Transcript Coriolis Effect

This Week (5)
Atmospheric Motions
Reading chapter 4 of your text
Study for Midterm on Friday in Discussion
•Addressing our assumptions
•Hadley Circulation, Coriolis Effect, Geostrophic Flow,
•Deserts, Tropical Rainy/Dry seasons, Jet Streams
•Surface Winds, Storms, Land/Ocean Contrasts
•Cyclones and anticyclones, sea breeze, monsoons
•Moist Convection and Clouds
•Vertical distribution of water vapor—hydrologic cycle
•Lapse Rate and the Greenhouse Effect
•Types of Clouds and Roles in Climate
Today—Addressing our assumptions
•Isn’t this class about climate?
•Energy-Imbalance: the incoming and outgoing energy flux
is not the same everywhere on Earth
•The atmosphere doesn’t just absorb outgoing radiation, it
transports heat (and moisture) and absorbs incoming
radiation
•The atmosphere isn’t uniformly the same temperature
A Brief Review
Over the past 4 weeks, we have primarily learned about
some of the physics which is important in the climate
system.
We used blackbody radiation physics to make predictions
about the global average temperature and what major
factors will influence that in the past and future.
(This topic is at the heart of “global warming”)
But, our definition of climate involved the “average of
weather”. Weather is the response of the Earth system to
energy imbalances, which we have so far ignored.
Energy Imbalance
Because of the curvature of the Earth, the equator
receives a greater solar radiation flux than the poles
I.e. more solar energy deposited per m2 per second at
the equator than at higher latitudes
Energy Imbalance
300
W/m2
EOUT (LW)
EIN (SW)
100
W/m2
Higher latitudes emit more radiation back to space than they receive
from the sun on average.
The equator emits less radiation back to space than receives from sun
Energy Imbalance
Examining the energy imbalance tells us the tropics
should be warmer than the higher latitudes.
(recall the energy balance model)
But it also says on average the northern latitudes should
be getting colder and colder, and the tropics should be
getting warmer and warmer.
There must be other processes (besides radiation)
which are helping maintain a closer energy balance
at each latitude.
The Atmosphere Moves
The atmosphere (and oceans) move in response to the
energy imbalance. This motion transports heat away from
the equator to higher latitudes helping to minimize the
imbalances.
To simulate climate, we’ll need to simulate the weather.
Questions
1. Where on Earth receives the largest daily average
solar energy flux?
2. Why are seasons more intense in higher latitudes than
in the tropics?
3. Give two forces that would cause air to move.
Making Air Move
Pressure Gradient Force
Air/water will move from a region of high
pressure to low pressure
Fluid with
horizontal
pressure
gradient
Applies to horizontal (parallel to earth’s surface)
and vertical (perpendicular to earth’s surface)
Buoyant Forces
Recall ideal gas law: density  1/T
buoyancy: when pressure gradient force in
vertical direction not equal to force due to gravity
When an air parcel’s density is lower/higher than
the surrounding air, it is “lighter”/”heavier” and
will rise/sink
convection
Pressure Gradients
Suggest Eq. To pole flow
PGF
The picture is roughly symmetric
for the Southern Hemisphere (SH)
General Circulation of the Atmosphere
An early picture based on Hadley’s 1735 paper:
Tropopause
divergence
subsidence
Uplift (convection)
subsidence
convergence
90
45
0
-45
-90
Cold high lats
Warm Tropics Cold high lats
higher pressure Lower pressure higher pressure
Intertropical Convergence Zone
Convergence and uplift leads to saturation/condensation
ITCZ moves around with season, and is not a continuous
band around globe.
Subsidence--Sinking
Sinking air is compressed,
therefore it warms, and becomes
sub-saturated w.r.t. water vapor.
In subsiding air, precipitation
suppressed.
The (original) Hadley Circulation
(1735)
COLD
HOT
COLD
Explains:
Intertropical
Convergence Zone
(ITCZ)
Wet Tropics
Dry Poles
But it has some
problems.
