Cloud Physics Summary - PowerPoint Presentation

Slide1

Cloud Physics Summary

SOURCE

:

www.geos.ed.ac.uk

/~

dstevens

/...DS_L3_Precipitation_development.pptSlide2

Precipitation development;

Warm and Cold clouds

>0

°

C

<0

°

CSlide3

Last lectures from me…

Cloud droplet formation (micro-scales)

Cloud/fog formation processes (macro-scales)This lecture – return to the micro-scalesSlide4

Cloud droplets and Raindrop sizes

How do droplets grow and become raindrops?

r = radius in

m

n = number

concentration

per litre

v = terminal

fall speed

in cm/sSlide5

Why doesn’t it always rain when there are clouds?

A: Updrafts can keep small cloud droplets suspended

Radius

(



m)

Terminal Velocity

(cm s

-1

)

Type of Particle

0.1

0.0001

Condensation (Aitken) nuclei

101Typical cloud droplet10070Large cloud droplet1000 = 1 mm650Typical raindrop2500 = 2½ mm900Large raindrop

Q

Need stronger updraughts to support larger drops…Slide6

What do rain drops look like?

Q

equivalent diameter (mm)

of rain drop

Drops break up for larger sizes;

Max. size ~8-10 mmSlide7

How do cloud droplets (radius = 10



m)

turn into rain drops (1 mm) ?

There are 2 main processes:

In ‘warm’ clouds with cloud top T > -15

°

C

In ‘cold’ clouds with cloud top T <-15

°

C

Initial growth by condensation, but this is limited by diffusion…

They never get a chance to grow into raindrops by condensation alone – this process would take D A Y S . . .

QSlide8

Raindrop formation by

collision and coalescence

in warm clouds

It takes about 10

6

small cloud

droplets (10

m

m) to form one

large raindrop (1000

m

m)Slide9

Stochastic model of collisions and droplet growth

‘Statistical’

Start with 100 drops

In 1 timestep, 10% grow

Next step, repeat…

End up with a logarithmic size distribution…

Actually, more complicated…Slide10

Cascade process

Raindrops reaching Earth’s surface rarely exceed 5 mm (5000

m

m). Collisions or

glancing blows between large raindrops break them into smaller drops.

Also surface tension is too weak to hold the larger drops togetherSlide11

Distribution of raindrop sizes – raindrop spectra

the

Marshall-Palmer

distribution

1

2

3

4

5

6

Drop diameter,

D

(mm)

1000

30002000400050006000

1

2

3

4

5

6

Drop diameter,

D

(mm)

1

100

10

1000

10000

different

rain rates

n

(D) =

n

o

e

-

Λ

D

No. of drops in each class size per m

3

n

o

= 8 x 10

3

;

Λ

= 4.1

R

h

-0.21

where

R

h

is the rainfall rate (mm h

-1

)Slide12

Depth of cloud influences type of rain

Stratus – thin cloud (<500 m) and has a slow upward

movement (< 0.1 ms

-1

).Growth by coalescence wouldn’t produce a dropletmore than about 200

m

m.

If RH below the cloud is high, then the droplets will arrive at the ground as

drizzle

, defined as diameter of drop < 500

m

m (0.5mm).

Thicker clouds, formed by convective motion, can have stronger updrafts and can keep larger cloud droplets aloft, permitting them to join (coalesce) with more droplets and grow to greater sizes. Slide13

1 Low – Nimbostratus (Ns)Slide14

3 Cumulonimbus (Cb)Slide15

Supplementary feature: virga

Slide16

Cold clouds (temperate latitudes and polewards).

Does water always freeze at 0

°

C ?

It depends … on its volume and the presence of

ice nuclei

.

Ice in your freezer in an ice tray – it’ll freeze at 0

°

C.

but a 1000

m

m (1mm) drop will not freeze until

T

≈ -11 °C.For ice to form all the water molecules must align in the proper crystal structure – in a large volume there is a high chance a few of the molecules will line up in the proper manner whereas in a small volume of water the chances are reduced, simply because there are fewer moleculesQASlide17

Ice nuclei

Ice or freezing nuclei aid the freezing process

c.f aitken nuclei (<0.2

m

m) for condensation nuclei.

1 cm

3

of pure water in a test tube wouldn’t freeze

until

T

was about -3 to – 5

°

C.

all

-421 in 102-351 in 104-301 in 105-201 in 106-10none0ProportionfrozenT (°C)

Proportion of

cloud droplets frozen

at different temperaturesSlide18

Ice nuclei

- are less common than Aitken nuclei

most effective ones have the same crystal shape

as ice crystals hence ice can form around and on them easily.

- kaolonite (clay) minerals are effective ice nuclei

- are most effective at about -10

°

C

because of the relative sparseness of ice nuclei, ice crystals

and supercooled water can coexist at the same time.

this last point is crucial in the formation of precipitation

in cold clouds as it gives rise to the

Bergeron

process.Slide19

vapour pressure

temperature

0

°

C

ice

Super-cooled

water

Bergeron process arises

since svp

ice

<svp

water

so ice grows at the expense of

supercooled water dropletsIf you look at the area in-between the two SVP curves you’ll see that an air parcel here would be unsaturated with respect to water but supersaturated with respect to ice. That means net evaporation will take place from the water but net condensation to the ice.Slide20

One of the reasons

you have to defrost

your freezer regularly…Slide21

Bergeron

processSlide22

Lab ice crystal growing from super-cooled water dropsSlide23
Slide24

Why are snowflakes hexagonal? …it’s complicated!

Angle ~104

°

+

+

-

Sheets of molecules – viewed from above

http://www.uwgb.edu/dutchs/PETROLGY/Ice%20Structure.HTMSlide25

Shape of H

2

O molecule and H-bonding gives rise to hexagonal crystals

Melting and re-freezing gives rise to vast variety of snow flakesSlide26
Slide27

Clouds can be a mixture of water droplets and iceSlide28
Slide29
Slide30
Slide31
Slide32
Slide33

Summary

Cloud particle size limited to a few mm by fall velocity

Droplets (μm) grow to raindrops (mm) by two main routes:Warm clouds: condensation, collision, coalescence (then break-up)Cold clouds: super-cooled water freezes on ice nuclei – producing larger ice particles – often melt en route to surfacePrecipitation can evaporate en routeSlide34

0000 Fri 06 NovSlide35

1200 FridaySlide36

0000 SaturdaySlide37

1200 SaturdaySlide38

0000 SundaySlide39

1200 SundaySlide40

0000 Monday