Transcript Tropical Cyclone JUSTIN

Wind and Rain
MCC Synoptic Week
11-15 March 2013, UCD
Surface Weather Systems
 Weather systems in the northern hemisphere generally move
from west to east due to the earth’s rotation. Movement of
tropical systems such as hurricanes are more variable.
 In the northern hemisphere, winds blow anti-clockwise
around lows such as depressions, and clockwise around highs.
 When the isobars (lines of equal pressure) become more
closely spaced, then winds increase. That is, the closer the
isobars over a particular area, the higher the wind speed.
Geostrophic wind
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Typically the wind speed at 2000 feet / 600m
Assume air parcel moves from rest
P is pressure gradient force
Co is Coriolis = 2 Ω SinΦ
Co acts at right angles
Low
P
1000
1004
Co
1008
High
Balanced Geostrophic flow
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Balance when P=Co, ie equal and opposite
Vg is the Geostrophic wind
Blows parallel to isobars in free atmosphere
Forecasters measure Vg from scale
1000
Low
P
Vg
1004
1008
High
Co
This is the chart for Monday
Cyclonic curved flow
• Ce is centrifugal force due to
circular motion
• Ce reduces P
• Co must reduce to maintain
balance
Low
P
• Vg must reduce to Vgr which
is the gradient wind
• Forecasters make correction
for curvature to get Vgr
• Example eye of a storm
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Ce
Co
High
Vgr
Vg
Anticyclonic curved flow
• Ce acts in unison with P
• Co must increase to
maintain balance
•Vg must increase to Vgr
• Forecaster makes
correction for radius of
curvature to get Vgr
• Example periphery of a
winter High
Low
P
Ce
Co
High
Vg
Vgr
Another complication !
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A difference between
curvature of isobars and
trajectories occurs when
systems in motion
Strongest winds on south flank
of eastwards moving
depression
Strongest winds on north flank
of westwards moving
depression
Similar for mobile anticyclones
L
L
Surface wind flow
Low
P
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Near ground friction(F)
reduces wind speed
Co must reduce
Balance upset
Vectors realign so that
P+Co=F
V-the real wind is reduced
and blows towards low
pressure
V
F

Vg
Co


High
Surface wind flow
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Over the Sea V=2/3 Vgr, and
is backed approximately 15
degrees to the
isobars(depending on
stability)
Over the Land V =1/2 Vgr
and is backed as much as 40
degrees to the
isobars(depending on
roughness of ground and
stability)
L
15
0
H
L
40 0
H
Measuring Wind
• A confusion of units!
– Beaufort Forces
– Knots (Nautical Miles per hour)
– Miles per hour
– Kilometres per hour
– Metres per second
Windy or Calm?
Admiral Francis Beaufort
•Born in Navan
•Hydrographer to the Royal Navy
•Devised one of the first wind
scales in 1805, from Force 0 to
Force 12
•http://www.mii.connect.ie/histor
y/beaufort/beaufort.html
Beaufort Scale on Land and Sea
Force
Wind
(Knots)
WMO
Classification
0
Less than 1
Calm
1
1-3
Light Air
2
4-6
Light Breeze
3
7-10
Gentle Breeze
4
11-16
Moderate
Breeze
5
17-21
Fresh Breeze
6
22-27
Strong Breeze
7
28-33
Near Gale
8
34-40
Gale
9
41-47
Strong Gale
10
48-55
Storm
11
56-63
Violent Storm
12
64+
Hurricane
Appearance of Wind Effects
On the Water
On Land
Sea surface smooth and mirror-like
Calm, smoke rises vertically
Smoke drift indicates wind direction, still
Scaly ripples, no foam crests
wind vanes
Small wavelets, crests glassy, no
Wind felt on face, leaves rustle, vanes
breaking
begin to move
Large wavelets, crests begin to break,
Leaves and small twigs constantly
scattered whitecaps
moving, light flags extended
Small waves 1-4 ft. becoming longer,
Dust, leaves, and loose paper lifted, small
numerous whitecaps
tree branches move
