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MET 60

Topic 5

Clouds and Cloud Microphysics

(and precipitation processes)

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1. Cloud types

Ci

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Cc

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Cs

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Ac

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As

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Ns

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Sc

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St

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Cu

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Cu

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Cb

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2. Cloud Microphysics

Things to cover:

a) When does condensation occur?

b) How does condensation occur?c) Laboratory observations (in text)

d) In-situ observations (in text)

e) Modeling of clouds (in text)

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a) When does condensation occur?

• When RH 100%, right?

• Wrong!

• More complicated…

• Answer is related to…

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b) How does condensation occur?

There are two ways:

(i) without help

Homogeneous nucleation

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(ii) with help

Heterogeneous nucleation

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Homogeneous nucleation

Where vapor molecules collide and form tiny embryonic liquid water droplets.

Held together by surface tension.

Given enough molecules, i.e., high RH values, nucleation can happen…

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High RH values?

We can define supersaturation where RH > 100%

In the lab with clean air and pure water, we need RH >> 100% (say 400%)

Depends on if condensation is relative to a flat or curved surface

Curved surface = surface of embryonic droplet

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Such high RH values are NOT OBSERVED

Max observed supersaturations are up to about 1%

Homogeneous nucleation does not contribute to cloud formation

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Heterogeneous nucleation

Where condensation occurs around an aerosol particle which serves as the nucleus of the water droplet.

Liquid water droplet

aerosol particle

(can be soluble or not)

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Atmospheric aerosol

We can take samples of air and see what aerosols are present (e.g., Aitken nucleus counter)

http://www.sciencemuseum.org.uk/objects/meteorology/1908-193.aspx

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• Take a sample of moist air

• Expand rapidly → cooling → supersaturation

• Vapor will condense onto aerosol particles → “cloud”

• Allow droplets to settle onto a surface

• Count droplets e.g., with microscope

Things to look for:

• Amount (number of particles)

• Size of particles

• Type of particles (salt? dust? chemical?)

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Figure 5.7 → aerosol sizes

Notes: we’re looking at a few things in Cht. 5

be familiar with sizes & ranges

Figure 5.8 → a “formation” mechanism!

Aerosol distributions from MODIS…

http://modis-atmos.gsfc.nasa.gov/MOD04_L2/sample.html

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Some of these aerosols serve as Cloud Condensation Nuclei

“CCN”

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Observations of CCN

Depends where you look!

Over land?

Over ocean?

Near surface?

Higher up?

Day/night?

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Typical particle sizes and numbers

Figure 5.9 → both (all aerosol)

Notes:

• Range of sizes is 10-2 m to 100 m

• MANY more small particles than large ones

• Aitken nuclei have diameter < 0.2 m

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Figure 6.5 looks just at CCN

More CCN in air over land than over ocean

clean Arctic air 30 per cm3

Marine air 100 per cm3

polluted Continental air 300 per cm3

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More CCN near the surface in continental air

5x reduction in CCN from boundary layer to “free atmosphere” (which starts at 1 km)

But maritime air…

number density roughly constant up to cloud base

Also, daily max at 6pm (makes sense)

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Origins of CCNs

Continental air…

Source = landSoil and dust particlesVolcanic injectionAnthropogenic sources

Table 5.3

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Origins of CCNs

Marine air…

Salt particles

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Recent stuff

• Fine dust particles from the Gobi

• Blown east of the Pacific towards us

http://seattletimes.nwsource.com/ABPub/2006/12/27/2003498327.pdf

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Warm versus Cold Clouds

A warm cloud has T > 0C throughout

A cold cloud has T < 0C in part or all

• e.g., cirrus (Ci) would have T < 0C throughout (cold cloud)

• e.g., cumulonimbus (Cb) would have T < 0C in the upper part (cold cloud)

• e.g., status (St) would have T > 0C throughout (warm cloud)

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Almost all our rain (in San Jose) comes from cold clouds

• e.g., Friday’s rain (11/27/09)

11/2809 00Z OAK sounding:

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OAK, 11/28/09 @ 00Z

FZL 800 hPa

cloud

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Warm Clouds

Three things to monitor in a warm cloud:

a) Liquid Water Content (LWC)grams per cubic meter

b) Cloud droplet concentrationNumber per cc

c) Droplet size distributionhistogram

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How to sample droplets?

Impaction studies

• Fly through a cloud

• Impact droplets on oil-covered slides etc.

• Preserve droplets in oil for later studies

Other

e.g., scattering (p.216)

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How to sample droplets cont…

Measure LWC via a heated wire (p. 216)

Measure optical depths via optical methods (p.216)

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Measurements!

Figure 6.6 → aircraft measurements of

• w (vertical velocity component)

• LWC

• droplet size spectra

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Cold Clouds

A cold cloud contains:

a) Liquid water droplets (T > 0)

b) Supercooled water (T < 0)

c) Ice particles

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Recall from Thermo that

SVP over water > SVP over ice

Fig. 3.9

Thus – vapor will migrate towards ice rather than liquid droplets

Fig. 6.36 = extreme case

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Conclusion = ice particles are VIP in growth process leading to precipitation

– Ice crystals form and start to grow via…

• Homogeneous nucleation?

– Happens when T -40C

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• Hetergeneous nucleation?

– Requires nucleation sites (just like CCN)

– Called ice nuclei

– Fewer in number than CCN

– Need to have ice-like structure

• See http://www.edinformatics.com/interactive_molecules/ice.htm

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– In a study: 87% of snow crystals collected on the ground contained clay mineral particles @ center

– of which ½ were kaolinite, which is from soil

– In urban air, 108 aerosol particles per liter» Of which ONE can act as an ice nucleus @

temp. -20C !!!

– Figure 6.31

• Figure 6.35 if time!

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3. Cloud Climatology

Q: How much of the globe is cloud covered?

Q: What types of clouds?

Q: Why do we care?– Role in radiation budget– Role in climate– Clouds have a net COOLING effect

• (based on satellite studies)

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We also care because of clouds’ role in climate change!

Warmed atmosphere & ocean

More vapor in atmosphere More clouds (maybe?) But which coulds?

Low? High?

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Observations of cloud cover:http://isccp.giss.nasa.gov/climanal.html

Notes:1. “cloud amounts vary by about 1-3% compared

to a mean value of 66.7% ”2. “cloud top pressures vary by about 10-40 mb

compared to a mean value of 578.1 mb”3. “cloud top temperatures vary by about 1-4 K

compared to a mean value of 261.7 K”4. “cloud optical thicknesses vary by about 0.1-

0.4 compared to a mean value of 3.4”5. “surface temperatures vary by about 2-3 K

compared to a mean value of 288.4 K”

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Look at:

1. Global cloud cover variation (“cloud amount” in Part 1)

2. In Part 7, play with different cloud types

3. Cu – 11.6%

4. Sc – 12%

5. St – 1.75%

6. Ci – 13.4%

7. Cb – 2.89%

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Look at:

1. VIS-IR low cloud cover: 25.49%

2. VIS-IR low cloud cover: 18.65%

3. VIS-IR low cloud cover: 22.23%