wind and rain - maths.ucd.ie€¦ · sea heaps up, waves 13-19 ft, white foam streaks off breakers...
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Wind and Rain
MCC Synoptic Week11-15 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.
1000
1004
1008
Low
High
P
Geostrophic wind• 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
Co
Balanced Geostrophic flow
Low
High
P
Co
Vg1000
1004
1008
• 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
This is the chart for Monday
Cyclonic curved flow
Clic k to add title
Clic k to ad d tex t
Low
HighCo
PVg
Ce
Vgr
•Ce is centrifugal force due to circular motion
• Ce reduces P
• Co must reduce to maintain balance
•Vg must reduce to Vgr which is the gradient wind
• Forecasters make correction for curvature to get Vgr
• Example eye of a storm
Anticyclonic curved flow
High
Low
P
Co
Ce
Vgr
Vg
•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
Another complication !• 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
• 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
P
FCo
Vg
High
1004
1008
1012
Low
V
Surface wind flow
• 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)
H
L
15 0
40 0
L
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/history/beaufort/beaufort.html
Beaufort Scale on Land and SeaForce
Wind(Knots)
WMOClassification Appearance of Wind Effects
On the Water On Land0 Less than 1 Calm Sea surface smooth and mirror-like Calm, smoke rises vertically
1 1-3 Light Air Scaly ripples, no foam crestsSmoke drift indicates wind direction, still wind vanes
2 4-6 Light BreezeSmall wavelets, crests glassy, no breaking
Wind felt on face, leaves rustle, vanes begin to move
3 7-10 Gentle BreezeLarge wavelets, crests begin to break, scattered whitecaps
Leaves and small twigs constantly moving, light flags extended
4 11-16Moderate Breeze
Small waves 1-4 ft. becoming longer, numerous whitecaps
Dust, leaves, and loose paper lifted, small tree branches move
5 17-21 Fresh BreezeModerate waves 4-8 ft taking longer form, many whitecaps, some spray
Small trees in leaf begin to sway
6 22-27 Strong BreezeLarger waves 8-13 ft, whitecaps common, more spray
Larger tree branches moving, whistling in wires
7 28-33 Near GaleSea heaps up, waves 13-19 ft, white foam streaks off breakers
Whole trees moving, resistance felt walking against wind
8 34-40 Gale
Moderately high (18-25 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks
Twigs breaking off trees, generally impedes progress
9 41-47 Strong GaleHigh waves (23-32 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility
Slight structural damage occurs, slate blows off roofs
10 48-55 Storm
Very high waves (29-41 ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility
Seldom experienced on land, trees broken or uprooted, "considerable structural damage"
11 56-63 Violent StormExceptionally high (37-52 ft) waves, foam patches cover sea, visibility more reduced
12 64+ HurricaneAir 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
40 25 22 11 6
Thomas Romney Robinson
• Born Dublin, 1792
• longtime director of the Armagh Astronomical Observatory
• 4-cup anemometer design, 1846
http://star.arm.ac.uk/history/instruments/Robinson-cup-anemometer.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• 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 charts-4hPa 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
X 997
X 999
X 1002
X 1013
X 1008
X 998
High
X 1005
Low
X 1008x1002
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 airAt 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
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 windsDeflection, channelling and shelter Effect on depressions and fronts
• Lee waves
• Locally induced winds Katabatic and anabatic windsValley wind circulations
• Downslope winds Föhn and Chinook windsBora wind
Deflection• Factors favouring deflection
over mountain barrier: Long barrierPerpendicular wind flow Concave barrierUnstable air
• Factors favouring deflection around mountain barrier: Short barrierOblique wind flowConvex barrier Stable air
Channelling
Gaps in barrier strengthen wind flowe.g. Mistral (between Alps & Massif Centrale)
Katabatic wind
• Down-slope wind, usually nocturnal
• Speed: a few knots
• Depth: typically ~100 m
• Best on even, gentle slopes
Cooling
• Day-time up-slope wind
• Speed: 5–10 knots
• Depth: up to 200 m
• Best on smooth, hot slopes
Anabatic wind
Heating
Föhn / Chinook winds
CoolWarm
Condensation & release of latent
heat
Wind Flow over Mountains
The Irish Meteorological Service www.met.ie
Mountain Waves from Above
The Irish Meteorological Service www.met.ie
Lenticular Altocumulus
The Irish Meteorological Service www.met.ie
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
RAIN Light Moderate Heavy
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
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 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
Showers - small Scale •20km
•last 10-20mins•Convective -
•develop over warm sea in winter
Fronts -give widespread rain
•Warm
•Cold
•Occlusion
•Air forced to rise•Stratus cloud forms on higher ground•Drizzle or rain likely
Forced Ascent
Convection - creates instability
WarmCooler
Cooler
Air in contact with high ground is warmer than free air at the same
height.
Warm air rising Warm air rising
Hot Hot
Convection
Hot Hot
Warm
Cooler Cooler
Showers and thunderstorms
• 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
Orographic Rainfall
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