1. L20 Tornadoes: cyclostrophic balance Ahrens Chapter 7/8: Precipitation Section on Hail Chapter 14/15: Lightning & Thunder

2. Edinburgh Storm 8 th July 2011 Photo: Brian Cameron

3. Edinburgh Floods 8 th July 2011 (Balcarres St, Morningside)

4. CAPE

5. Royal Meteorological Society Meetings Tuesday 8 th November, 5.30pm Institute of Geography, Drummond Street “A New Flood Warning System for Scotland” Joint presentation by Met Office/SEPA Everyone welcome (free tea+biscuits from 5pm)

6. Supercell thunderstorms Rotating updraught –Rotation causes the storm to be more robust – longer-lived, and therefore more dangerous Forms an area of low pressure at centre of rotation, called a mesolow Updraught centred on the low pressure Circulation around the low is in cyclostrophic balance…

7. Cyclostrophic balance Acceleration (= Force/mass) given by: v 2 /r v ~30 m s -1 r ~1000 m v 2 /r ~0.9 m s -2 Rotating air in a supercell generates an area of low pressure at the centre of the rotating updraught. The circulation is in ‘cyclostrophic balance’, where the pressure gradient force (PGF) is balanced by a ‘centrifugal force’ (really a centripetal acceleration) Centrifugal Force = PGF L v r Tornado/ supercell case

8. Doesn’t this look a bit familiar? Coriolis Force PGF L v ~ 10 m s -1 Geostrophic Balance r ~ 500 km Centripetal acceleration given by: v 2 /r v ~10 m s -1 r ~500000 m v 2 /r ~0.0002 m s -2 Centripetal acceleration much smaller than the supercell case. Coriolis force is due to planetary rotation Centrifugal force (or centripetal acceleration) is due to ‘local’ rotation Large-scale weather system

9. Coriolis Force Apparent force that acts on anything that moves in the Earth’s rotating frame of reference. Coriolis parameter, f: f is zero at equator, maximum at poles v = 10 m s -1 CF/m = 0.0011 m s -2 At 50 °N f = 1.1 x 10 -4 s -1  is the Earth’s rotation rate = 2  radians per day, or, in SI units (seconds):  = 2  /(24x60x60) per second = 7.27 x 10 -5 s -1

10. Comparing Coriolis & centrifugal forces Coriolis Force PGF L v ~ 10 m s -1 Geostrophic Balance r ~ 500 km Centripetal acceleration given by: v 2 /r v ~10 m s -1 r ~500000 m v 2 /r ~0.0002 m s -2 Coriolis force is due to planetary rotation Centrifugal force is due to ‘local’ rotation Coriolis acceleration given by: fv ~0.0011 m s -2 Is bigger, but in some cases the centripetal acceleration is important at synoptic scales; But Ignore for now!

11. Centrifugal Force = PGF Cyclostrophic balance L v Centripetal acceleration given by: v 2 /r v ~30 m s -1 r ~1000 m v 2 /r ~0.9 m s -2 Rotating air in a supercell generates an area of low pressure at the centre of the rotating updraught. The circulation is in ‘cyclostrophic balance’, where the pressure gradient force (PGF) is balanced by a centrifugal force Coriolis acceleration given by: fv ~0.0033 m s -2 Is much smaller than centripetal: can ignore Coriolis force for small scale rotations: storms/tornadoes

12. Summary of forces for rotating systems Supercell storms/tornadoes (~1 km across): –Cyclostrophic balance: –PGF vs. centrifugal force (ignore Coriolis) Synoptic weather systems (~1000 km): –Geostrophic balance: –PGF vs Coriolis Force (ignore centrifugal) Scale is all-important!

13. Back to Supercell storms Low pressure in rotating updraught can be so low that is causes saturation and forms a ‘funnel’ cloud (Drop in pressure is equivalent to ascent)

14. Tornadoes from supercell storms Funnel cloud Dust/debris stirred up at surface Pylon for scale

15. Supercells & Tornadoes in the UK Generally much less severe than a typical US supercell/tornado, nevertheless… The UK experiences around 40 tornadoes a year – they generally do not cause damage, or are not even noticed A couple of recent cases: –21 st March 2004 – S. Midlands –28 th July 2005 – Lincolnshire –Data & images from www.torro.org.uk

