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Precipitation development; Warm and Cold clouds. >0 ° C. <0 ° C. Last lectures from me…. Cloud droplet formation (micro-scales) Cloud/fog formation processes (macro-scales) This lecture – return to the micro-scales. Cloud droplets and Raindrop sizes. r = radius in m

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last lectures from me
Last lectures from me…
  • Cloud droplet formation (micro-scales)
  • Cloud/fog formation processes (macro-scales)
  • This lecture – return to the micro-scales
cloud droplets and raindrop sizes
Cloud droplets and Raindrop sizes

r = radius in m

n = numberconcentrationper litre

v = terminalfall speedin cm/s

How do droplets grow and become raindrops?

slide4

Q

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

A: Updrafts can keep small cloud droplets suspended

Need stronger updraughts to support larger drops…

slide5

Drops break up for larger sizes;

Max. size ~8-10 mm

Q

What do rain drops look like?

equivalent diameter (mm)

of rain drop

slide6

How do cloud droplets (radius = 10 m)

turn into rain drops (1 mm) ?

Q

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 . . .

  • There are 2 main processes:
  • In ‘warm’ clouds with cloud top T > -15 °C
  • In ‘cold’ clouds with cloud top T <-15 °C
slide7

Raindrop formation by

collision and coalescence

in warm clouds

It takes about 106 small cloud

droplets (10 mm) to form one

large raindrop (1000 mm)

slide8

‘Statistical’

Stochastic model of collisions and droplet growth

Start with 100 drops

In 1 timestep, 10% grow

Next step, repeat…

End up with a logarithmic size distribution…

Actually, more complicated…

slide9

Cascade process

Raindrops reaching Earth’s surface rarely exceed 5 mm (5000 mm). Collisions or

glancing blows between large raindrops break them into smaller drops.

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

slide10

6000

10000

5000

1000

different

rain rates

4000

100

3000

10

2000

1

1000

1

2

3

4

5

6

1

2

3

4

5

6

Drop diameter, D (mm)

Drop diameter, D (mm)

Distribution of raindrop sizes – raindrop spectra

No. of drops in each class size per m3

the Marshall-Palmer distribution

n(D) = noe-ΛD

no = 8 x 103 ; Λ= 4.1 Rh-0.21 where Rh is the rainfall rate (mm h-1)

slide11

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 droplet

more than about 200 mm.

If RH below the cloud is high, then the droplets will arrive at the ground as drizzle, defined as diameter of drop < 500 mm (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.

slide15

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

but a 1000 mm (1mm) drop will not freeze until T ≈ -11 °C.

Cold clouds (temperate latitudes and polewards).

Q

Does water always freeze at 0 °C ?

A

It depends … on its volume and the presence of ice nuclei.

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 molecules

slide16

Proportion of

cloud droplets frozen

at different temperatures

T (°C)

Proportion

frozen

0

none

-10

1 in 106

-20

1 in 105

-30

1 in 104

-35

1 in 102

-42

all

Ice nuclei

Ice or freezing nuclei aid the freezing process

c.f aitken nuclei (<0.2 mm) for condensation nuclei.

1 cm3 of pure water in a test tube wouldn’t freeze

until T was about -3 to – 5 °C.

slide17

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.
slide18

If 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.

Bergeron process arises

since svpice<svpwater

so ice grows at the expense of

supercooled water droplets

vapour pressure

Super-cooledwater

0 °C

ice

temperature

slide19

One of the reasons

you have to defrost

your freezer regularly…

slide20

Bergeron

process

slide23

+

Angle ~104°

+

-

Sheets of molecules – viewed from above

Why are snowflakes hexagonal? …it’s complicated!

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

slide24

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

Shape of H2O molecule and H-bonding gives rise to hexagonal crystals

summary
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 surface
  • Precipitation can evaporate en route