Moderate waves 4-8 ft taking longer
Small trees in leaf begin to sway
form, many whitecaps, some spray
Larger waves 8-13 ft, whitecaps
Larger tree branches moving, whistling in
common, more spray
wires
Sea heaps up, waves 13-19 ft, white
Whole trees moving, resistance felt
foam streaks off breakers
walking against wind
Moderately high (18-25 ft) waves of
greater length, edges of crests begin to
Twigs breaking off trees, generally
break into spindrift, foam blown in
impedes progress
streaks
High waves (23-32 ft), sea begins to
Slight structural damage occurs, slate
roll, dense streaks of foam, spray may
blows off roofs
reduce visibility
Very high waves (29-41 ft) with
Seldom experienced on land, trees broken
overhanging crests, sea white with
or uprooted, "considerable structural
densely blown foam, heavy rolling,
damage"
lowered visibility
Exceptionally high (37-52 ft) waves,
foam patches cover sea, visibility more
reduced
Air filled with foam, waves over 45 ft,
sea completely white with driving
spray, visibility greatly reduced
Velocity conversions
0.621
0.54
0.278
km/h
mph
Kts
m/s
Beaufort
2
1.2
1.1
0.6
1
4
2.5
2.2
1.1
1
5
3.1
2.7
1.4
1
6
3.7
3.2
1.7
2
8
5.0
4.3
2.2
2
10
6
5
3
2
15
9
8
4
3
20
12
11
6
4
25
16
13
7
4
30
19
16
8
4
35
22
19
10
5
Thomas Romney Robinson
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Born Dublin, 1792
longtime director of the Armagh
Astronomical Observatory
• 4-cup anemometer design, 1846
http://star.arm.ac.uk/history/instrum
ents/Robinson-cupanemometer.html
William Henry Dines
• Born London, 1855
• Dines Pressure Tube
Anemometer
• Pressure difference
between tube mouth and
sides moves float in sealed
chamber
• Allows instrument to be
remote to recorder.
onlinelibrary.wiley.com/doi/10.1256
/wea.38.05/pdf
Fundamentals of Wind
• Measured at 10m above the ground
• (Always be aware that Malin Head is much higher.
Treat wind readings from oil platforms, ships etc
with caution).
• Mean Speed – average over a ten-minute period
• Gust Speed – highest instantaneous wind speed
• Gusts normally do the damage!!
Wind Speed and Gusts
• Wind speed mentioned in marine observations,
forecasts, and warnings is the average speed over a 10
minute interval.
• Wind gusts may be up to 70% higher than the average
wind speed.
• For example, if the average wind speed is 25 knots,
occasional gusts up to 40 knots can be expected,
depending on stability of the air-mass.
Surface wind
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Speed: 1knot = 0. 514 m/s = 1. 15 mph
Direction: Direction from which wind blows measured
clockwise from true North
A veer is a clockwise change
A back is an anticlockwise change
Mean speed is average over 10 minute period
Gusts and lulls are rapid fluctuations due to obstacles
and instability which are called turbulence
Speed
Time
Pressure and drawing of
Isobars
• Plotted values are reduced to
MSL
• Isobars join areas of equal
pressure
• Back to wind, low pressure to
the left (Buy’s Ballots Law)
• On large Atlantic charts4hPa intervals
• On hourly charts –1 hPa
intervals
• A pascal =1 Pa = 1N/m2
• A hecto Pascal = 100 Pa = 10
N /m2
• 100Pa = 1mb = 1 hPa
High
X 1005
X 1002
Low
X 997
X 999
X 998
x1002
X 1008
X 1008
X 1013
The Sea Breeze
• An onshore breeze which develops in
coastal areas on a warm day.
• Differential heating between the land and
sea.
Sea breeze formation
Two columns of air
At dawn:
Sea breeze formation
As land heats up a
circulation develops
How… and When?
• Land temperatures need to be at least 3.5 oC warmer
than sea temperatures …
• They are very common and strong in tropical
regions
• In Ireland generally from March to late September.
Land breeze
• Another thermally driven circulation.
• Sea warmer than land at night.
• Usually weaker than the sea breeze.
• Very rarely exceeds 10 kt.
