# Precipitation development; Warm and Cold clouds >0 ° C<0 ° C.

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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 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 0.10.0001Condensation (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 123456 Drop diameter, D (mm) 1000 3000 2000 4000 5000 6000 123456 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 )

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 10 2 -35 1 in 10 4 -30 1 in 10 5 -20 1 in 10 6 -10 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 { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/12/3537525/slides/slide_18.jpg", "name": "vapour pressure temperature 0 °C ice Super-cooled water Bergeron process arises since svp 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 http://www.uwgb.edu/dutchs/PETROLGY/Ice%20Structure.HTM

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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