Electroscavenging of Condensation and Ice-Forming Nuclei

Brian A. Tinsley

University of Texas at Dallas

Tinsley@UTDallas.edu

WMO Cloud Modeling Workshop, Hamburg, 12-16 July, 2004

OutlineIonosphere-earth current density, Jz, charging clouds

Global cloud cover response to Jz at HCS crossings

Atmospheric transparency response to Jz at HCS crossings

Atmospheric vorticity response to Jz at HCS crossings

Implications of these and other observations

Electro-repulsion may reduce CCN scavenging loss rate

Electroscavenging of IFN may increase contact ice nucleation rate

Ion-mediated nucleation may produce CCN, assisted by space charge dependent on Jz

Conclusions

Production of space charge at cloud boundaries by downward ionosphere/earth current density Jz

Flow of Jz through conductivity gradients produces space charge (difference between concentrations of positive and negative ions).

Charge attaches to aerosols and droplets: affects microphysical interactions.

s

Observed Droplet Charges

Cloud Cover Responses to Jz Decreases at HCS Crossings 1991-1994

From Kniveton and Tinsley, in press, J. Geophys Res., 2004

Latitude Variation of Jz (and Ez near surface)

Percentage change in Jz (shaded) for Forbush decrease of 35% at high latitudes and 12% at low latitudes, for longitude 72.5 °E. Generators are assumed constant current. From Sapkota and Vareshneya, (1990).

Cloud Cover responses to Jz decreases at HCS

Crossings 1991-1994

Shading is dignificant at 95% level.From Kniveton and Tinsley, in press, J. Geophys. Res., 2004

Extinction by Aerosols at HCS Crossings

Upper panel: Years of low stratospheric aerosol loading. Lower panel: El Chicon years of high stratospheric aerosol loading. From Roldugin and Tinsley, J. Atmos. Solar Terr. Phys. in press.

VAI response to Solar Wind, Relativistic Electron Fux (and Jz) Decreases

Implications of Observations

• The responses to Jz changes at HCS crossings take place in the absence of significant cosmic ray (CR) flux changes.

• Similar responses of cloud cover, atmospheric transparency, and vorticity are found with CR changes, on day-to-day and decadal time scales.

• It may be that the Jz changes produced by the CR are more important for cloud microphysics than the ion concentration changes produced by the CR

EFFECT OF ELECTROSCAVENGING ON TRAJECTORIES

(Short range image force)

Long-range Electro-repulsion Reduces Losses of CCN from Phoretic and Brownian Scavenging

Variation of collision efficiency for droplets of radius 3m, 5m, and 12m, with charge on CCN of 1e-10e. Curves for 0e are for phoretic scavenging with 98% RH. Upper three curves are for like charges on CCN and droplet. Lower three curves are for unlike charges.

Effectiveness of Long-range Electro-repulsion in Reducing Scavenging of CCN by Phoretic Forces

and Brownian Diffusion

CCN from gas phase conversion are mostly between about .03m and .10m in radius, with the distribution falling rapidly with increasing radius.

The higher mobility of the smaller CCN for a given electrical force ensures that the long-range electrical repulsion prevents the CCN from being carried by the flow or phoretic forces or Brownian diffusion close enough to the droplet to be captured. The scavenging rate is reduced below the Brownian and phoretic rates. Thus increasing electric charge increases the smaller CCN concentrations.

(Tinsley et al., Atmospheric Research 59-60, 115-135, 2001).

Electroscavenging for Contact Ice Nucleation

Variation of collision efficiency for droplets of radius 10m, 15m, and 20m, with charge on IFN of 1e – 20e. Curves for 0e are for phoretic scavenging with 98% RH. Upper three curves are for like charges on IFN and droplet. Lower three curves are for unlike charges.

Charges on Aerosol Particles

The theory of charging of monodispersive aerosols in the presence of space charge , where = (n+ - n- + S+ - S- )e, yields values for equilibrium charges on aerosol particles that increase with . More accurately, charging depends on x, where x = (n++)/(n--). Here n+ and n- are the positive and negative air ion concentrations, and + and - are their respective mobilities. S+ and S- are the summed concentrations of charged aerosol particles and droplets.

For a plausible x = 0.3 at cloud base we have:~70% of CCN of radius 0.05m have mean charge –1e ~80% of CCN of radius 0.1 m have mean charge -2e ~80% of CCN of radius 0.2 m have mean charge –4e

Time dependent effects of flow of charge into cloud with Jz can drive x further from unity, and increase the mean aerosol charges. Also evaporation of charged droplets gives very high charges for aerosols.

One possibility for growth of ultrafine aerosol to CCN size (mass increase ~1000) is in the layers of space charge and volatiles near evaporating clouds. Tinsley and Yu, Geophysical Monograph 141, 2004

Ion-Mediated Nucleation of Ultrafine Aersol

Nucleation in 3 hours for a 20% increase in ionizing flux.

From Yu, JGR, 2002, overlaid by Kazil & Lovejoy, Dec. 2003 AGU

ConclusionsLayer clouds accumulate enough charge at cloud tops and cloud bases to affect microphysics.

Electroscavenging increases collection efficiencies of IFN (larger aerosol particles collected by larger droplets) through the short range image attractive force. For cloud top temperatures just below freezing and broad droplet size distributions, electroscavenging causing contact ice nucleation appears to be competitive with deposition nucleation.

Long range electro-repulsion reduces the losses of CCN (smaller particles) from phoretic or Brownian scavenging. This is with like charges on CCN and droplet and for clouds with narrow droplet size distributions and smaller droplets.

Parameterisation of these effects is needed for use in macroscopic cloud models