It’s not just a coastal thing
• Sea breezes can occasionally penetrate over
50km inland
• Sea breezes can enhance convection due to
convergence, particularly on peninsulas
Bureau
03/11/13
Sea breeze front
• Offshore wind opposes sea breeze
• Enhanced convergence
• Tightening temperature and humidity gradients
Sea Breeze Summary
• Nice cooling breeze on the coast.
• Can bring in offshore stratus to spoil a sunny day
right on the coast
• Useful for yachtsmen and inshore fishermen
• Enhanced convection can lead to some severe
weather.
A good sea breeze day
Mountain Airflow
• Modification of broadscale winds
Deflection, channelling and shelter
Effect on depressions and fronts
• Lee waves
• Locally induced winds
Katabatic and anabatic winds
Valley wind circulations
• Downslope winds
Föhn and Chinook winds
Bora wind
Deflection
• Factors favouring deflection
over mountain barrier:
Long barrier
Perpendicular wind flow
Concave barrier
Unstable air
• Factors favouring deflection around mountain barrier:
Short barrier
Oblique wind flow
Convex barrier
Stable air
Channelling
Gaps in barrier strengthen wind flow
e.g. Mistral (between Alps & Massif Centrale)
Katabatic wind
• Down-slope wind, usually nocturnal
Cooling
• Speed: a few knots
• Depth: typically ~100 m
• Best on even, gentle slopes
Anabatic wind
• Day-time up-slope wind
• Speed: 5–10 knots
• Depth: up to 200 m
• Best on smooth, hot slopes
Heating
Föhn / Chinook winds
Condensation &
release of latent
heat
Warm
Cool
Wind Flow over Mountains
The Irish
Meteorological Service
Mountain Waves from Above
The Irish
Meteorological Service
Bureau
03/11/13
Bureau
03/11/13
Bureau
03/11/13
Lenticular Altocumulus
The Irish
Meteorological Service
Fog, Rain, Drizzle and Showers
• Fog, Drizzle and Rain distinguished by DROP
SIZE
• If droplets are suspended in the air (not falling)
then we have FOG or MIST (drop size up to
0.2mm diameter)
• Falling droplets from 0.2mm to 0.5mm are termed
DRIZZLE
• Drops of greater size constitute RAIN
Rain and Drizzle Rates
DRIZZLE
Light
Moderate
Heavy
Intermittent
< 0.3 mm/hr
0.3-0.5 mm/hr >0.5 mm/hr
Continuous
< 0.3 mm/hr
0.3-0.5 mm/hr >0.5 mm/hr
RAIN
Light
Moderate
Intermittent
< 2.0 mm/hr
2.0-6.0 mm/hr >6.0 mm/hr
Continuous
< 2.0 mm/hr
2.0-6.0 mm/hr >6.0 mm/hr
Heavy
Rain and Showers
• Rain
– Primarily large geographical scale
– Origin in dynamical processes
• Showers
– Small spatial scale (500m – 20Km)
– Convective in origin
– Much higher rates of rainfall
– Can be embedded in larger scale rain bands
Fronts Versus Showers
Fronts -give widespread rain
•Warm
•Cold
•Occlusion
Showers - small Scale
•20km
•last 10-20mins
•Convective •develop over warm sea in winter
Forced Ascent
•Air forced to rise
•Stratus cloud forms on
higher ground
•Drizzle or rain likely
Convection - creates instability
Cooler
Warm air rising
Hot
Warm
Air in contact with high
ground is warmer than
free air at the same
height.
Cooler
Warm air rising
Hot
Convection
Showers and
thunderstorms
Warm
Cooler
Hot
Cooler
Hot
Orographic Rainfall
• There is a clear statistical
link between average
rainfall and altitude.
• The higher the site, the
heavier the rainfall.
• Mechanisms leading to the
increase.
– Forced Ascent
– Enhanced Convection
Irish Rainfall Rates
• Range from about 800mm/yr (Dublin) to about
3000mm/yr (Kerry Mountains)
• Very variable in nature
• Greatest rainfall totals:
– Hourly 97mm Co. Antrim 1887
– Daily 243.5mm Co. Kerry 1993
– Monthly 790mm Co. Waterford 1996
• Hourly totals of > 10mm are uncommon in Ireland
Rainfall Records
3,965 mm Ballaghbeena Gap, Ireland 1960
22,987 mm Cherrapunji, India, 1861