Measuring electrical properties of the lower troposphere using 1 enhanced meteorological radiosondes 2 3

In atmospheric science, measurements above the surface have long been obtained by carrying instrument 7 packages, radiosondes, aloft using balloons. Whilst occasionally used for research, most radiosondes around one 8 thousand are released daily only generate data for routine weather forecasting. If meteorological radiosondes are 9 modified to carry additional sensors, of either mass-produced commercial heritage or designed for a specific scientific 10 application, a wide range of new measurements becomes possible. A programme to develop add-on devices for 11 standard radiosondes, which retains the core meteorological use, is described here. Combining diverse sensors on a 12 single radiosonde helps interpretation of findings, and yields economy of equipment, consumables and effort. A self13 configuring system has been developed to allow different sensors to be easily combined, enhancing existing weather 14 balloons and providing an emergency monitoring capability for airborne hazards. This research programme was 15 originally pursued to investigate electrical properties of extensive layer clouds, and has expanded to include a wide 16 range of balloon-carried sensors for solar radiation, cloud, turbulence, volcanic ash, radioactivity and space weather. 17 For the cloud charge application, multiple soundings in both hemispheres have established that charging at the 18 boundaries of extensive layer clouds is widespread, and likely to be a global phenomenon. This paper summarises the 19 Christiaan Huygens medal lecture given at the 2021 European Geoscience Union meeting. 20


24
This paper is based on material presented in my Christiaan Huygens medal lecture at the 2021 meeting of the European also generate reference electric fields internally for calibration (Harrison and Marlton, 2020). The explanation for the positive electric potential consistently observed near the surface in fair weather is found 217 through the global atmospheric electric circuit, originally postulated by C.T.R. Wilson (1921Wilson ( , 1929. The global circuit 218 allows currents to flow from generating regions (driven by thunderstorms, shower clouds and vertical charge 219 exchange), to fair weather regions, through which the current passes to complete the circuit. The conduction from 220 generator regions occurs through the more strongly ionised parts of the atmosphere, and through the earth's surface,

221
which, compared with the atmosphere, is a relatively good conductor. As indicated above, the concept of the fair 222 weather branch of the circuit is well-established.

245
Conventionally, the electric Potential Gradient (PG), has been recorded at the surface in fair weather rather than the 246 electric field 9 . Under a persistent extensive layer cloud, the PG is found to be suppressed when the cloud base height 247 is lower than about 1000 m (Harrison et al, 2017a). By determining the cloud height using an optical time of flight 248 measurement, as provided by a laser ceilometer, variations of the cloud base height and PG can be compared. Figure   249 9. The PG is positive in fair weather. Although the electric field has the same magnitude as the PG, it has the opposite sign; positive charge moves downwards in fair weather.   return the measurements by radio. These devices, originally known as radiometeorographs, were developed in the 259 1920s to replace mechanical recording devices (e.g. Idrac and Bureau, 1927) and rapidly found widespread use 260 (Wenstrom, 1934). Commercial devices followed, notably developed by Vilho Väisälä (Väisälä, 1932), whose name 261 is carried by the Finnish company he founded.

263
Radiosondes have a well-established global role in obtaining routine meteorological data, and can, at some sites at

289
Reviewing previous approaches illustrates the range of different technologies which have been used, either by adapting 290 existing meteorological devices or, in some cases (e.g. the Lebedev instruments), developing a custom radiosonde. A 291 disadvantage of adaptation is that one or more channels of meteorological data may be lost to in providing telemetry 292 bandwidth for a new quantity. For applications which need the meteorological data, this is clearly undesirable. Instead,

293
if the routine radiosondes used in operational meteorology are harnessed to carry additional sensors without losing 294 their core meteorological data, a much greater opportunity for new measurements presents itself, allowing access to 295 the existing global launch and reception infrastructure of more than 1000 soundings daily. This has led to a new 296 strategy of making "piggy-back" systems which provide additional measurement capability on standard 297 meteorological radiosondes, whilst preserving the existing meteorological data. Furthermore, if the add-on devices 298 are made straightforward to use, more launch opportunities can be obtained worldwide from those familiar with using 299 meteorological radiosondes routinely, but who are not specialists in the research quantity sought.