Earth Rotates
Hadley’s ideas were important and are still utilized today.
To really explain the average wind directions and locations
of general subsidence, the fact that the air motions occur
on a rotating planet needs to be taken into account.
100 years after Hadley’s paper, Gustav de Coriolis, noted
that moving air or water on the planet tends to be
deflected from a straight path.
This Week (5)
Atmospheric Motions
Reading chapter 4 of your text
Study for Midterm on Friday in Discussion
•Addressing our assumptions
•Hadley Circulation, Coriolis Effect, Geostrophic Flow,
•Deserts, Tropical Rainy/Dry seasons, Jet Streams
•Surface Winds, Storms, Land/Ocean Contrasts
•Cyclones and anticyclones, sea breeze, monsoons
•Moist Convection and Clouds
•Vertical distribution of water vapor—hydrologic cycle
•Lapse Rate and the Greenhouse Effect
•Types of Clouds and Roles in Climate
IPCC In the News
Monday, January 29, 2007 Last Update: 4:45 PM ET
Scientists Gather to Finalize Climate Report
By JAMES KANTER and ANDREW C. REVKIN 2:58 PM ET
Today—General Circulation
•Hadley Circulation + Coriolis Effect
•A look at the major deserts
•Wet and dry seasons in the tropics
•Mid-latitude Circulation
•Polar Front
•Roaring 40s in SH
•Geostrophic Flow: When PGF = Coriolis “Force”
•Upper-level westerlies – jet stream
Coriolis Effect
Earth rotates from West to East (an
Easterly direction)
The equator is rotating with a larger
velocity than higher latitudes because a
larger circumference must be traveled in
the same time period (1 day).
coriolis effect movie
Air (or any object) on Earth always has
an easterly motion (relative to some
fixed point in space like a distant star).
Object from the equator and moving
northward, its easterly motion is faster
than the motion of the Earth at northern
latitudes so it looks like it bends right.
Coriolis Cartoon
Trajectory seen
by thrower
Coriolis Effect Summary
Coriolis effect applies only to moving objects (moving
relative to Earth’s rotation)
We analyzed motion in the N-S directions, but a similar
effect occurs for objects traveling in E-W directions (the
centrifugal force)
Objects (air or water, e.g.) moving in the NH will curve to
the right of their forward direction and to the left in SH.
The strength of the coriolis effect decreases from the
poles to the equator where it is zero.
Questions
1. Suppose Earth were a cylinder. Where
would there be a coriolis effect?
A
B
C
a)
b)
c)
d)
A only
B only
C only
A, B, and C
Coriolis Effect Modifies Idea of
Hadley Circulation
Modified Hadley Circulation
*
Horizontal motions
convergence:
divergence:
Vertical motions
convection:
subsidence:
coming together
spreading apart
rising air
sinking air
*not a fixed
value
Mean Surface Pressure
Note the location of
H’s for regions of
High Pressure (near
30N or 30S?)
Contours are lines of
constant pressure
(isobars). More
closely spaced lines
means a strong
pressure gradient.
Any hikers?
World’s Deserts
Not shown: Antarctica!
World’s largest deserts tend to be located under subsiding
branches of the Hadley cell –not only way for desert to exist.
World’s Deserts
Desert dust blows
from Western Sahara
and N. Morocco over
Canary Islands.
Dust from deserts
can be an important
source of nutrients to
ocean and land biota
(sometimes half a
world away).
Questions
1. What’s another mechanism for making/maintaining
a desert?
2. The Amazon’s rainy season runs from Nov. – April.
This period is Costa Rica’s dry season. Why?
Seasonal Shift in Hadley Circulation
ITCZ shifts N-S depending on season. Leads to wet and
dry seasons in the tropics.
