Stratiform cloud edge charging from cosmic ray ionisation Keri Nicoll and Giles Harrison University of Reading

Mechanism proposed to explain a possible link between cosmic rays and clouds via Global Electric Circuit (GEC). GEC mechanism involves vertical flow of cosmogenic ions through layers of stratiform cloud, generating charge at the edges. Charge transferred to droplets => cloud microphysical behaviour may be affected. As yet, no experimental evidence to confirm that the GEC mechanism is plausible: -not known whether vertical ion current flows through clouds - very few measurements of charge in stratiform cloud exist. 1. Introduction JzJz

Stratiform clouds charge at their upper and lower edges as result of vertical current flow, J z, in the Global Electric Circuit 2. Cloud charge generation Semi fair weather Region J c J c Semi fair weather Region J c J c Semi fair weather Region J c Semi fair weather Region J c JzJz JzJz JzJz

2. Cloud charge generation Vertical current flow, J z JzJz Vertical conduction current flow J z produces continuous supply of ions into cloud At cloud edge, ions attach to cloud droplets => conductivity decrease from clear air to cloud =>Vertical gradient in conductivity => Vertical gradient in electric field, dE z /dz Conductivity (σ) Electric field (E)Space Charge (ρ) dσ/dzdE/dz ρ Gauss’ Law of Electrostatics Therefore space charge, ρ, generated on cloud edges and transferred to cloud droplets

2. Cloud charge generation Cloud microphysical processes thought to be affected by charge: Increased chance of droplet freezing → droplet growth Coalescence between charged and uncharged droplets → droplet growth Activation (initial growth) of droplets occur at lower supersaturations → droplet growth Thus cloud droplet size distribution may be influenced by charge →Large scale cloud properties may be affected e.g. Lifetime Precipitation Horizontal extent COALESCENCE

2. Cloud charge generation Cloud type most affected – stratiform clouds of large horizontal extent Hypothesise that: (a)Broken cloud – J z flows around cloud (b) Overcast cloud - J z flows through cloud V i Jz (a)(b) Nicoll, K. A., and R. G. Harrison, Vertical current flow through extensive layer clouds, J. Atmos. Sol.-Terr. Phys., 71(17-18), , 2009.

2. Cloud charge generation Factors controlling magnitude of space charge generated at cloud edges: Thickness of boundary between clear air and cloud (dz) Altitude of cloud (charge lifetime) Cloud droplet number concentration (σ gradient) Magnitude of J z

Vertical current, J z, depends mainly on ionisation rate Variability in ionisation mostly from cosmic rays => J z modulated by cosmic ray flux 2.Cloud charge generation Solar-Terrestrial link Harrison and Usoskin (2010) Solar modulation in surface atmospheric electricity, J. Atm. Sol.-Terr. Phys. 72 (2010) 176–182 16% change in J z between CR max and CR min. JzJz Plausible that stratiform cloud edge charging is modulated by cosmic ray flux J z (pA m -2 ) B field (nT) Solar flare event Decadal timescaleHourly timescale Harrison and Ambaum, Enhancement of cloud formation by droplet charging. Proc. R. Soc. A –73, 2008.

Zhou and Tinsley, JGR (2007) – numerical model to predict stratiform cloud edge charging. Profiles for variation of J z. Dashed lines: J z = 4pAm -2, Solid lines: J z = 2pAm -2, dotted lines: J z = 1pAm -2, ρ is mean droplet charge (e) Higher charging expected for large values of J z 2. Cloud charge generation

Droplet charging on stratiform edges Large scale properties of clouds Ionisation Solar activity Cosmic ray flux JzJz Space charge on stratiform cloud edges Possible chain of processes linking solar activity to cloud cover: 2. Cloud charge generation

2. Project Plan Aim – remedy lack of knowledge about stratiform cloud edge charging Two step approach adopted: 1. Investigate whether J z flows through clouds- section 3 - surface measurements of J z and cloud from same site 2. Make in situ measurements of charge inside cloud - section 4 - development of balloon-borne charge sensor, the Cloud Edge Charge Detector (CECD) -findings from measurements of charge inside stratiform cloud

3. Current flow through clouds Very few sites around the world have ever measured J z, but fortunately the UK Met Office has, and Reading, since Long time series of J z measurements from UK Met.Office site at Kew (1909 to 1980), and shorter one from Lerwick ( ) have been recovered. Time series of J z measurements at Lerwick, Shetland and Kew, London Lerwick Clean air Kew Polluted air Reading Urban air

3. Current flow through clouds Categorise J z according to cloud conditions in which it was measured – given by DF criteria (DF ~ 1, overcast; DF ~0.2, clear). Analysis of J z in different cloud conditions from 3 different UK sites shows that J z is non zero in overcast cloud, J z must flow through the cloud Reading LerwickKew J z Clear (pAm -2 ) J z Broken (pAm -2 ) J z Overcast (pAm -2 ) Reading Lerwick Kew Nicoll, K. A., and R. G. Harrison (2009b), Vertical current flow through extensive layer clouds, J. Atmos. Sol.-Terr. Phys., 71(17-18), Cloud condition DF Overcast> 0.9 Broken0.4 > 0.9 Clear< 0.4

Balloon platform - high vertical resolution measurements - many measurements can be made for low cost - quick and easy to launch Sensor requirements - inexpensive - lightweight - minimise metal Voltage change on spherical electrode is measured using low leakage electrometer circuit Regular reset action ensures escape from saturation conditions Size of CECD box= 3.5cm x 3.5cm x 2.0cm, component cost is less than £ In situ measurements Cloud Edge Charge Detector (CECD) Nicoll K.A. and R.G. Harrison, A lightweight balloon-carried cloud charge sensor, Rev. Sci. Instrum., DOI: / , Electrode Electrometer circuit

