface in fair weather conditions. However, potential gradient measurements can be highly variable due to different local effects (e.g., pollution, convective processes). In order to try to minimize these effects, potential gradient measurements can be performed at remote locations where anthropogenic influences are small. In this work we present potential gradient measurements from five stations at high latitudes in the Southern and Northern Hemisphere. This is the first description of new datasets from Halley, Antarctica; and Sodankyla, Finland. The effect of the polar cap ionospheric potential can be significant at some polar stations and detailed analysis performed here demonstrates a negligible effect on the surface potential gradient at Halley and Sodankyla. New criteria for determination of fair weather conditions at snow covered sites is also reported, demonstrating that wind speeds as low as 3m/s can loft snow particles, and that the fetch of the measurement site is an important factor in determining this threshold wind speed. Daily and seasonal analysis of the potential gradient in fair weather conditions shows great agreement with the āuniversalā Carnegie curve of the global electric circuit, particularly at Halley. This demonstrates that high latitude sites, at which the magnetic and solar influences are often present, can also provide globally representative measurement sites for study of the global electric circuit