Search

Measurements of atmospheric electricity have been made at many sites over a long time, with the vertical Potential Gradient (PG) the most commonly observed quantity. In general, the PG responds to local influences from weather, aerosol effects on charge exchange, and variability in the global atmospheric electric circuit. Different methods have been used to classify PG data, for example through identifying days when conditions were considered relatively undisturbed, or by using meteorological information to identify days on which weather-related variability was negligible. Nevertheless, local effects can persist, especially in data obtained at continental sites. Hence, if long term changes in the global atmospheric electric circuit are to be investigated, the local effects need first to be reduced or, ideally, removed.Recent work has demonstrated a close relationship between the PG at some sites and ocean temperatures modulated by the El Niño Southern Oscillation, through the associated changes in the global atmospheric electric circuit ([1],[2], [3]). The expectation of such a relationship can be used to test methods of removing and reducing local effects in PG data. A method based on the Carnegie curve – the hourly variation known to be present in the global circuit – is discussed here. Through comparison of hourly PG data from a site with the Carnegie curve, outlier values lying beyond the usual range of global circuit changes can be identified and removed. The remaining data can then be used to construct new daily or monthly averages with reduced local variability, evaluated by comparison with global circuit changes associated with the El Niño Southern Oscillation. References[1] R.G. Harrison, K.A. Nicoll, M. Joshi, E. Hawkins: Empirical evidence for multidecadal scale Global Atmospheric Electric Circuit modulation by the El Niño-Southern Oscillation Environ Res Lett 17, 124048 (2022) https://iopscience.iop.org/article/10.1088/1748-9326/aca68c[2] N.N. Slyunyaev, N.V.I lin, , E.A. Mareev,.G. Price: A new link between El Nino - Southern Oscillation and atmospheric electricity, Environ. Res. Lett., 16, (2021) https://doi.org/10.1088/1748-9326/abe908 [3] R.G. Harrison, M. Joshi, K. Pascoe: Inferring convective responses to El Niño with atmospheric electricity measurements at Shetland Environ Res Lett 6 (2011) 044028 http://iopscience.iop.org/1748-9326/6/4/044028/

Ionisation in planetary atmospheres resulting from cosmic rays fragments atmospheric molecules resulting in the formation of free ions. The rate at which ions are produced varies with altitude and is determined by a combination of the cosmic ray flux and atmospheric density. The altitude at which this ion production rate peaks is known as the Pfotzer-Regener maximum which, on Earth, occurs at around 15-20 km. On Venus this maximum occurs at ~63 km, coinciding with the main cloud deck. This study investigates the effects enhanced ionisation may have on cloud droplets and their behaviour. Interactions between the ions produced and cloud droplets may have many consequences, including activation at lower saturation ratios, enhanced droplet coalescence and, for large charges, droplet breakup by Rayleigh instability.This work explores the effects of ionisation on water droplets in the laboratory and also simulates some of the conditions occurring in the clouds of Venus. The main element of the experimental apparatus is an acoustic levitator that can allow individual droplets to be electrically isolated and observed. Measurements are taken by a CCD camera and processed using LabView image acquisition software. The droplets can be subjected to enhanced ionisation from a corona source and perturbed by using a 10 kV/m electric field placed across the droplet causing it to be deflected relative to its charge. The principal findings on water droplets were that higher charge led to a slower evaporation rate; however, higher charge also led to increased incidence of Rayleigh explosions which were observed during several of the experiments. Overall, the effect of charge slowing evaporation did not lead to a longer droplet lifetime due to mass loss occurring from the periodic Rayleigh instabilities. In order to simulate conditions more like the clouds of Venus, sulphuric acid droplets were also examined. It was found that even very dilute sulphuric acid was extremely resistant to evaporation, suggesting that the clouds of Venus may have very long-lived droplet lifetimes. This has wide-reaching implications as cloud droplets on Venus have been suggested to act as a substrate for possible microbial life in the clouds.

Charge influences the properties of liquid droplets, such as their evaporation rates, hydrodynamic stability and sticking probabilities in droplet-droplet collisions. Introducing additional charge into an assembly of droplets therefore provides a possible method of influencing the droplet properties, and, in the case of a fog or cloud, a route to weather modification. The effect of charging on natural droplets has been investigated by releasing additional unipolar and bipolar ions into a natural surface fog, from both within the fog at the surface and above the fog, using an Uncrewed Aerial Vehicle (UAV). Droplet properties were monitored before and after the ion release using optical methods, together with the atmospheric electric field using a field mill. The atmospheric electric field measurements robustly demonstrate the present of the additional ions. Further, changes in the fog properties apparent when the additional ions are introduced are discussed.

