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

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

+ +++

+

- - - --


Jc

Jc

+ +++

+

- - - --


Jc

+ +++

+

- - - --


Jc

+ +++

+

- - - --

+ + +

+

+++ + +

+

+++ + +

+

+++ + +

+

+

- -- --

- - -- --

- - -- --

- - -- --

-

Ionosphere

Surface

Disturbed weatherregion

-

+


-

+

Precipitation

Cosmic rays

Ionisation

Radon Gas

Ionisation

Radon Gas

+ -+-

+ -

+ -+-

+ -

Cosmic rays

++++ + ++ +

++

- - - - -- - - -

JcJc

Ionosphere

Surface


-

+


-

+

Precipitation

Cosmic rays

Ionisation

Radon Gas

Ionisation

Radon Gas

+ -+-

+ -

+ -+-

+ -

Cosmic rays

++++ + ++ +

++

- - - - -- - - -

JcJc

Ionosphere

Surface


-

+


-

+


-

+


-

+

Precipitation

Cosmic rays

Ionisation

Radon Gas

Ionisation

Radon Gas

+ -+-

+ -

+ -+-

+ -

Cosmic raysCosmic rays

Ionisation

Radon Gas

Ionisation+ -+-

+ -

+ -+-

+ -

Cosmic rays

++++ + ++ +

++++++ + ++ +

++

- - - - -- - - -- - - - -- - - -

JcJcJcJc

Radon Gas

Jz

Jz

Jz


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


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

++

+-


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.


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


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. 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


Heig

ht (

km)

0 1 2 3 4 5

3.2

03.2

53.3

03.3

53.4

03.4

53.5

0


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


Heig

ht (

km)

0 1 2 3 4 5

3.2

03.2

53.3

03.3

53.4

03.4

53.5

0


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


Hei

ght (

km)

0 50 100 150 200

2.4

2.6

2.8

3.0

3.2


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


Hei

ght

(km

)

0 50 100 150 200

2.4

2.6

2.8

3.0

3.2


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


Heig

ht

(km

)

0 20 40 60 80 100 120

2.4

2.6

2.8

3.0

3.2


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

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

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