Final reviewFeedback

Processes in one system influences processes in another interconnected system by exchange of matter and energy.

Positive Feedback:Positive Feedback: Change in one system causes similar change in the other system. Can cause runaway instability.e.g., water vapor feedback, ice cover feedback

Negative FeedbackNegative Feedback: A positive change in one system causes a negative change in the other.e.g., cloud cover feedback

Composition of the Atmosphere

Major components: Nitrogen (N2), Oxygen (O2), Argon (Ar), Carbon dioxide (CO2),

Minute trace gases: water vapor (H2O),Methane (CH4), Ozone (O3), Nitrous Oxide (N2O)

Variable components: Water vapor, Aerosol, Ozone

Extent of the Atmosphere

Temperature F C, K, :unit o

273.16 t T 32); (t 9

5 t 32; t

5

9 t FF

Troposphere

Stratosphere

•Decreases 6.5C per km up to 11 km (lapse rate).•Nearly all weather happens in this layer.•Height of the tropopause varies with latitude with an average of 10 km. It is higher in the tropics and lower in the high latitudes.

Temperature is constant in the lower part of the layer, and then, increases with height due to O3 absorption of solar UV. ~ 99% of the atmosphere is below the stratopause.

Mechanisms of Energy Transfer

Conduction, convection, and radiation

Laws of blackbody radiation

Stefan-Boltzman law, Wien’s displacement law, Plank’s law

6000K

300K

Greenhouse Effect

Shortwave solar radiation is nearly transparent to the atmosphere, but longwave terrestrial radiation is trapped by greenhouse gases, causing the increase of surface temperature.

Atmospheric window Highly un-reactive greenhouse gases containing bonds of fluorine-carbon or fluorine-sulfur, such as Perfluorocarbons (CF4, C2F6, C3F8) and Sulfur Hexafluoride (SF6). These trace gases have strong absorption lines right in the atmospheric window.

Clouds can also absorb longwave radiation in the atmospheric window, and thus, can close up the window

Heat Budget of Earth’s Atmosphere

Controls of Temperature

LatitudeDifferential heating of land and water; Ocean currents; Altitude;Geographic position

Phase change and latent heat

Measuring water vapor in the air

Mixing ratio, r

Moist virtual effect

Saturation

Saturated water vapor pressure, E=E(T)

Relative humidity, h

Dew-point )e(e

First law of thermodynamics in the atmosphere, energy conservation law

Dry adiabatic process

K/100m 98.0 d Dry adiabatic lapse rate

Lapse rate of ambient environment

Lifting condensation level (LCL)

Moist adiabatic process

Moist adiabatic lapse rate ds s

Rain shadow effect

286.0)P

1000(T

Potential temperature is conserved during the dry adiabatic process

Potential temperature

Atmospheric Stability

i0T

iT

0T

T

=

<

Stable

i

i0T

iT

0T

T

=

=

Neutral

i

i0T

iT

0T

T

=

<

Unstable

i

stable , neutral; , unstable; , iii

unstablelly conditiona ,

stable; absolute ,

unstable; absolute ,

ds

d

s

Using potential temperature to determine atmospheric stability and instability

0

=

<

Stable

0z

=

Neutral

<

Unstable

i0

0=

0z

i0

0=

i0

0z

Cloud formation

Adiabatic cooling Water vapor -----> saturation

Cloud condensation nuclei (aerosol)

Cloud Types

Radiation fogFavorable condition: Clear sky; High relative humidity; Weak winds

High clouds: cirrus , cirrostratus, cirrocumulus

Middle clouds: altostratus, altocumulus

Low clouds: shallow cumulus, stratus, stratocumlus

Height:

Bergeron Process (mixed phase clouds)

Mixed phase clouds

E e E s

evaporatecondensate

The saturated vapor pressure above ice crystals is lower than that above supercooled water droplets. Es < E

Cloud affects diurnal variation of surface temperatures

Collision-Coalescence Process (warm clouds)

a.Collision-Larger drops fall faster than smaller drops, the larger drops overtake the smaller drops to form larger drops until rain drops are formed.

b.Coalescence-The merging or "sticking together" of cloud droplets as they collide.

The collision-coalescence mechanism is more efficientin environments where large cloud droplets are plentiful.

Where? tropics

The Bergeron process is more efficient for mixed clouds withplentiful ice crystals and supercooled water droplets.

Where? Mid-latitudes

Atmospheric pressure P: Force F acting on unit area due to the weight of the atmosphere.

Unit: Pascal (Pa); hPa; mb;

Hydrostatic balance

The upward pressure gradient force balances the downward gravity.

1 Pressure decreases with height

Pressure gradient force

Gravitational force2 Pressure depends on temperature3 How does a pressure system change with height?

Low pressure center coldwarm warm

Deep system

Horizontal pressure gradient force

Coriolis force: an apparent forced acting on a moving object

Coriolis deflection depends on latitude

Z

Z

Geostrophic Balance

The horizontal pressure gradient force balances the Coriolis force.

Geostrophic adjustment

High pressure

Low pressure

Coriolis force

Pressure gradient force

Geostrophic Wind

Gradient wind Balance

LPcentFcoriF

V

H

V

coriFP centF

Anti-clockwise (cyclonic) Clockwise (anti-cyclonic)

The balance among horizontal pressure gradient force, Coriolis force, and centrifugal force.

