Climate & Introduction to Earth System Science Meteorology ...

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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 07Chapter 07

Global Scale Winds


Lecture 14


Simple Introductory Examples:Transport of heat to equalize temperature

Explain on blackboard

• Bar of steel: one end in the fridge, one in the oven.

• Tank of water: heated at one end, cooled at the other.


Voyage of ChristopherColumbus

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Fitzroy &

Darwin

BeagleVoyage


Fig. 7.10

Composite image (clouds & surface temperature). Note the line of clouds along the ITCZ

Introduction to Meteorology & Climate Introduction to Meteorology & Climate


Mean Monthly Cloud Cover (%) in JanuaryMean Monthly Cloud Cover (%) in January


Mean Monthly Cloud Cover (%) in JulyMean Monthly Cloud Cover (%) in July

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GLOBAL CIRCULATIONGeorge Hadley first suggested in 1735 the general concept of atmospheric circulation – a single cell, to explain the existence of the easterly winds at the surfaceCold air at pole - high pressure at surface. Warm air at equator - low pressure at surface. Pressure gradient force at surface will move air from pole to equator at surface. Return path at high altitudes. Coriolis force produces easterlies


Hadley Hadley CellCell


GLOBAL CIRCULATIONIn reality, we have three cells, with boundaries at about 30º and 60º latitude.

This results in sinking air at 30ºN and 30ºS. But sinking suppresses cloud development and precipitation. Hence most of the worlds deserts occur along these latitudes.

These are the horse latitudes.

Circulation at the surface moves from 30ºto the equator – easterly winds.


Three cell

model


Lecture 15


GLOBAL CIRCULATIONGLOBAL CIRCULATIONBetween 30º and 60º the circulation at the surface is from 30º to 60º, giving rise to westerly winds.Between 60º and the Poles, the flow is from the Pole to 60º, leading to easterly winds at the surface.Upwelling at the equator and at 60º –precipitation.The Doldrums.

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Conservation of Angular Momentum

Demonstration:Nut-on-a-string


Conservation of Angular Momentum:

As the parcel of air moves from the equator towards the poles, its velocity increases to conserve angular momentum.

At the same time the Coriolis force acts to produce a strong westerly wind – the sub-tropical jet stream.



JET STREAMSJET STREAMS

As the air moves from the equator to 30º at high altitudes, its velocity increases to conserve angular momentum.

At the same time the Coriolis force acts to produce a strong westerly wind – the sub-tropical jet stream

A similar jet forms at 60º – the polar jet, or polar front jet.


SUBTROPICAL AND POLAR JET STREAMS IN RELATION TO THE THREE CELLS

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Cloud band from Pacific to Florida follows Sub-tropical jet


Average Global Sea-level Pressure

Extra-tropical WesterliesIcelandic Low; Bermuda HighTrade winds; ITCZDoldrums; Horse latitudes



Average Global Winds

Extra-tropical WesterliesIcelandic Low; Bermuda HighTrade winds; ITCZDoldrums; Horse latitudes


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Lecture 16


WESTERLIES

In the upper troposphere:

High pressure over equator, low pressure over poles.Net flow from the equator to the poles.This flow plus Coriolis effect produces westerlies.Winds are (nearly) geostrophicPressure gradient increases with altitude. So does the wind speedJet streams are part of the westerlies



WAVES IN THE WESTERLIES

Dish pan experiment.C. G. Rossby.Waves along the jet streams are known as Rossby waves.Three to six of them around the globe. The air flow along the edge of the waves can be rapid, however the waves move slowly – about 15º per day. Higher jet stream speeds in the winter. Jets shifts south in the winter, north in the summer.


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(A) Zonal flow pattern – air flows parallel to latitudes.

(B) Meridional flow pattern.

(C) Combination of the two flows.

Index Cycle



Long Waves & Short Waves


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Rossby waves in a dish-pan


WESTERLIES AND THE HEAT BUDGET

Major function of atmospheric dynamics is to move heat from the equator to the poles.

But how can winds move heat when the predominate wind direction is zonal (E to W, or W to E)?

The meanderings of the jet streams continually mix cold and warm air, thus transporting heat.


POLEWARD TRANSPORT OF HEAT BY THE OCEANS AND ATMOSPHERE


Mean position of the ITCZ

in January and July


THE SW MONSOON

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Summer Monsoon: Wet

Winter Monsoon: Dry


Model Simulation of

General Circulation

[…/sony/Movies/NUGAM ]


Conclusion

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