Climate & Introduction to Earth System Science Meteorology ...maths.ucd.ie/met/cess/LectureNotes/PDFs/MAPH10050-CH06.pdf · Earth System Science ... THE MECHANISM OF GEOSTROPHIC FLOW

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Introduction to Meteorology & Climate

Climate & Earth System Science

Peter LynchMeteorology & Climate Centre

School of Mathematical SciencesUniversity College Dublin


Introduction toMeteorology & Climate

MAPH 10050

Peter Lynch

Meteorology & Climate CentreSchool of Mathematical SciencesUniversity College Dublin


Chapter 06

Atmospheric Forces and Wind


Introduction to Meteorology & Climate Introduction to Meteorology & Climate

Newton’s LawsNewton’s Laws

First Law (Law of Inertia):First Law (Law of Inertia):

A body at rest tends to stay at rest A body at rest tends to stay at rest while a body in motion tends to stay while a body in motion tends to stay in motion, in motion, travellingtravelling at a constant at a constant speed and in a straight line, until speed and in a straight line, until acted upon by an acted upon by an outside forceoutside force

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Newton’s LawsNewton’s Laws

Second Law (Law of Momentum):Second Law (Law of Momentum):

A force exerted on an object causes it A force exerted on an object causes it to accelerate in the direction of the to accelerate in the direction of the force.force.

Force equals Mass times Acceleration:Force equals Mass times Acceleration:

F = m aF = m a


Sum offorces


PARALLELOGRAM OF FORCES


FORCES THAT MOVE THE AIRFORCES THAT MOVE THE AIR

(1)(1) Gravitational force:Gravitational force:

Is directed downward , normal Is directed downward , normal to the Earthto the EarthEqual to the mass of the air Equal to the mass of the air times the gravitational times the gravitational acceleration: acceleration: mgmg


(2) Pressure (2) Pressure gradient gradient

forceforce


(2) PRESSURE GRADIENT FORCE(2) PRESSURE GRADIENT FORCE

Is a function of the Is a function of the pressure differencepressure difference and and air density:air density:

F = (1/F = (1/ρρ) ) ∂∂p/∂p/∂nnwherewhere

F = Pressure gradient force per unit massF = Pressure gradient force per unit massρρ = Density of air= Density of airp = Pressure p = Pressure n = Distance normal to isobarsn = Distance normal to isobars

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Mean sea-level pressure

analysis for Sunday, 1st

February, 1953




(3) APPARENT FORCES (3) APPARENT FORCES IN THE ATMOSPHEREIN THE ATMOSPHERE

CoriolisCoriolis ForceForce

Centrifugal Force/Centripetal Centrifugal Force/Centripetal accelerationacceleration


CORIOLIS EFFECTCORIOLIS EFFECT

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Roundabout film



•D

ublin

٠٠٠٠٠٠٠


CORIOLIS EFFECTCORIOLIS EFFECT

Weather map shows that winds go Weather map shows that winds go around a low pressurearound a low pressure area, not across area, not across the isobars. the isobars.

This deviation is a result of the This deviation is a result of the Earth's Earth's rotationrotation and has been named the and has been named the corioliscoriolis effect (effect (corioliscoriolis force)force)

This force always acts at right angles to This force always acts at right angles to the direction of motion.the direction of motion.


• Occurred 31 Jan/1 Feb, 1953.

• Greatest storm surge on recordfor the North Sea

• 100,000 hectares flooded andmore than 300 people lost theirlives In eastern England

• 5 dykes burst and 1,800 peopledrowned in Holland.

• The Princess Victoria sank inthe North Channel.

THE 1953 STORM


Mean sea-level pressure


February, 1953

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Mean sea-level wind


February, 1953


Sea-level pressure and wind


A POPULAR MISCONCEPTION

In the Southern Hemisphere,

the water going down the

plug-hole spins in

the opposite direction.


