Download ESS15 Lecture 10

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Wherewearesofar.
ESS15 Lecture 10
Winds 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
Today’s lecture will give you an in depth appreciation for:
•
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.
•
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!
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.
Navigating the planet - latitude.
Latitude of zero is at the equator.
positive
latitude
First, we need to remember how to navigate
around Earth.
0
negative
latitude
Common unit:
deg N
Specific locations on the Earth’s surface correspond
to unique (lat, lon) coordinates.
Navigating the planet - longitude.
Longitude of zero is near England
This location’s coordinates are ~
(30 deg N, 200 deg E)
Common unit:
deg E
0
Increasing
longitude.
Increasing
longitude.
negative
latitude
Example: Map of surface elevation.
Maps are data (numbers) visualized as colors
at different latitude, longitude coordinates.
Green:
in between values
Increasing latitude
Blue:
low values
(low places)
Increasing longitude
Red:
high values
(tall places)
Example - map of vegetation amount.
Brown:
low values
(deserts)
Cool fact: there’s a planetary scale “latitudinal” pattern to
where the Earth’s deserts occur!
Green:
high values
(forests)
You are here
~ 30 deg N
dry
dry
dry
equator
wet
~ 30 deg S
wet
dry
Cool fact: there’s a planetary scale “latitudinal” pattern to
where the Earth’s deserts occur!
Increasing latitude
30S
EQ
30N
Pattern:
dry
wet
dry
Ever wonder: Why doesn’t it
rain very much in the O.C.?
Has everything to do with what causes
this mysterious global pattern.
Stay tuned for a
scientifically
accurate rap!
Energy imbalances in the planet.
wet
dry
At low latitudes, near the equator, sun rays strike the earth
surface more perpendicularly, so more Watts are absorbed
there than at the poles.
The principle is the same as why solar panels track the sun to
absorb maximum Watts by staying perpendicular to its rays.
This is called “differential forcing” —> different solar radiation
absorbed at different latitudes.
Energy balance is different in different places!
•
To see this, useful to map “net” radiation.
•
Definition: “Net” radiation
•
Observed energy Imbalances
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.
Circulation of the atmosphere and oceans are
driven by energy imbalances
Energy Imbalances
Winds and Weather
and the atmospheric
circulation.
Isaac Newton
What makes the wind blow?
•
•
Objects stay put or move
uniformly in the same
direction unless acted on
by a force
(gravity, pressure gradient, & friction) push the air
around
•
Acceleration is a result of
the sum (net) of forces, in
the vector sense
Two apparent forces due to rotation
•
•
•
Three real forces
(Coriolis and centrifugal)
Large-scale flow is dominated by gravity/pressure and
Coriolis … friction and centrifugal are also important
locally
Forces acting on the air.
Air is pushed from regions of higher to lower pressure - makes
a wind.
Pressure gradient
force (pushing)
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.
Pressure gradient
force makes the
wind
•
(think bike tire leak)
Forces acting on the air.
Pressure gradient
force
Altitude (km)
•
•
Gravity
(falling)
low pressure
Apparent forces:
high pressure
Coriolis
force
Centrifugal
force
Friction
(rubbing against
the surface)
Whydoespressure
varyhorizontally?
•
Elevation changes
cause pressure
differences
•
These are balanced by
gravity and don’t cause
wind to blow
•
Why does pressure vary horizontally?
P2
Butwhydoespressure
varybetweenloca3ons
whichareatthesame
eleva3on?
P1
ThoughtExperiment:
Nowcooltheleftcolumn
andheattheright
Considertwocolumnsofairwiththe
sametemperatureanddistributionofmass
Theheatedcolumn
expands
500mb
Thecooled
columncontracts
original500mblevel
500mblevel
500mb
1000mb
1000mb
1000mb
1000mb
Heightofthe500mbsurfacechanges;
thesurfacepressureisunchanged
The 500 mb surface is
displaced upward in the
warmer column
The level corresponding to
500 mb is displaced downward
in the cooler column
new 500 mb
level in warm
air
original 500 mb level
new 500 mb
level in cold
air
The surface pressure
remains the same since both columns still contain
the same mass of air.
1000 mb
A pressure difference in the horizontal direction develops above the surface
original 500 mb level
Low
High
new 500 mb
level in cold
air
1000 mb
Air moves from high to low pressure in the middle of the column, causing the surface pressure to change.
The 500 mb surface is
displaced upward in the
warmer column
The 500 mb surface is
displaced downward in
the cooler column
1000 mb
1000 mb
new 500 mb
level in warm
air
The surface pressure remains the same since both columns still contain the same mass of air.
So air moves from high to low pressure at the surface…
Where would we have rising motion?
original 500 mb level
Low
1003 mb
High
997 mb
original 500 mb level
Low
High
High
Low
1003 mb
997 mb
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.
Local example of a thermal circulation
cell – theday8mecoastalseabreeze.
This is a microcosm of how the atmosphere
converts differential heating into motion
Local example of a thermal circulation
cell – theday8mecoastalseabreeze.
Solar radiation
Solar radiation
Less heating
A: OCEAN
B: LAND
i-clicker: Which air heats up faster during the day,
the stuff over (A) the ocean or (B) the land?
A: OCEAN
More heating
B: LAND
In other words, “differential heating”
of air across the coastline.
Local example of a thermal circulation
cell – theday8mecoastalseabreeze.
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!
That is, the circulation cell moves heat energy from the hotter
to the colder place.
So, surfers get up early in the morning because
The atmosphere uses circulation cells to counteract energy imbalances
in the air across coastlines. Produces offshore winds in the night / early AM.
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.
And where’s my professor rap about why it doesn’t rain in the O.C.?
Energy Imbalances
from differential heating
IftheEarthdidn’trotate,itwouldbeeasyfor
theglobalflowofairtobalancetheenergy
throughathermalcircula8oncell.
•
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.Andthis
reallymaHerstotheglobalcircula8on!
The pattern of the Hadley Cell explains why the deserts are
where they are.
Pattern:
30S
EQ
30N
dry
wet
dry
Increasing
latitude
We live
underneath
a descending
branch of
The Hadley
Cell
Why it doesn’t rain in the O.C.
(science)
•
Differential forcing (different solar radiation at different
latitudes) drives energy imbalances in the Earth
system.
•
Why it doesn’t rain in the O.C
(hip hop)
•
Best believe my science tight i’m talking differential
forcing
The air tries to move the heat poleward via a thermal
circulation cell.
•
(Different solar radiation that the latitudes
absorbing)
•
But because the Earth is spinning and the result is a
Hadley cell with descending, dry air near 30S & 30 N.
•
- add the fact the Earth be spinning Hadley
circulation soaring
•
That’s why it doesn’t rain much in the OC.
•
•
That’s why most deserts are located at similar latitudes.
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.
Thanks.
Next time:
The Coriolis force.