Analysis of the variation of the potential gradient (PG) at ground level is important to monitor the global electric circuit and the different solar and geophysical phenomena affecting it. However, this is challenging since several factors (e.g., meteorological) produce perturbations in the potential gradient. For this reason, timeseries and spectral analysis of PG at several stations are required. In this work, for the first time we performe the spectral analysis of the potential gradient recorded at several sites located at Vostok, Concordia, Halley and Casleo (South Hemisphere), and Sodankyla and Reading (North Hemisphere). In order to find the main periodicities and how the amplitude of those periods change as a function of time we use the Lomb-Scargle Periodogram and the Wavelet Transform, respectively. For all PG sites the periodicities of 0.5, 1, ~180 and 365-day were found. It was also found evidence of the ~27- and ~45-day periods. Further analysis using the cross-wavelet transform for PG versus cosmic rays, PG versus Madden-Julian Oscillation index, and PG versus meteorological parameters, shows that the 27- and 45-day periods are likely related to the solar rotation and Madden- Julian Oscillation, respectively. Moreover, for the 27-day period we found that the relationship is stronger during the occurrences of co-rotating interaction regions.

The occurrence and characteristics of rainfall events in arid and water scarce regions are of great interest to many, as it is vital to understand the efficient use of this finite resource, for example in terms of water management, agriculture, irrigation, and domestic food security. Fundamental to this is understanding the numerous environmental aspects that affect the generation and persistence of rain. These include the presence of cloud droplets, activation and growth processes, temperature and relative humidity of the within and below cloud regions, and the cloud base height. Not only must what causes rainfall to be initiated be understood, but also the conditions that allow that rain to reach the surface. This work examines the conditions required for a successful rain event (i.e. one in which rainfall reaches the ground) to occur in the arid desert region of Al Ain, in the United Arab Emirates (UAE) (annual rainfall 76mm). The high surface temperatures and dry air mean that rain events at Al Ain commonly occur as virga, as the rain droplets cannot survive evaporation under the local atmospheric conditions. Here we examine individual rainfall events using backscatter data from a laser ceilometer, in conjunction with C-band radar data, to further understand the processes required for successful rain generation. During the 2 year period of study, there was a total of 57.5 hours of rain (rainfall 0.5% of the time), with a total of 105 rainfall events. Here we examine the effect on rainfall of (a) the initial size of the droplets falling from the cloud base, which must be large enough to survive the fall to the surface; and (b) the effect of the below cloud thermodynamic profile on the evaporation of the droplet (which also depends on the height of the cloud base). Preliminary conclusions find that smaller droplets, higher cloud bases, smaller cloud depths, and lower cloud base temperatures all favour the occurrence of a rain event terminating as virga before it reaches the surface. Understanding the details of why many potential rainfall events don’t reach the surface can ultimately lead to the more efficient implementation of rainfall enhancing measures such as cloud seeding.

Countries in arid and desert climates that have small amounts of rainfall each year use cloud seeding techniques to enhance the little rainfall that is present. Typically, this is achieved by seeding the cloud with hygroscopic nuclei to increase the rainfall. A possible alternative method is to inject the cloud with electric charge, which has been shown in models to alter the droplet size and distribution and influence rainfall properties.Here, in-situ observations of the electrical and optical properties of clouds are described from a desert site. These are used to inform droplet growth models. For this, a yearlong campaign, during which 10 weather balloons carrying electric charge and optical sensors were launched through fog layers from Abu Dhabi airport, United Arab Emirates. Here we present 2 case studies. The first is a clear air ascent comparison between the desert site at Abu Dhabi and a temperate site in northern Finland. The second is a fog comparison between Abu Dhabi and a temperate site in the United KingdomThe results show that the fogs in Abu Dhabi are highly charged with a charge density of 0.1-1 nC m-3 as opposed to the charge densities of fogs in Northern Hemisphere temperate regions which have a typical charge density of 10 pC m-3. The droplet concentration in the Abu Dhabi fog case study is significantly smaller, approximately 150 cm-3 as opposed to droplet concentrations of 300-400 cm-3 in fog over a temperate site.The results suggest that dust contributes strongly to the atmospheric electrical conditions in the UAE region, due to charging of the dust tribo-electrically. This dust charge may also affect the droplet distribution within the fog. These new measurements of the vertical profile of charge through fog layers in desert climates will be used to improve understanding in droplet growth models.