Frictional force

1

gfV

gVfF

V

fF

gfV

L

V

fF

PcoriF

centFH

V

fF

coriFP

centF

Due to the friction, the flow near surface blow across the isobars pointing from high pressure to low pressure

High pressure: divergence at the surface

Low pressure: convergence at the surface

Relationship of Flow Pattern Aloft to Surface Cyclones and Anticyclones

+

- --

Thermal wind: wind difference between two levels

p1F

1V

hPa 1010P01

hPa 1001P02

c1Fhigh

low

Z

9

1001-1010P0

A

Warm

Cold

A

B

hPa 8811201001P12

hPa 9101001010P11

low

high

hPa 19

881910

PPP 12111

2V

c2F

p2F

12T VVV

Mid-latitude jet stream (using thermal wind relation to explain)

African easterly jet (using thermal wind relation to explain)

Ferrel cell: indirect cell

Polar cell: direct cell

Three cell model: (a) Hadley cell, (b) ITCZ (c) trade-wind, (d) mid-latitude westerly, (e) subtropical high

Monsoon refers to a wind system that exhibits a pronounced seasonal reversal in direction.

Asian Monsoon The North American Monsoon

Absolute vorticity=Planetary vorticity + Relative vrticity f a

:effect Planetary vorticity changes with latitude

fCyclone 0

f is defined as positive

Anti-cyclone 0

Planetary vorticity f, relative vorticity , absolute vorticity

Depth

vorticityAbsolute vorticityPotential is conserved

constantH

f

Potential vorticity

Rossby wave: large scale (planetary) wave due to the change of planetary vorticity with latitude. It can only propagate westward.

Wind driven Surface Current

Ekman effect and coastal upwelling and equatorial upwelling

Deep ocean density driven circulation

Walker circulation

The Walker Circulation refers to an east-west circulation of the atmosphere above the tropical ocean in the zonal and vertical directions, with air rising above warmer ocean regions (normally in the west), and descending over the cooler ocean areas (normally in the east). Its strength fluctuates with the change in sea surface temperature.

El Niño and La Niña

El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific that is closely related to the change in the Walker circulation and has important consequences for weather and climate around the globe.

Stronger Walker circulation

Weakened Walker

circulationEl Niño condition

La Niña condition

Teleconnections via atmospheric Rossby waves

Impact of ENSO on Global Climate

mT and cP are most important to the weather east of the Rockies. cP dominates in winter, often bringing cold weathers. mT is responsible for most precipitation in the eastern US.

mP from North Pacific is mostimportant to weather along thePacific coast.

cA and cP can sweep with relativeease far southward into US becausethere are no major barriers to theirmovement.

Air Mass

Lake-Effect Snow: Cold Air over Warm Water

Warm Fronts

Cold Fronts

Stationary Front

Occluded Front

S N

warm cold

density surface

.

Release of potential energy stored in a tilted system to kinetic energy

Dryline

ThunderstormsCharacterized by strong up- and down- movements, producing lightening and thunder, generating gusty winds, heavy rain, and hail. Winds of a thunderstorm do not follow the inward spiral of a cyclone.

Stages of development of a thunderstorm

Severe Thunderstorm

Definition (NWS), wind: 58 mph or hailstone: D=1.9 cm or tornado.

Local circulation, sea breeze

Supercell ThunderstormDiameter: 20 - 50 km; depth: 20 km

Rotated updraft

A squall line is a line of thunderstorms that have a common lifting mechanism.

radarMicrobursts

Doppler EffectDoppler radar

Squall lines

+ + + + + ++

-

--

--

-

Averaged fair-weather electric field near surface is about 130 V/m

The system can be viewed as an electric circuit in which electrified clouds are the generator (batteries).

Thunderstorm

The Earth's Electrical StructureEarth's electrical charge

The Earth is electrically charged. The Earth surface has a net negative charge, while an equal and positive charge resides in the upper atmosphere.

What charges the Earth?

The atmosphere is not a perfect insulator. Negative charge leaks from the Earth and rises to the electrosphere. So, the Earth's charge would dissipate in less than an hour if there were no recharge!

Lightning recharges the Earth by delivering negative charges to the surface.

Lightning

Step-leader: a faint, negatively charged channel with about 50 meters in length and 1 microsecond in duration

Streamers: bolts of ground-to-air lightning

Return stroke: an electric current wave propagates up the established channel as a bright pulse

Dart leader: After return stroke, if additional charge is available at the top of the leader channel, another leader can propagate down the established Channel

THUNDER

Acoustic shock wave caused by the extreme heat generated by a lightning flash.

Tornadoes

The central vortex of a tornado is typically about 100-600 m in diameter. The averaged wind speed is about 96 mph, but can be as strong as or greater than 220 mph. The central pressure in a tornado can be lower than normal atmospheric pressure by over 100 hPa.

Tornados have extremely large horizontal pressure gradient. It is governed by the balance between the inward pressure gradient force and outward centrifugal force.

Hurricane structure What determines the movement of a hurricane?

X

BoundaryLayer

EkmanPumping

Hurricane Secondary circulation

DivergingSpin down

ConvergingSpin upBuoyancy

Pressure grad.force

Centrifugalforce

Coriolisforce

L

It is the convective clouds that generate spin up process to overcome the spin down process induced by the Ekman pumping