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 = number concentration per litre v = terminal fall speed in 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 Condensation (Aitken) nuclei 101Typical cloud droplet 10070Large cloud droplet 1000 = 1 mm650Typical raindrop 2500 = 2½ mm900Large 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 °C 2.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 coalescence in warm clouds It takes about 10 6 small cloud droplets (10  m) to form one large 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 Drop diameter, D (mm) Drop diameter, D (mm) 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 where R h is the rainfall rate (mm h -1 )

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  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 process c.f aitken nuclei (<0.2  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-42 1 in in in in none0 Proportion frozen T (°C) Proportion of cloud 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 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 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 reasons you have to defrost your 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

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 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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