cloud microphysics

Cloud MicrophysicsSOEE3410 : Lecture 4

Ken Carslaw

Lecture 2 of a series of 5 on clouds and climate• Properties and distribution of clouds• Cloud microphysics and precipitation• Clouds and radiation• Clouds and climate: forced changes to clouds• Clouds and climate: cloud response to climate

change

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Aims of Lecture 4

Understand:

• What determines the number and size of drops in a cloud

• The two main processes that can initiate rain in clouds


Recommended reading for this lecture

• A Short Course on Cloud Physics, R. R. Rogers and M. K. Yau, 3rd ed., Butterworth-Heinemann– Some very readable chapters

– Physics L-0 Rog (Reference, short, long)

• Several cloud physics books in the library worth flicking through

• Short article from ISCCP http://isccp.giss.nasa.gov/climanal8.html


What is Cloud Microphysics?

• Properties of a cloud on the micro-scale (i.e., micrometres)

• Includes droplet concentrations, sizes, ice crystal formation, droplet-droplet interactions, rain drop formation, etc.


Microphysics and Climate

• Cloud drop number (CDN) influences cloud albedo, or reflectivity (next lecture)– “1st indirect effect” of aerosols on climate

• CDN/size influences precipitation efficiency (and therefore cloud lifetime/distribution and cloud fraction)– “2nd indirect effect” of aerosols on climate

• Ice formation affects latent heat release, precipitation intensity, cirrus properties,etc.


Condensation NucleiStarting Point for Drop Formation

• Droplets form by condensation of water vapour on aerosol particles (condensation nuclei, CN) at very close to 100% RH

• Without CN, humidities of >300% are required for drop formation

• Droplets form on some (a subset of) CN– Cloud Condensation Nuclei (CCN)

• Typical CN concentrations (100-10,000 cm-3). Typical CCN (20-500 cm-3)

• CN are composed of– Salt particles from sea spray– Natural material (inorganic and organic mixtures)– Human pollution (sulphuric acid particles, etc)


Cloud Formation

Either:

• Air rises and cools to saturation (100% RH) and then supersaturation (>100% RH)– By adiabatic expansion

• Air cools by radiative energy loss or advection over a cold surface (fogs)


Increase in humidity in a rising air parcel

temperature

wate

r vapour

con

centr

ati

on

100% RH line

Air initially at 70% RH

Air rises, cools, RH increases

100% RH (saturation, dew point)

Droplets form(RH>100%)

Droplets grow, remove water vapour


Droplet “activation”

• Small particles require higher humidities because surface tension of small droplets increases the pressure of water vapour over their surface

• Consequence: droplets form on large particles first

sea salt

ammoniumsulphate


Droplet “activation”

Typically100-1000 cm-3

Typically 1000-10000 cm-3

maximum supersaturation in cloud equates to minimum radius of activation

growth


Factors affecting droplet number

• Aerosol particle size– larger particles activate at lower humidities

• Particle chemical composition– Some substances are more ‘hygroscopic’

• Aerosol particle number concentration– Simple

• Cloud-scale updraught speed– Higher speed = more drops

}Humanactivitiesaffect these


Droplet number vs. aerosol size and number

• Fixed updraught speed

log(N)

DiameterSolid contours = CDN; colours = aerosol mass (g m-3)


Droplet Evolution Above Cloud Base

Hei

ght a

bove

clo

ud b

ase

(m)

Supersaturation (%)

0

20

40

60

80

0 0.4 0.6Drop conc’n (cm-3)

0

20

40

60

80

0 200 400Ave’ radius (m)

0

20

40

60

80

0 2 4 6Liquid water content (g m-3)

0

20

40

60

80

0 0.1 0.2

updraught = 0.5 ms-1

updraught = 2.0 ms-1

Decreasingsupersat’n as droplets grow, suppresses new droplets

(S = %RH-100)


Diffusional Droplet Growth

Radius time

1 2.4 s

2 130 s

4 1000 s

10 2700 s

20 2.4 hr

30 4.9 hr

40 12.4 hr

NaCl particle (10-14 g mass); initial radius = 0.75 micron; RH = 100.05%; p = 900 mb; T = 273 K

.typical CNr=0.1, V=10-4

large dropr=50, V=27

typical dropr=10, V=1

typical raindrop: r=1000, V=650

transition dropr=50, V=27

Droplets grow by diffusion of water vapour

const

S

dt

drr (S = %RH-100)


Diffusional Droplet Growth

• Leads to narrowing of droplet size distribution, but not observed

• Possible reasons:– Giant CN

– Supersaturation fluctuations

– Mixing

const

S

dt

drr

Ndrop

Diameter

Ndrop

Diameter

cloud base

cloud top

cloud base

cloud top

Diffusion only Observed


Definition of “Precipitation-Sized” Droplet

• How big must a droplet be before it can be considered a “raindrop”

Initial radius

Distance fallen

1 m 2.0 m

3 m 0.17 mm

10 m 2.1 cm

30 m 1.69 m

0.1 mm 208 m

0.15 mm 1.05 km

Distance a drop falls before evaporating.Assumes isothermal atmosphere withT=280 K, RH=80%

Definition of a drizzle drop


“Warm Rain” Formation

• Rain formation without ice phase

• Additional process needed to grow droplets to precipitation size

• Collision and coalescence

Narrow distributions not very efficient for collision

Some large drops initiate collision-coalescence

20 m droplets needed to initiate rapid collision-coalescence

20 m droplets needed to initiate rapid collision-coalescence


Droplet Evolution with Collision-Coalescence

05

1015

2025

30

time (

mins)

10-3 10-2 10-1 100

Radius (cm)

10 m


Summary of “Warm Cloud” Microphysics

• Precipitation is favoured in clouds with – Large liquid water content (i.e., deep cumulus)

– Broad drop spectrum

– Large drops (must be larger than ~20 m)

– Large vertical extent (=long growth/collision times)


Precipitation Formation Through Ice Processes

Ice forms on ice nuclei (IN)

• Silicates (soil dust, etc.)

• Clays

• Fungal spores

• Combustion particles (soot, etc.)

• Other industrial material


Ice formation Processes

Contact nucleationfreezing

Immersionfreezing

(Rate proportionalto drop volume)

Between–10 oC and –39 oC

Result = very few crystals

Homogeneousfreezing

Below –39 oC

Result = complete freezing of all drops


The Growth Advantage of Ice Crystals

At –20 oC at 100% RH Sice = 24%

Compare with typical Sliq = 0.05-0.5% !

Few crystals grow at expense of drops

Air is Marginally supersaturated with respect to liquid water in a rising cloud thermal

Highly supersaturated with respect to ice

Subsequent growth from accretion and aggregation


Atmospheric Ice Nuclei Concentrations


Effect of Freezing on Cloud Development

• Intensification of rain

• Release of latent heat aloft (giving further buoyancy)


Questions for lecture 4

• Define what is meant by the activation of cloud drops

• Why do more cloud drops form in clouds with higher updraught speeds?

• When you leave this lecture, what sort of clouds are visible and what is their typical droplet concentration?

• Based on the figure on slide 12, what droplet concentration would occur in an atmosphere with a) an aerosol concentration of 50 cm-3 of diameter 0.05 m and b) 200 cm-3 and 0.1 m diameter? Explain the difference.

• Explain why condensation of water on growing droplets is not enough to initiate rain

cloud microphysics

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