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

Aplin, Karen

Harrison, R Giles

Nicoll, Keri

English

arXiv

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 

Nicoll, Keri A.

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                          Harrison, R. G., Nicoll, K. A., & Aplin, K. (2017). Remote sensing of cloudbase charge. In Proceedings of the 2017 Annual Meeting of the ElectrostaticsSociety of America Electrostatics Society of America.Peer reviewed versionLink to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-termsProc. 2017 Annual Meeting of the Electrostatics Society of America 1 Remote sensing of cloud base charge   R. Giles Harrison*, Keri A. Nicoll* and Karen L. Aplin+  *Department of Meteorology University of Reading, UK +Department of Physics, University of Oxford, UK e-mail: r.g.harrison@reading.ac.uk Abstract—Layer clouds are abundant in the Earth’s atmosphere. Such clouds do not be-come 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. I. INTRODUCTION Thunderstorms are widely appreciated to be strongly electrified clouds, but it is less widely known that other clouds also carry charge, although to a lesser extent. Horizontal layer clouds containing liquid water are abundant across the planet and affect the balance of incoming and outgoing radiation which determines atmospheric temperature. Even without the mixed phases of water and turbulent motions which are key ingredients of thunderstorm electrification, liquid water layer clouds become charged at their upper and lower boundaries. This is due to the vertical current flow always present in the atmos-phere through the cloud from the global atmospheric electric circuit, which is also the source of the atmospheric electric field always observed in cloudless fair weather condi-tions. The importance of droplet charging in such clouds comes from theoretical consid-erations indicating an enhancement of droplet coalescence, and may ultimately shorten the timescale to rain formation. Layer cloud edge charging results from the interaction of the vertical current with the vertical conductivity transition at the cloud edge. The current flowing is essentially a global parameter, and the cloud edge properties a local meteorological parameter. Charg-ing of the upper and lower horizontal edges of extensive layer clouds has been observed at sites in both hemispheres using balloon-carried electrometers, demonstrating negative charge in the cloud base and positive charge at the cloud top [1]. Opportunities for such measurements are however limited, and each balloon sounding provides only a single pass through a cloud.  For low altitude clouds, the charge in the cloud base can affect the electric field meas-ured at the surface. If the distance from the surface to the cloud is known, the charge in the cloud base can be determined. Such a remote sensing approach for cloud base charge Proc. 2017 Annual Meeting of the Electrostatics Society of America 2 is evaluated here, using cloud and surface electric field measurements obtained at the University of Reading Atmospheric Observatory. II. ATMOSPHERIC MEASUREMENTS Surface measurements of the vertical atmospheric electric field Ez, are, by convention, recorded as the Potential Gradient (-Ez) which is positive in fair weather. At Reading University Atmospheric Observatory, the Potential Gradient (PG) is measured continu-ously using an all-weather field mill (JCI model JCI131), together with cloud base height using an infra-red laser time-of-flight ceilometer (Vaisala model CL31). Geometrical dis-tortions on the electric field measured by the field mill are corrected to that of an open site by comparison with measurements from a stretched wire antenna, which is long-established standard method [2, 3]. The ceilometer retrieves the amount of backscattered power at different time intervals after a pulse of laser light is emitted upwards, allowing a vertical profile of the atmosphere’s properties beneath a cloud to be imaged: the greatest backscatter is generated by the cloud droplets, which is used to determine the position of the cloud edge and the cloud base height (see fig 1a).  0400800hour (UTC)cloud base (m)0 6 12 18 2404080PG (Vm1)0 6 12 18 24(a)(b)(c)our UTC Fig. 1. Time series (plotted against hour of day) from 20th February 2016 at Reading, during persistent cloud with cloud base below 1000 m.  (a) Backscatter retrieved using a CL31 ceilometer, (b) cloud base height de-rived by the ceilometer algorithm from the backscatter and (c) Potential Gradient (PG) measured at the surface using a JCI131 field mill.  (The cloud base height and PG were recorded as 5 minute average values.) Proc. 2017 Annual Meeting of the Electrostatics Society of America 3 For extensive and persistent layer clouds having cloud base heights below about 1 km, the PG and cloud base height can be well correlated (fig 1b and c). This because the changes in height occur more slowly than the timescale for the cloud base charge to change, and therefore the electrostatic effect is similar to that of a fixed charge being moved towards and away from the measuring sensor. Changes in cloud base height can result from a variety of causes, but commonly they are associated with variations in the daily cycle of solar heating, with timescales of hours. In comparison, the timescale of the cloud edge charging is relatively rapid, and typically of order 200 to 500s [4]. III. ELECTROSTATIC CONSIDERATIONS The correlation between fig 1b and fig 1c can be understood in terms of the induced ef-fect of the steady cloud base charge. Consequently, as the cloud base lowers, the surface PG will be suppressed by the negative charge in the cloud base. Conversely, as the cloud base rises, the suppressing effect on the PG is reduced, until the cloud base charge is suf-ficiently distant for no effect to be observable and the PG returns to its undisturbed fair weather value. This response can be modelled in the simplest case by assuming a single fixed charge of some appropriate geometry in the cloud base. Several possible represen-tations can be considered, such as a point charge, line charge and charged disk (fig 2).  