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1. Hail growth; Thunderstorm electrification2. Cyclostrophic balance in tornadoes
Ahrens Chapter 7/8: Precipitation
Section on Hail
Chapter 14/15: Lightning & Thunder
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
Typical hail pellets~0.5 cm
Grapefruit-sized hailstones~10 cm
Serious Hazard
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
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…
Lowest part of cloudoften weakly positivelycharged
Lightning discharge
• Air has a low conductivity, but it can only cope with a gradient in charge (an electric field) up to ~106 volts per metre – beyond that it discharges: Lightning.
• 3 types of lightning:– 1a Within cloud– 1b Cloud to air– 2 Cloud to ground – most
energetic
Produce cloud flashes}
Lightning time sequence:1. Stepped Leader
Initial chargedistribution
Preliminarybreakdown inlower cloud – neutralizes the positive charge in cloud base
‘Stepped Leader’ advances in ~50 m steps, in 1 stime period between steps ~50 s
Downwards spread of negative charge induces a positive charge at ground
2. Attachment and 1st Stroke
Stepped leadercontinues 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
3. Dart leader, subsequent strokes
New regions of negative charge in the cloud are connected
Dart leader moves down the mainpath 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
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)
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.
Days with thunder (1971-2000)
Distribution of lightning
Global Electrical Circuit
Thunderstormsare the ‘batteries’driving the circuit
Blue Jets, Red Sprites, etc.
These are allfairly recentlydiscovered electricalphenomena, closelyassociated withthunderstorms, that probably play a rolein the global electricalcircuit.
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
Supercell, Kansas, rotating updraught
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…
Cyclostrophic balance
Acceleration (= Force/mass)given by: v2/r
v ~30 m s-1
r ~1000 m
v2/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
CentrifugalForce =
PGF
Lv
r
mv2
r
Tornado/supercellcase
Doesn’t this look a bit familiar?
CoriolisForce
PGF
Lv ~ 10 m s-1
Geostrophic Balance
r ~ 500 km
Centrifugal accelerationgiven by: v2/r
v ~10 m s-1
r ~500000 m
v2/r ~0.0002 m s-2
Centrifugal cceleration much smaller than the supercell case.
Coriolis force is due to planetary rotation
Centrifugal force is due to ‘local’ rotation
Large-scaleweathersystem
Coriolis Force
sin2f
Apparent force that acts on anythingthat moves in the Earth’s rotating frameof reference.
Coriolis parameter, f:
f is zero at equator, maximum at poles
mfvCF v = 10 m s-1
CF/m =0.0011 m s-2
At 50°Nf = 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
Comparing Coriolis & centrifugal forces
CoriolisForce
PGF
Lv ~ 10 m s-1
Geostrophic Balance
r ~ 500 km
Centrifugal accelerationgiven by: v2/r
v ~10 m s-1
r ~500000 m
v2/r ~0.0002 m s-2
Coriolis force is due to planetary rotation
Centrifugal force is due to ‘local’ rotation
Coriolis accelerationgiven by: fv
~0.0011 m s-2
Is bigger, but in some casesthe centrifugal acceleration isimportant at synoptic scales;But Ignore for now!
CentrifugalForce =
PGF
Cyclostrophic balance
Lv
r
mv2Centrifugal accelerationgiven by: v2/r
v ~30 m s-1
r ~1000 m
v2/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 accelerationgiven by: fv
~0.0033 m s-2
Is much smaller thancentrifugal: can ignoreCoriolis force for small scalerotations: storms/tornadoes
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!
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)
Tornadoes from supercell storms
Funnel cloud
Dust/debrisstirred upat surface
Pylonfor scale
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:– 21st March 2004 – S. Midlands– 28th July 2005 – Lincolnshire– Data & images from www.torro.org.uk
Damage in Oxfordshire
Also accompanied by 2cm diameter hail
UK supercell storm: 28th July 2005
Nottingham Skew T-log P – large CAPE
Path of two supercells:Right-moving is the strongest
Nr Peterborough, 28 July 2005
Damage nr. Peterborough
Farnborough, Dec. 2006
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
Analysis: 0000 Wed 11 Nov
1200 Wed 11 Nov
0000 Thu 12 Nov
1200 Thu 12 Nov
0000 Fri 13
1200 Fri 13
0000 Sat 14
1200 Sat 14