This Week (5)
Atmospheric Motions
Reading chapter 4 of your text
Study for Midterm on Friday in Discussion
•Addressing our assumptions
•Hadley Circulation, Coriolis Effect, Geostrophic Flow,
•Deserts, Tropical Rainy/Dry seasons, Jet Streams
•Surface Winds, Storms, Land/Ocean Contrasts
•Cyclones and anticyclones, sea breeze, monsoons
•Moist Convection and Clouds
•Vertical distribution of water vapor—hydrologic cycle
•Lapse Rate and the Greenhouse Effect
•Types of Clouds and Roles in Climate
Surface Winds and Pressure
In the tropics air flows towards the ITCZ, impacted by
Coriolis effect and friction. These are the “trades”
Subsidence leads to surface highs and divergence both
N and S
subsidence
subsidence
Mid-latitude circulation
Strong T gradients
LP
HP
HP
Northward-traveling warm air runs into southwardtraveling polar air. (A similar situation in SH – less land)
Warm air rides up on top of colder air creating a front of
uplift.
The polar front zone is wavy in the horizontal
A Jet Stream Is A Geostrophic Wind
The PGF between 45 –
75 N generates strong
flow towards pole. See
the
Coriolis Effect
balances PGF so flow
becomes westerly.
The j indicates
“location” of polar jet
stream. Direction of
flow is into the page
j
Polar-Front Jet in NH
Blue arrows
indicate wind
speed (size) and
direction at a
pressure level of
300 mb.
What does 300
mb mean in terms
of altitude?
Upper-level Flow tends to be Geostrophic
The high-altitude branches of circulation cells feel little
friction, and can be well organized flow.
As air flows down pressure gradient, Coriolis Effect
becomes more and more significant curving air mass to
right of forward direction
PGF
Coriolis
Flow direction
Low Pressure
Geostrophic Flow: When PGF
balanced by Coriolis Effect
High Pressure
Upper-level Flow tends to be Geostrophic
Centers of high or low pressure aloft induce clockwise and
counterclockwise geostrophic flow around the centers
W/out friction, flow becomes parallel to isobars.
In NH flow counterclockwise
around Lowcyclonic flow
L
Low Pressure Center
In NH flow clockwise around
Highanticyclonic flow
H
High Pressure Center
Surface Flow Impacted by Friction
Forces
PGF
Coriolis
Friction
Flow Directions
Geostrophic Flow direction
Actual Flow direction
Low Pressure
VG
High Pressure
Friction causes flow to move away from high pressure,
but towards low pressure.
Surface-level Flow affected by Friction
Centers of high or low pressure at surface induce flow that
spirals out or in, respectively.
Convergence—Surface Storm
L
Low Pressure Center
Divergence
H
High Pressure Center
Tropical Cyclone—Hurricane Gordon
Midlatitude Cyclones
Land-Ocean Contrasts
Heat capacity of water is 3 to 4 times greater than dry soil
land T changes more than ocean for same E input
Ocean can mix heat down into large thermal mass reservoir
land heats/cools more rapidly than the ocean
Land tends to have a higher albedo than ocean …but…
Radiation can penetrate deeper into ocean before absorbed
absorbed energy in ocean distributed over greater
mass/depth
Resulting T contrasts give rise to:
diurnal sea breeze circulation, continentality in seasonal
variability, and monsoonal circulation
Continentality
Seasonal variation in land temperatures much
larger than over oceans.
Feeds into circulation changes between seasons.
Strongest continentality in mid-latitude NH, much
weaker in mid-latitude SH due to less land.
Seasonal variability in SH mid-latitude circulation is
much less pronounced than in NH.
Diurnal (Daily) Sea Breeze
Day
Night
Monsoon Circulation
The cause is the same as
the diurnal sea breeze but
on a larger, seasonal scale.