16. UK Tornadoes by Month Percentages 1960 - 1999 (Reynolds) From Royal Met Soc talk by Nigel Bolton, 2010

17. Damage in Oxfordshire Also accompanied by 2cm diameter hail

18. UK supercell storm: 28 th July 2005 Nottingham Skew T-log P – large CAPE Path of two supercells: Right-moving is the strongest

19. Nr Peterborough, 28 July 2005

20. Damage nr. Peterborough

21. Images of the Day, 28/07/05

22. Farnborough, Dec. 2006

23. Spatial scale of storms Tornadoes are generally very localised, but can cause severe damage on small scales (100’s metres) Met: Weather and Climate (next semester), covers larger scale storms: tropical cyclones (hurricanes), also mid- latitude cyclones in more detail. Hurricane Katrina, August 2005

24. Summary: Tornadoes Small scales (~1 km across) Flow close to cyclostrophic balance: –PGF in balance with a centrifugal force –strictly: PGF creates a centripetal acceleration –Ignore Coriolis force at these small scales –cf. Geostrophic balance in synoptic weather systems Tornadoes are an important hazard in the UK (as well as other parts of the world): they can cause very localised severe damage

25. Hail formation Starts with small ice crystal surrounded by abundant supercooled droplets – within a cloud with strong updraughts Growth by riming Once initial crystal shape is lost: graupel

26. Typical hail pellets ~0.5 cm Grapefruit-sized hailstones ~10 cm Serious Hazard

28. Hailstone structure Hail can grow quite rapidly (5-10 minutes) As it grows, requires larger and larger updraught velocities to support it One path is approximately horizontally across the cloud, growing as it traverses the updraught, then plummeting as it enters the downdraught Alternating light/dark layers due to different growth stages – dark layers have bubbles trapped; ‘wet’ growth vs ice (colder) growth

29. Cloud electrification Need a cold cloud – i.e. contains ice Radar data indicates graupel or hailstones As hail falls through cloud, bumping into other cloud particles, it tends to become negatively charged Exact mechanism is not clear, but falling hail tends to make the lower cloud negatively charged, and leaves the upper part of the cloud positively charged…

30. Lowest part of cloud often weakly positively charged

31. Lightning discharge Air has a low conductivity, but it can only cope with a gradient in charge (an electric field) up to ~10 6 volts per metre – beyond that it discharges: Lightning. 3 types of lightning: –1aWithin cloud –1bCloud to air –2Cloud to ground – most energetic Produce cloud flashes }

32. Lightning time sequence: 1. Stepped Leader Initial charge distribution Preliminary breakdown in lower cloud – neutralizes the positive charge in cloud base ‘Stepped Leader’ advances in ~50 m steps, in 1  s time period between steps ~50  s Downwards spread of negative charge induces a positive charge at ground

33. 2. Attachment and 1 st Stroke Stepped leader continues advance When within ~10-100 m of the highest objects, a discharge moves up from the ground to meet the downwards advancing stepped leader Once connected, large flow of electrons to ground – the ‘return stroke’ - Intense flash

34. 3. Dart leader, subsequent strokes New regions of negative charge in the cloud are connected Dart leader moves down the main path followed by the first stroke, sending more electrons downwards Further stroke; Usually 3 or 4 strokes to discharge the cloud. Charge can build up again in as little as 10s

35. NB Time-lapse photograph – many processes superimposed! Can see a range of stepped leaders, together with the path of the main stroke (re-used for subsequent strokes)

36. Thunder Return stroke raises air temperature in the channel it passes through to >30,000K very quickly – air has no time to expand, so pressure rises, and air expands rapidly. Creates a shock-wave, which then creates a sound- wave a little further away: thunder Travels at speed of sound: 330 m s -1 (i.e. 1 mile in ~5 seconds) Stepped leaders also create thunder, but much less than the main stroke Sound waves tend to be refracted upwards, limiting the range over which thunder can be heard to within ~25 km of the source.

37. Days with thunder (1971-2000)

38. Distribution of lightning

39. Global Electrical Circuit Thunderstorms are the ‘batteries’ driving the circuit

40. Blue Jets, Red Sprites, etc. These are all fairly recently discovered electrical phenomena, closely associated with thunderstorms, that probably play a role in the global electrical circuit.

41. Summary Hail –Produced in cold clouds, multiple ascent and descent cycles with growth by riming Lightning –Falling hail is negatively charged, leaving upper cloud positive, lower cloud negative –Stepped leader; Return stroke; Dart leader; subsequent strokes –Global electrical circuit driven by thunderstorms Thunder –Sound wave from 30000K heating by lightning stroke