CENTRIFUGAL

FORCE


THE MECHANISM OF GEOSTROPHIC FLOW


GEOSTROPHIC FLOW GEOSTROPHIC FLOW

A A balance is reachedbalance is reached between the between the CoriolisCoriolis force and the pressure force and the pressure gradient force.gradient force.

If there is no friction, this occurs If there is no friction, this occurs when the movement of air is when the movement of air is parallelparallelto the isobarsto the isobars..

At this point there is no net force on At this point there is no net force on the air parcel the air parcel -- no acceleration no acceleration -- it it now moves with now moves with constant velocityconstant velocity..

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GEOSTROPHIC FLOW …GEOSTROPHIC FLOW …

Under these idealized conditions the Under these idealized conditions the airflow is said to be in airflow is said to be in geostrophicgeostrophicbalancebalance..

Wind generated is called the Wind generated is called the geostrophicgeostrophic windwind..

Normally only applies to winds aloft.Normally only applies to winds aloft.

Surface winds are subject to frictionSurface winds are subject to friction..



THE GEOSTROPHIC WINDTHE GEOSTROPHIC WIND

An exact balance between PGF and An exact balance between PGF and CoriolisCoriolis forceforce

PP+1

P+2

fVCF =

npPGF∂∂

−=ρ1

nn

High Pressure

Low Pressure

V


GRADIENT WINDSGRADIENT WINDSWinds around centers of high or low Winds around centers of high or low pressure follow curved paths in order pressure follow curved paths in order to stay parallel to the isobars.to stay parallel to the isobars.These winds are called These winds are called gradient gradient windswinds..Low pressure centers are called Low pressure centers are called cyclonescyclones -- rotation is counterrotation is counter--clockwise clockwise –– the same as the earth. the same as the earth. Centers of high pressure are called Centers of high pressure are called antianti--cyclonescyclones..


THE GRADIENT WIND

LOWPGFCoriolis

Force

V

Cyclonic flow

HIGHCoriolis ForcePGF

V

AnticyclonicAnticyclonic flowflow

SubSub--geostrophicgeostrophic SuperSuper--geostrophicgeostrophic

CFF CFF


FRICTIONAL DRAG NEAR THE SURFACEFRICTIONAL DRAG NEAR THE SURFACE

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THE SURFACE WINDTHE SURFACE WIND

Friction causes the wind back and reduce

PP

P+P+ΔΔPP

PGF

CFCFFFFF

vv



SURFACE WINDSSURFACE WINDSFriction affects winds only close to the Friction affects winds only close to the earth's surface.earth's surface.

Here we must balance Here we must balance three forcesthree forces --CoriolisCoriolis, pressure gradient and friction., pressure gradient and friction.

Net effect is a Net effect is a net inflownet inflow around a around a cyclone, an effect known as cyclone, an effect known as convergenceconvergence..

Around an anticyclone there is a Around an anticyclone there is a net outflownet outflow, , divergencedivergence..

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Aneroid BarometerAneroid Barometer



SEA BREEZESSEA BREEZES

Are the result of differential heatingof the ocean and the landDuring the day the land heats up quickly while the ocean heats up slowlyHigh temperature over the land, lower temperature over the oceanAt the surface - high pressure over the ocean, low pressure over the land - causes wind at the surface to flow from the ocean to the land.


Sea Breeze Effect


LAND BREEZELAND BREEZE

At night the land cools rapidly At night the land cools rapidly to a to a temperauretemperaure below that below that of the ocean. of the ocean.

Wind reverses Wind reverses -- flows from flows from the land to the ocean at the the land to the ocean at the surface (land breeze)surface (land breeze)

Introduction to Meteorology & ClimateFig. 6.27

Land Breeze Effect: Night-time

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15:45 UTC


18:15 UTC


20:15 UTC


21:43 UTC


Conclusion

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Climate & Introduction to Earth System Science Meteorology ...maths.ucd.ie/met/cess/LectureNotes/PDFs/MAPH10050-CH06.pdf · Earth System Science ... THE MECHANISM OF GEOSTROPHIC FLOW