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Stratiform cloud edge charging from cosmic ray ionisation

Stratiform cloud edge charging from cosmic ray ionisation. Keri Nicoll and Giles Harrison University of Reading. 1. Introduction. Mechanism proposed to explain a possible link between cosmic rays and clouds via Global Electric Circuit (GEC).

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Stratiform cloud edge charging from cosmic ray ionisation

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

  2. 1. Introduction • 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. Jz

  3. Ionosphere Ionosphere Ionosphere + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Cosmic rays Cosmic rays Cosmic rays Cosmic rays Cosmic rays Cosmic rays Cosmic rays Cosmic rays Disturbed Disturbed Disturbed Disturbed Disturbed Disturbed Disturbed Disturbed J J J J J J J J c c c c c c c c weather weather weather weather weather weather weather weather region region region region region region region region + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - Ionisation Ionisation Ionisation Ionisation Ionisation Ionisation Ionisation Ionisation - - - - - - - - + + + + + + + + - - - - - - - - - - - - - - - - Precipitation Precipitation Precipitation Radon Gas Radon Gas Radon Gas Radon Gas Radon Gas Radon Gas Radon Gas Radon Gas - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Surface Surface Surface 2. Cloud charge generation Stratiform clouds charge at their upper and lower edges as result of vertical current flow, Jz, in the Global Electric Circuit + + + + + + + + + + + + + + + + + + + + J J Jz c c + + + + + + + + + + + + + + + + + + + + + + + Jz - - - - - - - - - - - - - - - - - - - - - - - - Jz J J J J c c c c Semi fair weather Semi fair weather Semi fair weather Semi fair weather Region Region Region Region - - - - - - - - - - - - - - - - - - - -

  4. 2. Cloud charge generation ρ dσ/dz dE/dz Vertical current flow, Jz + + + + + - - - - - Jz Conductivity (σ) Electric field (E) Space Charge (ρ) Vertical conduction current flow Jz 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, dEz/dz Gauss’ Law of Electrostatics Therefore space charge, ρ, generated on cloud edges and transferred to cloud droplets

  5. 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

  6. (a) (b) V i Jz Jz 2. Cloud charge generation • Cloud type most affected – stratiform clouds of large horizontal extent • Hypothesise that: • Broken cloud – Jz flows around cloud • (b) Overcast cloud - Jz flows through cloud Nicoll, K. A., and R. G. Harrison, Vertical current flow through extensive layer clouds, J. Atmos. Sol.-Terr. Phys., 71(17-18), 2040-2046, 2009.

  7. 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 (σgradient) • Magnitude of Jz

  8. 2.Cloud charge generation Solar-Terrestrial link Vertical current, Jz, depends mainly on ionisation rate Variability in ionisationmostly from cosmic rays => Jz modulated by cosmic ray flux Decadal timescale Hourly timescale Jz (pA m-2) Jz Solar flare event B field (nT) Harrison and Usoskin (2010) Solar modulation in surface atmospheric electricity, J. Atm. Sol.-Terr. Phys. 72 (2010) 176–182 16% change in Jz between CR max and CR min. Harrison and Ambaum, Enhancement of cloud formation by droplet charging. Proc. R. Soc. A 464 2561–73, 2008. Plausible that stratiform cloud edge charging is modulated by cosmic ray flux

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

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

  11. 2. Project Plan • Aim – remedy lack of knowledge about stratiform cloud edge charging • Two step approach adopted: • 1. Investigate whether Jz flows through clouds- section 3 • - surface measurements of Jz 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

  12. 3. Current flow through clouds Very few sites around the world have ever measured Jz, but fortunately the UK Met Office has, and Reading, since 2006. Long time series of Jz measurements from UK Met.Office site at Kew (1909 to 1980), and shorter one from Lerwick (1978-1984) have been recovered. Lerwick Clean air Kew Polluted air Reading Urban air Time series of Jz measurements at Lerwick, Shetland and Kew, London

  13. 3. Current flow through clouds • Categorise Jz according to cloud conditions in which it was measured – given by DF criteria (DF ~ 1, overcast; DF ~0.2, clear). • Analysis of Jz in different cloud conditions from 3 different UK sites shows that • Jz is non zero in overcast cloud, • Jz must flow • through the cloud Lerwick Kew Reading Nicoll, K. A., and R. G. Harrison (2009b), Vertical current flow through extensive layer clouds, J. Atmos. Sol.-Terr. Phys., 71(17-18), 2040-2046.

  14. 4. In situ measurements Cloud Edge Charge Detector (CECD) • 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 £50. Electrode Electrometer circuit Nicoll K.A. and R.G. Harrison, A lightweight balloon-carried cloud charge sensor, Rev. Sci. Instrum., DOI:10.1063/1.3065090, 2009.

  15. 4. In situ measurements Cloud Edge Charge Detector (CECD) CECD has several modes of operation: 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 CECD Data Transfer DAS RS92 radiosonde 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. Temperature and RH sensor

  16. 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.

  17. 4. CECD Results Balloon flight through extensive layer of stratiform cloud Cloud droplet diameter measured by FAAM aircraft K. A. Nicoll and R. G. Harrison, Experimental determination of layer cloud edge charging from cosmic ray ionisation, Geophys. Res. Lett., 37, L13802, doi:10.1029/2010GL043605 (2010) 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) Cloud droplet diameter, d, measured by FAAM aircraft : red d<5μm, orange 5μm<d<10μm, green 10μm<d<15μm, blue 15μm<d<20μm, and purple d>20μm

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

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

  20. 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

  21. 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 = 1.6x10-19C • Cloud base • Median droplet charge =2.6e • Maximum droplet charge = 31e • Cloud top • Median droplet charge =2.4e • Maximum droplet charge = 15e Cloud base Cloud top Cloud base Cloud top

  22. 4.CECD Results Individual droplet charges 4.CECD Results Individual droplet charges • Are derived droplet charges large enough to affect cloud microphysical processes in stratiform cloud? More measurements of individual droplet charges in stratiform cloud required to investigate this fully Variability in clouds

  23. 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-3have 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

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

  25. 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, 126111

  26. Thank you

  27. Sensor calibration using two parallel plates and an oscillating voltage source to create dE/dt. 4. In situ measurements Calibration • 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: • Aeff = 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/Aeff0) Calibration graph (3) Simulating known dE/dt, and measuring dV/dt allows calculation of constant.

  28. 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

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