Cloud processes leading to rainfall generation are suspected to be influenced by droplet charge. Droplet charging is abundant, and even in layer clouds, charging of droplets readily occurs at the horizontal cloud-air boundary. Droplet charging in such circumstances is proportional to the vertical current driven through the cloud by the global electric circuit. Small global circuit variations from natural influences, such as solar modulation of cosmic rays can be used to investigate this, but an alternative is presented by artificial introduction of ionisation. The atmospheric nuclear weapons test period, which reached its peak 1962-1964, caused exceptional anthropogenic disturbance to the global circuit, through the increased ionisation from steady sedimentation of stratospheric radioactive debris.Measurements of the vertical current Jz made at Kew Observatory near London (51°28′N, 0°19′W) were several times greater than normal during 1962-1964, as a result of the widespread extra ionisation in the lower atmosphere. At Lerwick, Shetland (60°09′N, 1°08′W) where deposition of radioactive material occurred, the atmospheric electrical parameters were strongly affected by the enhanced ionisation. To investigate tropospheric ionisation effects on local cloud processes, rainfall days at Lerwick in 1962-64 have been analysed by considering reduced and enhanced ionisation periods. During the enhanced ionisation, the Lerwick rainfall distribution shifted towards heavier rainfall and is significantly different from the rainfall distribution for reduced ionisation days; the Lerwick cloud was also significantly optically thicker during the enhanced ionisation. This contrasts with other years of the Kew record, when Jz was relatively undisturbed. Whilst the ionisation conditions of 1962-64 were exceptional, controlled methods of enhancing tropospheric ionisation by non-radioactive means - such as corona emission - may nevertheless be promising for local rainfall modification, or even geoengineering of cloud properties.

Characterisation of the vertical variation in the atmospheric electric field has been made for many decades, but normally in an ad-hoc manner, using instrumented weather balloons or manned aircraft, which are expensive to fly. Such vertical measurements are required to measure the ionospheric potential and to characterise electric fields with clouds (both thunderstorm and non thunderstorm clouds) to understand the charging processes within them. Advances in electronics and battery technology has meant that small Unmanned Aerial Vehicles (UAVs) have now become available as a new science platform. These measurement platforms address many of the problems associated with manned aircraft while allowing in-situ measurements with an increased level of control and repeatability when compared to weather balloons. Despite their potential advantages, one of the main challenges to using UAVs for atmospheric electricity research is the lack of small, lightweight sensors which are commercially available. To overcome this barrier, this work describes the development of a lightweight, miniaturised electric field sensor to be integrated with a small UAV (The sensor has been designed to allow measurements of the electric field intensities typical of non-thunderstorm low altitude (

A tethered balloon system (TBS) has been developed and is being operated by Sandia National Laboratories (SNL) on behalf of the U.S. Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) User Facility in order to collect in situ atmospheric measurements within mixed-phase Arctic clouds. Periodic tethered balloon flights have been conducted since 2015 within restricted airspace at ARM’s Advanced Mobile Facility 3 (AMF3) in Oliktok Point, Alaska, as part of the AALCO (Aerial Assessment of Liquid in Clouds at Oliktok), ERASMUS (Evaluation of Routine Atmospheric Sounding Measurements using Unmanned Systems), and POPEYE (Profiling at Oliktok Point to Enhance YOPP Experiments) field campaigns. The tethered balloon system uses helium-filled 34 m3 helikites and 79 and 104 m3 aerostats to suspend instrumentation that is used to measure aerosol particle size distributions, temperature, horizontal wind, pressure, relative humidity, turbulence, and cloud particle properties and to calibrate ground-based remote sensing instruments. Supercooled liquid water content (SLWC) sondes using the vibrating wire principle, developed by Anasphere Inc., were operated at Oliktok Point at multiple altitudes on the TBS within mixed-phase clouds for over 200 hours Sonde-collected SLWC data were compared with liquid water content derived from a microwave radiometer, Ka-band ARM Zenith radar, and ceilometer at the AMF3, as well as liquid water content derived from AMF3 radiosonde flights. The in situ data collected by the Anasphere sensors were also compared with data collected simultaneously by an alternative SLWC sensor developed at the University of Reading, UK; both vibrating wire instruments were typically observed to shed their ice quickly upon exiting the cloud or reaching maximum ice loading. Tethered balloon fiber optic distributed temperature sensing measurements were also compared with AMF3 radiosonde temperature measurements. Combined, the results indicate that TBS distributed temperature sensing and supercooled liquid water measurements are in reasonably good agreement with remote-sensing and radiosonde-based measurements of both properties. From these measurements and sensor evaluations, tethered balloon flights are shown to offer an effective method of collecting data to inform and constrain numerical models, calibrate and validate remote sensing instruments, and characterize the flight environment of unmanned aircraft, circumventing the difficulties of in-cloud unmanned aircraft flights such as limited flight time and in-flight icing.