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

How do droplets grow and become raindrops?

r = radius in m

n = numberconcentrationper litre

v = terminalfall speedin cm/s

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

10 1 Typical cloud droplet

100 70 Large cloud droplet

1000 = 1 mm 650 Typical raindrop

2500 = 2½ mm 900 Large raindrop

Need stronger updraughts to support larger drops…

What do rain drops look like?

equivalent diameter (mm) of rain drop

Drops break up for larger sizes;Max. size ~8-10 mm

How do cloud droplets (radius = 10 m) turn into rain drops (1 mm) ?

There are 2 main processes:1. In ‘warm’ clouds with cloud top T > -15 °C2. 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 . . .

Raindrop formation by collision and coalescencein warm clouds

It takes about 106 small cloud droplets (10 m) to form onelarge raindrop (1000 m)

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…

Cascade process

Raindrops reaching Earth’s surface rarely exceed 5 mm (5000 m). Collisions or glancing blows between large raindrops break them into smaller drops.

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

Distribution of raindrop sizes – raindrop spectra

the Marshall-Palmer distribution

1 2 3 4 5 6Drop diameter, D (mm)

1000

3000

2000

4000

5000

6000

1 2 3 4 5 6Drop diameter, D (mm)

1

100

10

1000

10000

differentrain rates

n(D) = noe-ΛD

No.

of

drop

s in

eac

h cl

ass

size

per

m3

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

Depth of cloud influences type of rain

Stratus – thin cloud (<500 m) and has a slow upwardmovement (< 0.1 ms-1).

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

1 Low – Nimbostratus (Ns)

3 Cumulonimbus (Cb)

Supplementary feature: virga

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 (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 molecules

Ice nuclei

Ice or freezing nuclei aid the freezing processc.f aitken nuclei (<0.2 m) for condensation nuclei.1 cm3 of pure water in a test tube wouldn’t freeze until T was about -3 to – 5 °C.

all-42

1 in 102-35

1 in 104-30

1 in 105-20

1 in 106-10

none0

Proportionfrozen

T (°C)Proportion ofcloud droplets frozen

at different temperatures

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.

vapour

pre

ssure

temperature

0 °Cice

Super-cooledwater

Bergeron process arisessince svpice<svpwater

so ice grows at the expense of supercooled water droplets

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.

One of the reasonsyou have to defrostyour freezer regularly…

Bergeron

process

Lab ice crystal growing from super-cooled water drops

Why are snowflakes hexagonal? …it’s complicated!

Angle ~104° +

+

-

Sheets of molecules – viewed from above

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

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

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

Clouds can be a mixture of water droplets and ice

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

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