Post on 27-Dec-2015
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
Jz
Stratiform clouds charge at their upper and lower edges as result of vertical current flow, Jz, in the Global Electric Circuit
2. Cloud charge generation
Semi fair weatherRegion
Jc
Jc
+ +++
+
- - - --
Semi fair weatherRegion
Jc
Jc
+ +++
+
- - - --
Semi fair weatherRegion
Jc
+ +++
+
- - - --
Semi fair weatherRegion
Jc
+ +++
+
- - - --
+ + +
+
+++ + +
+
+++ + +
+
+++ + +
+
+
- -- --
- - -- --
- - -- --
- - -- --
-
Ionosphere
Surface
Disturbed weatherregion
-
+
Disturbed weatherregion
-
+
Precipitation
Cosmic rays
Ionisation
Radon Gas
Ionisation
Radon Gas
+ -+-
+ -
+ -+-
+ -
Cosmic rays
++++ + ++ +
++
- - - - -- - - -
JcJc
Ionosphere
Surface
Disturbed weatherregion
-
+
Disturbed weatherregion
-
+
Precipitation
Cosmic rays
Ionisation
Radon Gas
Ionisation
Radon Gas
+ -+-
+ -
+ -+-
+ -
Cosmic rays
++++ + ++ +
++
- - - - -- - - -
JcJc
Ionosphere
Surface
Disturbed weatherregion
-
+
Disturbed weatherregion
-
+
Disturbed weatherregion
-
+
Disturbed weatherregion
-
+
Precipitation
Cosmic rays
Ionisation
Radon Gas
Ionisation
Radon Gas
+ -+-
+ -
+ -+-
+ -
Cosmic raysCosmic rays
Ionisation
Radon Gas
Ionisation+ -+-
+ -
+ -+-
+ -
Cosmic rays
++++ + ++ +
++++++ + ++ +
++
- - - - -- - - -- - - - -- - - -
JcJcJcJc
Radon Gas
Jz
Jz
Jz
2. Cloud charge generation
Vertical current flow, Jz
Jz
+ + + ++
- - - --
+++ +++ +
- --
- --
Conductivity (Sm 1
)
Heig
ht
Electric field (Vm 1
)
Heig
ht
charge density (pCm 3
)H
eig
ht
.(a) (b) (c) (d) (e) (f)
Electrode Voltage (V)
Hei
ght
dV/dt (Vs 1
)
Hei
ght
charge density (pCm 3
)
Hei
ght
(2)(1)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
Conductivity (σ) Electric field (E) Space Charge (ρ)
dσ/dz dE/dz ρ
0
dz
dEz
tz dz
dJ
10
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 affectede.g. LifetimePrecipitationHorizontal extent COALESCENCE
++
+-
2. Cloud charge generation
Cloud type most affected – stratiform clouds of large horizontal extent
Hypothesise that:
(a) Broken cloud – Jz flows around cloud
(b) Overcast cloud - Jz flows through cloud
Vi
JzJz
(a) (b)
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.
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 Jz
tz dz
dJ
10
Vertical current, Jz, depends mainly on ionisation rate
Variability in ionisation mostly from cosmic rays => Jz 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 Jz between CR max and CR min.
J z
Plausible that stratiform cloud edge charging is modulated by cosmic ray flux
J z (
pA
m-2)
B
field
(n
T)
Solar flare event
Decadal timescale Hourly timescale
Harrison and Ambaum, Enhancement of cloud formation by droplet charging. Proc. R. Soc. A 464 2561–73, 2008.
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, ρ is mean droplet charge (e)
Higher charging expected for large values of Jz
2. Cloud charge generation
Droplet charging on stratiform
edges
Large scale properties of
cloudsIonisation
Solar activity
Cosmic ray flux
Jz
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 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
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.
Time series of Jz measurements at Lerwick, Shetland and Kew, London
LerwickClean air
KewPolluted air
ReadingUrban air
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 thatJz is non zero in overcast cloud,
Jz must flow through the cloud
Clear Broken Oc
1.0
1.5
2.0
(a) Kew Jc
Cloud conditions
Jz (
pAm2
)
Clear Broken Oc
20
025
03
003
504
004
50
(b) Kew PG
Cloud conditions
PG
(
Vm1
)
Clear Broken Oc
0.51
.01
.52
.02
.53
.03
.54
.0 (c) Lerwick Jc
Cloud conditions
Jz (
pAm2
)
Clear Broken Oc
501
001
502
002
503
003
50 (d) Lerwick PG
Cloud conditions
PG
(
Vm1
)
(a) Kew Jz (b) Kew PG
Clear Broken Oc
0.5
1.0
1.5
2.0
2.5
(a)
Cloud conditions
Jz ( p
Am
2)
Clear Broken Oc
05
01
00
15
0
(b)
Cloud conditions
PG
(Vm
1)
(a) Reading Jz (b) Reading PG
Clear Broken Oc
1.0
1.5
2.0
(a) Kew Jc
Cloud conditions
Jz (
pAm2
)
Clear Broken Oc
20
025
030
035
040
045
0
(b) Kew PG
Cloud conditions
PG
(
Vm1
)
Clear Broken Oc
0.51
.01
.52
.02
.53
.03
.54
.0 (c) Lerwick Jc
Cloud conditions
Jz (
pAm2
)
Clear Broken Oc
501
001
502
002
503
003
50 (d) Lerwick PG
Cloud conditions
PG
(
Vm1
)
(a) Lerwick Jz (b) Lerwick PGReading Lerwick Kew
Jz Clear (pAm-2)
Jz Broken (pAm-2)
Jz Overcast (pAm-2)
Reading 1.15 1.24 1.24
Lerwick 2.52 2.49 2.12
Kew 1.36 1.45 1.40
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.