301
The associated programme of work at Reading has mostly built on the Vaisala range of radiosondes, largely because 302 the related equipment was already available at the University. Many other commercial radiosondes are available 303 internationally, and the principles developed in using a programmable interface to support a range of sensors and 304 communicate with the radiosonde are very flexible, and amenable to other commercial systems too.

316
The first experiments were with the analogue RS80 radiosonde. The RS80 used a sequence of audio tones to send its 317 PTU measurements, and it also provided an additional channel to relay LORAN (a LOng RAnge Navigation system 318 using very low frequency) positioning signals, later entirely superseded by satellite GPS (Global Positioning System).

319
As LORAN was not implemented in the Reading Meteorology Department's radiosonde system, this offered a spare 320 channel to send additional data, through an analogue voltage-to-frequency converter varying within a few percent of 321 100 kHz. This signal was recovered as a voltage, by passing the modulated 100 kHz signal to a phase-locked loop

322
(PLL), and the tracking voltage logged with a 12bit analogue to digital converter. Time stamping of the radiosonde 323 data and PLL data on separate logging computers allowed the two different files to be aligned. Regular switching to 324 full scale at the radiosonde end was also applied to allow correction for non-linearity and thermal drift.

361
Some of the sensors devised for various atmospheric measurements, motivated originally by the cloud charge 362 application, are now described.

365
Measurement of atmospheric charge using a radiosonde requires a sensing electrode and electrometer able to measure 366 the charge collected or induced, with some sort of data telemetry as described above. The electric potential of the 367 radiosonde changes as it rises through the atmosphere, but more slowly than that of a small sensing electrode, causing 368 a current to flow transiently which can be measured. One electrode configuration which has some simplicity, suitable 369 for large electric fields, is a corona needle. Figure 8a shows a corona sonde from 1998, in which a needle electrode 370 was connected to a current amplifier, following the electronic principles of section 2.2. It is not, however, a convenient 371 arrangement to launch, not least because of the proximity of the needle to an inflated rubber balloon. Rounded 372 electrodes are preferable, with, the connection between the electrode and the electrometer as short as possible to reduce 373 leakage. A novel capsule well suited to this application is found within a "Kinder Egg", housing a self-assembly toy 374 contained within the confectionery egg. This capsule is manufactured from hydrophobic material, is strong enough to 375 resist modest impacts, and is water-tight, offering some protection to any electronics mounted within it.   Figure 8c shows a later version of the charge detector, using smaller spherical electrodes, again a mass-produced item, However, the smaller size led to more difficulties with saturation. It was found more satisfactory to measure the current 395 flowing, arising from changes in the electrical environment through which the sensor passed (Nicoll, 2013). In Figure   396 8c, two sensors were connected to electrometers with different ranges, to allow different cloud charges to be 397 investigated, as the optimum range had to be established empirically. Figure 8d shows The Eyjafjallajökull plume measurements were made following an urgent request from the UK Government, for which 577 a special expedition was undertaken (Figure 13a, b), using the devices designed for the work in Cape Verde ( Figure   578 13c). The sounding demonstrated the presence of small particles aloft, which was not associated with cloud ( Figure   579 13d, e). Due to the haste 14 , the charge sensors used in Cape Verde were not removed. This was fortuitous, as it allowed 580 charge in the plume to be observed (Figure 13f), which, given the distance from the eruption in Iceland, would have 581 been generated during the atmospheric transport of the plume. approach for in situ sensing is to collect the ash and determine the mass concentration directly. One method is to use 592 a vibrating rod method (or "oscillating microbalance"), as also used for supercooled water collection. As the mass 593 accreted on the rod increases, its natural oscillation frequency decreases. With accurate frequency measurements and 594 knowledge of the collection efficiency, the concentration encountered can be found.

598
In the ash collection mode, adhesive is first applied to the vibrating rod. Figure 14 shows the effect of introducing 599 pumice into a region monitored by the optical cloud sensor, also allowing collection by the rod of the oscillating 600 microbalance. Clearly, physical collection will require more material than for optical detection, but, as impaction is 601 the process which presents the hazard to aircraft engines, the application to airspace security is much more direct.