Copyright © 2010 R. R. Dickerson & Z.Q. Li 1 AOSC 620 Cloud Nucleation Russell Dickerson 2014...

Copyright © 2010 R. R. Dickerson & Z.Q. Li

1

AOSC 620Cloud Nucleation

Russell Dickerson2014

Rogers and Yau, Chapt. 6


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Questions on the Effects of Aerosols on Clouds and Precipitation

● Why do many people think aerosols inhibit deep convective cloud formation?


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Opposing Effects of Aerosols on Clouds and Precipitation

● How do radiative and micro-physical effects of aerosols compete?

How does suppression of precp change buoyancy?

How does freezing change buoyancy?


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Opposing Effects of Aerosols on Clouds and Precipitation

● How do radiative and micro-physical effects of aerosols compete?

How does suppression of precp change buoyancy?

negative impact.

How does freezing change buoyancy?

a) if normal precp then freezing enhances buoyancy.

b) If suppressed precp (too many ccn) then freezing generates even more buoyancy.

Liquid Water Cloud VDR Yorks et al., 2011


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Opposing Effects of aerosols on Clouds and Precipitation

(Rosenfeld et al., Science 2008)Radiative Effects:

● Aerosols aloft shield the Earth’s surface from radiation and stabilize the atmosphere wrt convection and the moisture is advected away. (Park et al., JGR, 2001; Ramanathan et al., Science, 2001)

● Increased numbers of CCN slow the conversion of droplets into raindrops and inhibit precipitation, but ingestion of large particles such as sea salt appears to enhance precip. (Radke et al.,

Science, 1989; Rosenfeld et al., Science, 2002)

● Total water vapor is conserved so suppression of precip here means more rain there.


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The Rain according to Rosenfeld (microphysical effects)

● The extra CCN in hazy air make for more, smaller droplets in the early stages of a convective cloud.

● The smaller droplets travel higher and more reach colder levels where they are more likely to release latent heat of freezing and increase buoyancy – haze means more instability for the same amount of rain.

● Even though aerosols slow the conversion of cloud droplets into rain drops, convection is eventually invigorated.

● With cold-based clouds (< 0 oC) most of the water is frozen already and there is no enhancement of precip.

Published by AAAS

D. Rosenfeld et al., Science 321, 1309 -1313 (2008)

Fig. 2. Evolution of deep convective clouds developing in the pristine (top) and polluted (bottom) atmosphere

Wet (Pseudo-Adiabatic) Parcel Theory (no mixing).

● If all the water in excess of the saturation vapor pressure immediately condenses and precipitates out, then buoyancy is zero all the way up; this is the reference for CAPE calculations.

● If all the water is held in the cloud, then buoyancy becomes more negative with altitude.

● If all the water in excess of the saturation vapor pressure immediately condenses and freezes at T < – 4oC then buoyancy is enhanced.

● If precip is suppressed until the parcel reaches T = – 4oC then buoyancy is enhanced further.

The following figure shows an example with the LCL at 960 hPa and 22oC.

Published by AAAS


Fig. 3. The buoyancy of an unmixed adiabatically raising air parcel

Energy released in J kg-1.

←Cloud base

no precp.

suppressed precp.

All precp frozen


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Who wins – radiation or microphysics?

Particles in the accumulation mode with a diameter around 500 nm are most effective at increasing AOT, but CCN can be almost any size – it is the number that matters.

Does CCN correlate with AOT?

Published by AAAS D. Rosenfeld et al., Science 321, 1309 -1313 (2008)

Fig. 1. Relations between observed aerosol optical thickness at 500 nm and CCN concentrations at supersaturation of 0.4% from studies where these variables have

been measured simultaneously, or where data from nearby sites at comparable times were available


● From this empirical relationship we can estimate the number of CCN as a function of AOT.

● If the count of CCN is 104 cm-3 then AOT ~ 1.0 and radiation reaching the Earth’s surface is reduced by an e-folding.

● CAPE reaches a maximum at CCN ~ 1200 cm-3 (AOT ~ 0.25) ;

adding more aerosols will inhibit convection.

