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

Vg

1000

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

Click to add title

Click to add text

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

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 Sea

  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

03/11/13Bureau of

Meteorology

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

03/11/13Bureau of

Meteorology

03/11/13Bureau of

Meteorology

03/11/13Bureau of

Meteorology

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