This Week (5)
Atmospheric Motions
Reading chapter 4 of your text
Study for Midterm on Friday in Discussion
•Addressing our assumptions
•Hadley Circulation, Coriolis Effect, Geostrophic Flow,
•Deserts, Tropical Rainy/Dry seasons, Jet Streams
•Surface Winds, Storms, Land/Ocean Contrasts
•Cyclones and anticyclones, sea breeze, monsoons
•Moist Convection and Clouds
Today—Convection, Clouds, Climate
•My mid-term evaluation
•Vertical Heat transport—conduction and convection on
firmer ground
•Cementing the link between convection, lapse rate and
greenhouse effect
•Cloud Formation and Types
•Clouds affect on climate
Midterm Evaluation
To help improve this course and to increase the amount you
learn, please take a moment to answer the following
questions. Remain anonymous (or not, up to you).
1. “This course is __________ than I thought it would be.”
2. Is the current mix of power point and writing on the
board/overhead useful?
3. Does Prof. Thornton write legibly and speak clearly
during lecture?
4. “If Prof. Thornton would just _________ during lecture,
I would understand the material better.”
Continentality-Find the Continents
Contours show annual temperature range
Moist Deep Convection—Where?
divergence
What is Buoyancy?
Buoyancy refers to the density of an object in a fluid,
relative to the density of that fluid.
PGF
Fluid (rf)
PGF
object
(rob)
Here “density” means
density of substance : g of
substance per cm3 of that
substance.
Fbuoyancy = PGF - Fg
Fg
Fg
If density of object > density of fluid, force due to gravity will
be a larger than the PGF, and object will sink.
If density of object < density of fluid, PGF > Fg and it will rise.
What is Buoyancy
If the object and the fluid are the same gas, as they
are in the atmosphere, the density becomes just
proportional to 1/T.
Consider air over a paved parking lot with a T = 303 K.
If the surrounding air had T = 300 K, air over the
parking lot would feel an acceleration upwards of 10
cm/s2.
Typical convergence produces vertical velocities of ~
0.1 cm/s (very slow compared to buoyant motion!).
Convection is the result of
buoyant forces
Convection is the vertical transport of heat
How do convection and buoyant forces get started?
Conduction (sensible
heat transfer) warms air
over surface.
A hypothetical situation
(never really observed)
~ 2 km
0 km
Dry adiabatic slope
from expansion work
T
Slope greater than dry adiabatic
lapse rate because of surface heating
Air parcels become
buoyant or are lifted
mechanically to where
they are buoyant.
Rise following less steep
lapse rateunstable
vertical motion
Moist Convection
Heat eventually
transferred to
surrounding
atmosphere
Condensation
Latent Heat Release
More buoyant
“Dry” convection
Convection and the Energy Budget
A large amount of energy is carried by convection
Convection and Greenhouse Effect
H2O vapor’s vertical distribution affects radiating
temperature, and moist convection impacts lapse rate.
New skin altitude
with more GHGs
Alt
255K
“Skin” altitude
T
Convection/Advection/Water Cycle
Advection: horizontal transport of energy or mass
Warm Cloud Formation (similar for ice
clouds but require much colder temperatures)
1. Start with air parcel (a volume of air with
similar properties) containing water vapor
and aerosol particles.
2. Lift parcel up
•
Heating at surface, thermals
•
Upslope wind forced by mountains
•
Converge of mass by large scale
circulation
3. Parcel cools by rising, reaches saturation
4. Condensation occurs on aerosol particles =>
cloud forms
More aerosol particles  more cloud droplets
No aerosol particles  no cloud droplets
Some Cloud Types
Cumulus and
cumulonimbus from space
Cirrus-high, thin ice clouds
Marine
stratocumulus
aviation contrails
Clouds and Climate—a complex problem
Surface Warming
High thin Cirrus: Not so reflective, but absorb and emit at cold T
Surface Cooling
Low Warm Clouds: Absorb IR but emit like warm surface. Very reflective
Activity
•Think of a location other than Seattle where you have
lived.
Characterize the climate of that region. E.g. Was it rainy in
the summer, dry in the winter, or vice versa? Was it humid,
tropical with lush vegetation, or was it arid and dry?
Always foggy at night and in the morning, but clear and
sunny during the day?
Come up with ideas as to why the climate might have been
that way.