Bell (GSFC) et al., (JGR, 2008; “Why do tornados and hailstorms rest on weekends?” 2011) showed a weekday/weekend effect.

Published by AAAS


Fig. 4. Illustration of the relations between the aerosol microphysical and radiative effects


● From this empirical relationship we can estimate the number of CCN as a function of AOT.

● If the count of CCN is 104 cm-3 then AOT ~ 1.0 and radiation reaching the Earth’s surface is reduced by an e-folding.

● CAPE reaches a maximum at CCN ~ 1200 cm-3 (AOT ~ 0.25) ; adding more aerosols will inhibit convection.

Bell (GSFC) et al., (JGR, 2008) showed a weekday/weekend effect.

From Rosenfeld and Bell, 2011

Copyright © 2010 R. R. Dickerson & Z.Q. Li 19

Let’s get quantitative; Rogers & Yau, Chapt 6.


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Phase Change &Nucleation Process

(inhibited by surface tension)

LiquidCondensation

VaporEvaporation

SolidDeposition

SublimationVapor

SolidFreezing

MeltingLiquid


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Condensation

• In theory, a cloud droplet may not be formed until pure water vapor is over saturated by a few hundreds per cent.

• In nature, super-saturation rate rarely exceeds a few tenths per cent.

• The reason lies in the presence of plentiful of water cloud nuclei.


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Deposition

• In theory, a cloud droplet may be frozen at a temperature at 0oC.

• In nature, super-cooled water droplets of temperature well below the freezing point are often observed.

• The reason lies in the lack of ice water cloud nuclei.


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The coverage of this lecture

• Derivation of equilibrium water vapor pressure for a small droplet of pure water vs pure bulk water;-Homogenous nucleation

• Derivation of equilibrium water vapor pressure for a small droplet of solution water vs pure water.-Heterogeneous nucleation

• Aerosol and CCN


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Questions to be addressed:

1. How is an embryonic cloud droplet formed and maintained?

2. Why do cloud droplets have a rather narrow range in size?

3. How can a cloud exist for certain period of time?


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* Surface tension = work required to increase surface area by one unit.

* Store potential energy.

* Volume of liquid tends to assume minimum area-to-volume.

* Small masses Spherical droplets.

For a droplet to form by condensation from the vapor, the surface tension, , must be overcome by a strong gradient of vapor pressure.

The Clausius-Claperon equation describes the equilibrium condition for bulk water and its vapor, which does not apply to small droplet.

Homogeneous Nucleation


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Surface tension causes internal pressure

The surface tensions for a solute is lower than that of pure water by up to one-third, which was attributed to dissolved organics or ions.


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•Surface energy associated with curved surface has impact on equilibrium vapor pressure and rate of evaporation.

•Let equilibrium vapor pressure over a flat surface be es.

•And over a curved surface be esr.

•Consider droplet in equilibrium with environment, temperature = T and vapor pressure = ec

Derivation of the Kelvin (1870) Equation- Curvature effect on saturation


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31

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Kelvin’s Equation, R&Y Eq 6.1


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The relative humidity and supersaturation (both with respect

to a plane surface of pure water) for pure water droplets.


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STRr

wvc ln

2

An embryonic cloud droplet (molecular cluster) can be formed by collision of water vapor molecules. Once it exists, it may grow or decay depending on ambient water vapor pressure.

S = e/es(∞).e>esr, the droplet tends to grow, e<esr, the droplet tends to decay. So, the droplet must be big enough for it to endure.We will show that the critical radius (S is supersaturation) is:


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Köhler curve

S* - critical saturation ratio

r* - critical radius

Haze ←

→ Activated nucleus

Kelvin Curve

Fair Weather CumulusFair weather cumulus

1 pm EST July 7, 2007,

a smoggy day

1.0

0.5

0.0


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43

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43

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50

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43

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Kelvin CurveKöhler Curve


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Next lecture will show where these trends come from.


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Copyright © 2010 R. R. Dickerson & Z.Q. Li 1 AOSC 620 Cloud Nucleation Russell Dickerson 2014...

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Transcript of Copyright © 2010 R. R. Dickerson & Z.Q. Li 1 AOSC 620 Cloud Nucleation Russell Dickerson 2014...