A self-calibrating electrometer for atmospheric charge measurements from a balloon platform

Charged aerosol particles and water droplets are abundant throughout the lower atmosphere, and may influence interactions between small cloud droplets. This note describes a small, disposable sensor for the measurement of charge in non-thunderstorm cloud, which is an improvement of an earlier sensor [K. A. Nicoll and R. G. Harrison, Rev. Sci. Instrum. 80, 014501 (2009)]. The sensor utilizes a self-calibrating current measurement method. It is designed for use on a free balloon platform alongside a standard meteorological radiosonde, measuring currents from 2 fA to 15 pA and is stable to within 5 fA over a temperature range of 5 °C to −60 °C. During a balloon flight with the charge sensor through a stratocumulus cloud, charge layers up to 40 pC m−3 were detected on the cloud edges

Remote sensing of cloud base charge

Layer clouds are abundant in the Earth's atmosphere. Such clouds do not become sufficiently strongly charged to generate lightning, but they show weak charging along the upper and lower cloud boundaries where there is a conductivity transition. Cloud edge charging has recently been observed using balloon-carried electrometers. Measurement of cloud boundary charging without balloons is shown to be possible here for low altitude (<1km) charged cloud bases, through combining their effect on the surface electric field with laser time of flight cloud base height measurements, and the application of simple electrostatic models.Comment: Proceedings of the Electrostatics Society of America conference, Ottawa, June 201

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

Programmable data acquisition system for research measurements from meteorological radiosondes

A programmable data acquisition system to allow novel use of meteorological radiosondes for atmospheric science measurements is described. In its basic form it supports four analogue inputs at 16 bit resolution, and up to two further inputs at lower resolution configurable instead for digital instruments. It also provides multiple instrument power supplies (+8V, +16V, +5V and -8V) from the 9V radiosonde battery. During a balloon flight encountering air temperatures from +17°C to -66°C, the worst case voltage drift in the 5V unipolar digitisation circuitry was 20mV. The system liberates a new range of low cost atmospheric research measurements, by utilising radiosondes routinely launched internationally for weather forecasting purposes. No additional receiving equipment is required. Comparisons between the specially instrumented and standard meteorological radiosondes show negligible effect of the additional instrumentation on the standard meteorological data

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 ±10 fA to ±3 μA and across ±20 ◦C show it has an excellent logarithmic response over six orders of magnitude from 1 pA to 1 μA 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 ∼0.5 μA

Coordinated weather balloon solar radiation measurements during a solar eclipse

Solar eclipses provide a rapidly changing solar radiation environment. These changes can be studied using simple photodiode sensors, if the radiation reaching the sensors is unaffected by cloud.Transporting the sensors aloft using standard meteorological instrument packages modified to carry extra sensors, provides one promising but hitherto unexploited possibility for making solar eclipse radiation measurements. For the 20th March 2015 solar eclipse, a coordinated campaign of balloon-carried solar radiation measurements was undertaken from Reading (51.44N, 0.94W), Lerwick (60.15N, 1.13W) and Reykjavik (64.13N, 21.90W), straddling the path of the eclipse.The balloons reached sufficient altitude at the eclipse time for eclipse-induced variations in solar radiation and solar limb darkening to be measured above cloud. Because the sensor platforms were free to swing, techniques have been evaluated to correct the measurements for their changing orientation. In the swing-averaged technique, the mean value across a set of swings was used to approximate the radiation falling on a horizontal surface; in the swing-maximum technique, the direct beam was estimated by assuming the sensing surface becomes normal to the solar beam direction at a maximum swing. Both approaches, essentially independent,give values that agree with theoretical expectations for the eclipse-induced radiation changes

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 days’ 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

A miniature oscillating microbalance for sampling ice and volcanic ash from a small airborne platform

A lightweight and low power oscillating microbalance for in situ sampling of atmospheric ice and volcanic ash is described, for airborne platforms. Using a freely-exposed collecting wire fixed at only one end to a piezo transducer, the instrument collects airborne material. Accumulated mass is determined from change in natural frequency of the wire. The piezo transducer is used in a dual mode to both drive and detect the oscillation. Three independent frequency measurement techniques are implemented with an on-board microcontroller: a frequency sweep, a Fourier spectral method, and a phase-locked loop. These showed agreement to ±0.3 Hz for a 0.5 mm diameter collecting wire 120 mm long, flown to 19 km altitude on a weather balloon. The instrument is well suited to disposable use with meteorological radiosondes, to provide high resolution vertical profiles of mass concentration

A self-calibrating wide range electrometer for in-cloud measurements

Charge is observed in clouds of all forms, which influences their development and properties. In-cloud charge measurements require a wide dynamic range instrument, extending from charge in aerosols and dusts to that present in thunderstorms. Unexpectedly large charge densities (>200 pCm-3) have recently been detected in layer clouds using balloon-carried linear electrometers. These, however, lead to instrument saturation if sufficient sensitivity for aerosol and droplet charge is maintained. Logarithmic electrometers provide an alternative, but suffer strong non-linear thermal effects. This is a limitation for balloon-carried instruments which encounter temperature changes up to ~100 °C, as full thermal compensation requires complexity inappropriate for disposable devices. Here, a novel hybrid system is described, combining linear and logarithmic electrometers to provide extended dynamic range (±50 pA), employing the negligible (±4%) total temperature drift of the linear device to provide in situ calibration of the logarithmic device. This combination opens up new measurement opportunities for charge in clouds, dusts and aerosols