HRFcQDHFcQLHFcQP(a) (b) (c)  Fig 2. Representation of an extensive layer cloud containing cloud base charge inducing a potential gradient Fc at the surface. The cloud base charge is alternately regarded as (a) a point charge QP at height H, (b) part of an infinite line charge QL at H, and (c) a disk charge at H of charge density QD with finite radius R. The reduction Fc of the surface PG associated with the cloud base charge at height H is given in the point charge representation by 204 HQF Pc                  (1), in the line charge representation by HQF Lc02                 (2)  and the disk charge representation by    2/122012 RHHQF Dc               (3).  The corresponding point, line and disk charges are given by QP, QL and QD respective-ly, with R the radius of the disk charge. 0 is the permittivity of free space, which is con-Proc. 2017 Annual Meeting of the Electrostatics Society of America 4 sidered a good approximation for the medium, as, even in cloudy air, the fraction by mass of water vapour is small. IV. RESULTS Plotting the ceilometer measurements of cloud base height against those of the simul-taneously obtained Potential Gradient allows the electrostatic effect to be investigated further, fig 3. This clearly shows the effect on the PG of the cloud base approaching the surface, but as diminishing effect on the PG as the cloud base moves away. At cloud base heights of about 1000m, an influence of the cloud base charge is no longer detectable, and the PG approaches that of a typical fair weather value.  Fig. 3. Cloud base height plotted against simultaneous, co-located measurements of vertical potential gradient (PG) at the surface.  (Measurements for 20th February 2016 at Reading determined as 1 min averages, yielding 1349 values with H between 200m and 3000m). Model fits for the point, line and disk charge representation are also included. Equations (1) to (3) have been fitted to the data in fig 3 using the non-linear regression functions of the R statistical package [6]. In each case the fits were made allowing for the final asymptotic value (i.e. the undisturbed potential gradient F0) as a free parameter. For the point and line charge cases, two parameters were therefore determined, and in the disk charge case, three parameters. Table 1 shows the parameters found for each model, together with the standard errors on the fit: all the fits were statistically significant at p<0.001 or better. TABLE 1: FITTED PARAMETERS Model F0 (Vm-1) Cloud base charge Cloud disk radius (m) Point charge 93.5 ± 0.8 -(0.96± 0.02) mC - Line charge Disk charge 116.3 ± 1.3 101.0 ± 1.4 -(1.7 ± 0.04) C m-1 -(4.8 ± 0.6) nC m-2 - 329 ± 32 Proc. 2017 Annual Meeting of the Electrostatics Society of America 5 V. DISCUSSION The cloud base charge found by the different charge models all provide fair representa-tion of the variations in the data, and confirm the original assumption that the cloud base charge can effectively be regarded as constant on the timescale of the cloud height varia-tions. However, the point charge model is somewhat unphysical for a cloud base where it is known theoretically that the charge is distributed along the lower cloud edge and hence the line or disk charge is more appropriate. Neither of these is entirely satisfactory though, as, in the line charge case, the requirement for an infinite uniform surface is clearly not met, and, in the disk charge case, there is an additional parameter (the disk radius) to be determined, so the fit is more difficult to obtain statistically.  Nevertheless, an advantage of the disk model is that it is bounded, and provides an es-timate of the surface charge density in the cloud base. This can then be compared with the previous balloon-borne measurements which provide the volumetric charge density. Further, as the cloud base will often show some structure due to wave structures and tur-bulent processes, the existence of a finite length scale as required is physically plausible. For wind speeds at the cloud base of ~ 1 ms-1 and charging times of ~ 500 s, the associat-ed length scale of ~ 500 m is comparable with that determined for R from the three pa-rameter fit in Table 1. The parameters of the disk charge model can therefore be regarded as physically the most useful in remote sensing of cloud base electrical properties. REFERENCES [1] K.A. Nicoll, and R.G. Harrison, “Stratiform cloud electrification: comparison of theory with multiple in‐cloud measurements” Quarterly Journal of the Royal Meteorological Society, 142(700), 2679-2691, 2016. [2] R.G. Harrison, “The Carnegie Curve” Surveys in Geophysics 34, 2, 209-232, 2013. [3] A.J. Bennett and R.G. Harrison, “In situ calibration of atmospheric air conductivity measurements” Re-view of Scientific Instruments 77, 016103, 2006. [4] L. Zhou and B.A. Tinsley, “Time dependent charging of layer clouds in the global electric circuit” Ad-vances in Space Research 50, 828-842, 2012. [5] J.D. Jackson, “Classical Electrodynamics”, Wiley, 1962. [6] R Core Team. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.  URL http://www.R-project.org/ 

Remote sensing of cloud base charge

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Remote sensing of cloud base charge

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