ESS15 Lecture 10Winds and weather The Coriolis force

Global circulations of atmosphere & ocean Weather vs. Climate

Earth’s energy imbalances, winds, and the global circulation of the atmopshere.

Please read Chapter 6 from Archer Textbook

Theforestandthetrees. Wherewearesofar.

• To appreciate future climate change we need to understand climate.

• Climate is place and energy flows.

• What forces sculpt it to be the way it is?

• Until we get this, it’s pointless for me to lecture about carbon taxes and coal plants.

• We’re getting very close!

• Why surfers get up early in the morning.

• Why it doesn’t rain much in the O.C.

• Origins of a mysterious geographic pattern in where Earth’s deserts are located.

Today’s lecture will give you an in depth appreciation for:

Energy balance of the whole Earth.

But nobody lives on the “whole Earth”.

To understand why OC’s climate is different than other locations, need to look at energy

imbalances within the planet.

First, we need to remember how to navigate around Earth.

negative latitude

positive latitude

0

Latitude of zero is at the equator.

Common unit:deg N

Navigating the planet - latitude.

Increasinglongitude.

Longitude of zero is near England

Common unit:deg E

Navigating the planet - longitude.

Specific locations on the Earth’s surface correspond to unique (lat, lon) coordinates.

Increasinglongitude.

negative latitude

0

This location’s coordinates are ~(30 deg N, 200 deg E)

Maps are data (numbers) visualized as colors at different latitude, longitude coordinates.

Example: Map of surface elevation.

Blue: low values

(low places)

Red: high values (tall places)

Green: in between values

Increasing longitude

Incr

easi

ng la

titud

e

Example - map of vegetation amount.Brown:

low values (deserts)

Green: high values

(forests)

Cool fact: there’s a planetary scale “latitudinal” pattern to where the Earth’s deserts occur!

equatorwet wet wet

drydrydry

dry dry

~ 30 deg N

~ 30 deg S

You are here

Cool fact: there’s a planetary scale “latitudinal” pattern to where the Earth’s deserts occur!

wet drydryEQ 30N30SIncreasing latitude

Has everything to do with what causes this mysterious global pattern.

Ever wonder: Why doesn’t it rain very much in the O.C.?

Stay tuned for a scientifically accurate rap!

Pattern:

Energy imbalances in the planet.

At low latitudes, near the equator, sun rays strike the earth surface more perpendicularly, so more Watts are absorbed there than at the poles.

This is called “differential forcing” —> different solar radiation absorbed at different latitudes.

The principle is the same as why solar panels track the sun to absorb maximum Watts by staying perpendicular to its rays.

Energy balance is different in different places!

• To see this, useful to map “net” radiation.

• Definition: “Net” radiation

• Solar in minus longwave out (W/m2)

• If positive: More Watts from sun than Watts out to space —> energy constantly building up.

• If negative: More Watts cooling to space than coming in from sun —> energy constantly being lost.

Observed energy Imbalances

Circulation of the atmosphere and oceans are driven by energy imbalances

Energy Imbalances

Winds and Weather and the atmospheric

circulation.

• Objects stay put or move uniformly in the same direction unless acted on by a force

• Acceleration is a result of the sum (net) of forces, in the vector sense

Isaac Newton

• Three real forces

(gravity, pressure gradient, & friction) push the air around

• Two apparent forces due to rotation

• (Coriolis and centrifugal)

• Large-scale flow is dominated by gravity/pressure and Coriolis … friction and centrifugal are also important locally

What makes the wind blow?

Forces acting on the air.

Pressure gradient force (pushing)

Air is pushed from regions of higher to lower pressure - makes a wind.

• Pressure gradient force makes the wind

• (think bike tire leak)

i-clicker survey.Why doesn’t higher pressure air at sea level get blown up to lower pressure air at the top of Mount Whitney?

• A: Gravity

• B: Friction

• C: The Coriolis force

• D: All of the above.Al

titud

e (k

m)

high pressure

low pressure

Forces acting on the air.

Pressure gradient force Gravity

(falling)Friction

(rubbing against the surface)

Apparent forces:

Coriolis force Centrifugal

force

Whydoespressure varyhorizontally?

• Elevation changes cause pressure differences

• These are balanced by gravity and don’t cause wind to blow

• Butwhydoespressurevarybetweenloca3onswhichareatthesameeleva3on?

P1

P2

Why does pressure vary horizontally?