CECD has several modes of operation: (a)Induction - charge induced in the CECD electrode due to space charge, typically in and around clouds. This creates a displacement current in the CECD, which is measured by the CECD electrometer circuit. (b) Impaction - charge may be transferred directly to the CECD electrode by collisions with charged droplets or particles. Charge sensor flown alongside Vaisala RS92 radiosonde 4. In situ measurements Cloud Edge Charge Detector (CECD) CECD attached to specially developed Digital Acquisition System (DAS), which transfers data from the CECD to extra channel on standard sonde. CECD data sent over radio link synchronously with pressure, temperature, RH and GPS position data from sonde at 1Hz. Data Transfer CECD DAS RS92 radiosonde Temperature and RH sensor

4.CECD Results Flight through stratiform cloud CECD flight through stratocumulus layer on 18/02/09 FAAM aircraft, measuring cloud droplet number concentration also flew through same cloud layer three hours before. Nicoll, K.A., R.G. Harrison, Experimental determination of layer cloud edge charging from cosmic ray ionisation, Geophys. Res. Lett., 37, L13802, 2010.

4. CECD Results Cloud droplet diameter, d, measured by FAAM aircraft : red d 20μm Cloud droplet diameter measured by FAAM aircraft Meteorological parameters measured by RS92 radiosonde Charge present on cloud base Max ρ ~ 35pC m -3 Depth of charge layer ~ depth of cloud base (~100m) K. A. Nicoll and R. G. Harrison, Experimental determination of layer cloud edge charging from cosmic ray ionisation, Geophys. Res. Lett., 37, L13802, doi: /2010GL (2010) Balloon flight through extensive layer of stratiform cloud

4.CECD Results Flight 08/07/09 Ascent and descent through same extensive Sc cloud layer Ascent Descent Burst position

4.CECD Results Flight 08/07/09 Ascent Descent Space charge at cloud top on both ascent and descent Max ρ~100 pC m -3 Horizontal separation ~ 65km Suggests same charging mechanism is responsible

4.CECD Results Summary of all CECD flights Several CECD flights have been made through stratiform cloud Space charge was detected on 2/3rds of measured cloud edges Median ρ at cloud base ~ 20 pC m -3 Median ρ at cloud top ~ 17 pC m -3 Max ρ ~255 pC m -3

4.CECD Results Individual droplet charges Can estimate charges carried by individual cloud droplets by making several assumptions: All space charge is carried by cloud droplets Charge is distributed evenly between cloud droplets (regardless of droplet size) Estimate cloud droplet number concentration, n ~50 cm -3 ρ = Nje ρ = derived space charge density N=cloud droplet no. conc. j = average no. charges carried by droplets e=electronic charge = 1.6x C Cloud base Median droplet charge =2.6e Maximum droplet charge = 31e Cloud top Median droplet charge =2.4e Maximum droplet charge = 15e Cloud baseCloud top Cloud baseCloud top

4.CECD Results Individual droplet charges Are derived droplet charges large enough to affect cloud microphysical processes in stratiform cloud? MechanismAuthorDroplet charge required (e) Required droplet charge present in stratiform cloud? CoalescenceKhain et al (2004) Freezing Tinsley (1989, 1991, 2000) 5-50 ActivationHarrison and Ambaum (2008) More measurements of individual droplet charges in stratiform cloud required to investigate this fully Variability in clouds

5. Conclusions Experimental confirmation of theory that charge exists on edges of layer clouds using specially developed balloon borne charge sensor. Charges of the order hundreds of pC m -3 have been observed Charge not present on all layer cloud edges => Criteria for cloud edge charging: 1. Vertical current must flow through cloud i.e. cloud must be of large horizontal extent 2. Cloud must have existed for sufficient time to become charged (~30mins to 1 hour) 3. Depth of boundary between clear air and cloud is sharp 4. No appreciable turbulent mixing inside cloud

5. Conclusions Solar activity, cloud and climate Droplet charging on stratiform edges Large scale properties of clouds Ionisation Solar activity Cosmic ray flux JzJz Space charge on stratiform edges likely? Further investigation of possible mechanisms is required

6. Ionisation sensor Small geiger tube used to detect atmospheric ionisation (from cosmic rays and radioactivity) Electronic circuit generates 500V power supply from 9V to operate geiger tube Balloon flight in 2005 showed expected profile of increase in ionisation with height due to cosmic rays (and agreed well with that predicted by theory) LND714 Geiger tube (sensitive to beta and gamma radiation) R.G. Harrison, (2005) Meteorological radiosonde interface for ion production rate measurements Rev.Sci. Instrum. 76, 12,

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Sensor calibration using two parallel plates and an oscillating voltage source to create dE/dt. 4. In situ measurements Calibration Simulating known dE/dt, and measuring dV/dt allows calculation of constant. From Maxwell, a time varying electric field, E, induces a current density, j, in a conductor according to : (1) Change in electrode voltage, V given by: A eff = effective area through which field acts, C=capacitance of electrode (2) Follows that gradient of graph of dE/dt vs dV/dt gives the constant (–C/A eff  0 ) (3) Calibration graph

4. In situ measurements Calculation of space charge Space charge, , is difference between net positive and net negative charge per unit volume. Given by Gauss’ Law: (4) By combining equation (3) with (4), , can be calculated from: Where w is the ascent rate and C/Aeff is calculated from calibration. (5) Space charge, , is measured in pC m -3 1pC m -3 = 6.25 electronic charges cm -3