Cloud condition
DF
Overcast > 0.9
Broken 0.4 > 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 £50.
4. 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:10.1063/1.3065090, 2009.
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
CECDDASRS92 radiosonde
Temperature and RH sensor
4.CECD Results Flight through stratiform cloud
(a) (b)
(a) (b) (c)
Portsmouth(CECD sonde)
Yeovil(aircraft)
Larkhill(sonde)
Nottingham(sonde)
-18 -16 -14 -12 -10 -8
3.23.3
3.43.5
3.63.7
3.8
Temperature (deg C) (Sonde)
Height (k
m)
Temperature (deg C) (Aircraft)
-16 -14 -12 -10 -8 -6
0 10 20 30 40 50 60
3.23.3
3.43.5
3.63.7
3.8
Cloud droplet no. conc. (cm 3)
Height (k
m)
0 1 2 3 4 5
3.23.3
3.43.5
3.63.7
3.8
CECD Electrode Voltage (V)
Height (k
m)
(c) (d)
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<5μm, orange 5μm<d<10μm, green 10μm<d<15μm, blue 15μm<d<20μm, and purple d>20μm
Cloud droplet diametermeasured 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:10.1029/2010GL043605 (2010)
Balloon flight through extensive layer of stratiform cloud
(a) (b)
(c) (d)
0 10 20 30 40 50 60
3.2
03.2
53.3
03.3
53.4
03.4
53.5
0
Cloud droplet no. conc. (cm 3)
Heig
ht (
km)
0 1 2 3 4 5
3.2
03.2
53.3
03.3
53.4
03.4
53.5
0
CECD Electrode Voltage (V)
Heig
ht (
km)
0 5 10 15 20
3.20
3.25
3.30
3.35
3.40
3.45
3.50
Conductivity (fSm 1)
Hei
ght
(km
)
Vertical conductivity gradient (fSm 2)-0.15 -0.10 -0.05 0.00 0.05
-40 -30 -20 -10 0
3.20
3.25
3.30
3.35
3.40
3.45
3.50
Space charge (pCm 3)
Hei
ght
(km
)
Space charge (ecm 3)-250 -200 -150 -100 -50 0
(a) (b)
(c) (d)
(a) (b)
(c) (d)
0 10 20 30 40 50 60
3.2
03.2
53.3
03.3
53.4
03.4
53.5
0
Cloud droplet no. conc. (cm 3)
Heig
ht (
km)
0 1 2 3 4 5
3.2
03.2
53.3
03.3
53.4
03.4
53.5
0
CECD Electrode Voltage (V)
Heig
ht (
km)
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
-6 -4 -2 0 2
2.4
2.6
2.8
3.0
3.2
Temperature (deg C)
Hei
ght (
km)
RH (%)
0 20 40 60 80 100
0 1 2 3 4 5 6
2.4
2.6
2.8
3.0
3.2
Electrode Voltage (V)
Hei
ght (
km)
0 50 100 150 200
2.4
2.6
2.8
3.0
3.2
Space charge (pCm 3)
Hei
ght (
km)
(a) (b) (c)
(d) (e) (f)
-6 -4 -2 0 2
2.4
2.6
2.8
3.0
3.2
Temperature (deg C)
Hei
ght
(km
)
RH (%)
0 20 40 60 80 100
0 1 2 3 4 5 6
2.4
2.6
2.8
3.0
3.2
Electrode Voltage (V)
Hei
ght
(km
)
0 50 100 150 200
2.4
2.6
2.8
3.0
3.2
Space charge (pCm 3)
Hei
ght
(km
)
(a) (b) (c)
(d) (e) (f)
-6 -4 -2 0 2
2.4
2.6
2.8
3.0
3.2
Temperature (deg C)
Heig
ht
(km
)
RH (%)
0 20 40 60 80 100
0 1 2 3 4 5 6
2.4
2.6
2.8
3.0
3.2
Electrode Voltage (V)
Heig
ht
(km
)
0 20 40 60 80 100 120
2.4
2.6
2.8
3.0
3.2
Space charge (pCm 3)
Heig
ht
(km
)
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 = Njeρ = derived space charge densityN=cloud droplet no. conc.j = average no. charges carried by droplets e=electronic charge = 1.6x10-19C
Cloud baseMedian droplet charge =2.6eMaximum droplet charge = 31e
Cloud topMedian droplet charge =2.4eMaximum droplet charge = 15e
Cloud base Cloud top
Mean droplet charge at cloud base (e)
De
nsi
ty
0 5 10 15 20 25 30
0.0
00
.05
0.1
00
.15
Mean droplet charge at cloud top (e)
De
nsi
ty
0 5 10 15
0.0
00
.05
0.1
00
.15
0.2
0
Cloud base Cloud top
4.CECD Results Individual droplet charges4.CECD Results Individual droplet charges
Are derived droplet charges large enough to affect cloud microphysical processes in stratiform cloud?
Mechanism Author Droplet charge required
(e)
Required droplet charge present in stratiform cloud?
Coalescence Khain et al (2004) 20-100
FreezingTinsley (1989, 1991, 2000)
5-50
Activation Harrison and Ambaum (2008)
100-1000
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
cloudsIonisation
Solar activity
Cosmic ray flux
Jz
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, 126111
Thank you
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)0 0E
jt
��������������
• Change in electrode voltage, V given by:
Aeff = effective area through which field acts, C=capacitance of electrode
(2)0z
eff
dE dVA C
dt dt
• Follows that gradient of graph of dE/dt vs dV/dt gives the constant (–C/Aeff0)
0
z
eff
dE C dV
dt A dt
(3)
Y=49.3x+74.6R2=1.00
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
dz
dEz0
dt
dV
wA
C
eff
1