ThoughtExperiment:Considertwocolumnsofairwiththe

sametemperatureanddistributionofmass

1000mb 1000mb

500mblevel

Nowcooltheleftcolumn

andheattheright

1000mb

500mb

500mb

1000mb

Theheatedcolumnexpands

Thecooledcolumncontracts

original500mblevel

Heightofthe500mbsurfacechanges;thesurfacepressureisunchanged

1000 mb

new 500 mb level in warm air

new 500 mb level in cold air

1000 mb

The 500 mb surface is displaced upward in the warmer column

The level corresponding to 500 mb is displaced downward in the cooler column

original 500 mb level

The surface pressure remains the same since both columns still contain the same mass of air.

1000 mb

new 500 mb level in warm air

new 500 mb level in cold air

1000 mb

The 500 mb surface is displaced upward in the warmer column

The 500 mb surface is displaced downward in the cooler column

original 500 mb level

The surface pressure remains the same since both columns still contain the same mass of air.

A pressure difference in the horizontal direction develops above the surface

HighLow

1003 mb 997 mb

original 500 mb level

Air moves from high to low pressure in the middle of the column,

causing the surface pressure to change.

HighLow

1003 mb 997 mb

original 500 mb level

So air moves from high to low pressure at the surface…

HighLow

High Low

Where would we have rising motion?

Thought Experiment Review

• Starting with a uniform atmosphere at rest, we introduced differential heating

• The differential heating caused different rates of expansion in the fluid

• The differing rates of expansion resulted in pressure differences aloft along a horizontal surface.

• The pressure differences then induced flow (wind!) in the fluid

• The convective circulation cell transfers heat away from the hotter column and towards the colder column.

This is a microcosm of how the atmosphere converts differential heating into motion

Local example of a thermal circulation cell – theday8mecoastalseabreeze.

A: OCEAN B: LAND

i-clicker: Which air heats up faster during the day, the stuff over (A) the ocean or (B) the land?

Solar radiation

Local example of a thermal circulation cell – theday8mecoastalseabreeze.

A: OCEAN B: LAND

Solar radiation

More heatingLess heating

In other words, “differential heating” of air across the coastline.

Local example of a thermal circulation cell – theday8mecoastalseabreeze.

That is, the circulation cell moves heat energy from the hotter to the colder place.

i-clickersurvey

Which way does the coastal sea breeze blow in day / night?

a. land àocean / land à ocean b. land à ocean / ocean à land c. ocean à land / ocean à land

d. ocean à land / land à ocean e. there is no rule on it!

The atmosphere uses circulation cells to counteract energy imbalances in the air across coastlines. Produces offshore winds in the night / early AM.

So, surfers get up early in the morning because

And where’s my professor rap about why it doesn’t rain in the O.C.?

Ok, but how on Earth does this stuff relate to the question why there’s a global geographic pattern in deserts?

Recall “Differential heating” also happens on planetary scales.

Energy Imbalances from differential heating

IftheEarthdidn’trotate,itwouldbeeasyfortheglobalflowofairtobalancetheenergythroughathermalcircula8oncell.

• Thermal convection would lead to formation of convection cell in each hemisphere

• Energy transported from equator toward poles

• Surface wind in Irvine would always blow from the North

What it would look like if the planetary wind patterns looked like a coastal cell.

ButtheEarthdoesrotate–aCoriolisforce.AndthisreallymaHerstotheglobalcircula8on!

We live underneath a descending branch of The Hadley Cell

wet drydryEQ 30N30S Increasing

latitudePattern:

The pattern of the Hadley Cell explains why the deserts are where they are.

• Differential forcing (different solar radiation at different latitudes) drives energy imbalances in the Earth system.

• The air tries to move the heat poleward via a thermal circulation cell.

• But because the Earth is spinning and the result is a Hadley cell with descending, dry air near 30S & 30 N.

• That’s why it doesn’t rain much in the OC.

• That’s why most deserts are located at similar latitudes.

Why it doesn’t rain in the O.C. (science)

Why it doesn’t rain in the O.C (hip hop)

• Best believe my science tight i’m talking differential forcing

• (Different solar radiation that the latitudes absorbing)

• - add the fact the Earth be spinning Hadley circulation soaring

• Up in tropics, down in O.C. that’s the reason it ain’t pouring.

Why it doesn’t rain in the O.C (hip hop)

• Best believe my science tight i’m talking differential forcing

• (Different solar radiation that the latitudes absorbing)

• - add the fact the Earth be spinning Hadley circulation soaring

• Up in tropics, down in O.C. that’s the reason it ain’t pouring.

Next time: The Coriolis force.

Thanks.