Midterm I Review

The most common atmospheric circulation

structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Topics we have discussed

• Overviews I: Extreme weather and climate

• Overview II: Success and failure of weather and climate prediction

• Overview III: Why is it so difficult to predict weather and climate?

• Evolution of the atmosphere

• The incoming solar energy

• What cause the four seasons?

• What is the greenhouse effect?

• Vertical structure of the atmosphere

• What set the atmosphere in motion?

• How does air move around the globe?

The mission of the atmospheric sciences is to understand and predict weather, climate, and

related disasters

Overview I: Extreme weather and climate• Atmosphere: A mixture of gas molecules,

microscopically small particles of solid and liquid, and falling precipitation

• Meteorology: The study of the atmosphere and the processes that form weather

• Weather: The state of the atmosphere at a given time and place

• Climate: The statistical properties of the atmosphere. (i.e. averages and variability)

• Weather- and climate-related disasters: tropical cyclones, tornados, floods, droughts, winter storms, extreme heat, extreme cold, lightning, El Nino, global warming

• Impacts of weather/climate on agriculture, business, international relationships, history, science, philosophy, public health, psychology, social work, education, …

Weather- and Climate-related Disasters

Weather/climate and Psychology, Public Health, Economy, Transportation, Education, …

Overview II: Success and failure of weather and climate prediction

• The modern climatology (meteorology) was born in the 1940s (a very young science!), but has been growing very fast! Now we have a global observational network with many satellites, ships, radars and surface stations, as well as very comprehensive prediction models running on the world’s fastest supercomputers.

• The current status of weather and climate predictions: (1) weather prediction good to 10 days, (2) tropical cyclone prediction good in track but not in intensity, (3) climate prediction good to two seasons, (4) climate change projections have a 3-fold difference in magnitude.

Observing the atmosphere

World’s major climate modeling centers

Overview III: Why is it so difficult to predict weather and climate?

• The main reasons of the difficulties: (1) Teleconnection problem, (2) Feedback problem, and (3) Subgrid-scale problem, (4) Limitation of concept/theory/model.

Problem I: Different parts of the world are strongly connected to each other (The “Teleconnection Problem”)

Global atmospheric flow

Factors affecting US weather and climate

El NinoMadden-Julian Oscillation

Amazon

ArcticN. Atlantic

Atlantic/ Sahel

Any location is affected by all the other locations, and in turn is

affecting all the other locations

Problem II: Different components of the earth system (atmosphere, land, ocean, ice,

clouds, etc) are strongly interacting with each other

(The “Feedback Problem”)

Problem III: The global climate models divide the earth into many small pixels (called grids), but the earth

system composes of both very big objects (such as the whole Pacific Ocean) and very small objects (such as the cloud droplets), making it very difficult to draw

them on the same page (The “Subgrid-Scale Problem”)

Evolution of the atmosphere– The standard units of measurements (SI)– Earth’s three atmospheres:

1st: 4.6 billion years ago, H, He

Transition: formation of magnetic field, volcano activities

2nd: 4 billion years ago, CO2, H2O, N2

Transition: emergence of life, formation of ocean

3rd: 400 million years ago, O2

Important event: formation of seven continents– What is the residence time? What is the difference between

the permanent and variable gases? Name 3 of each. What are the most and second most abundant gases?

– Given that variable gases are so rare, why are they considered at all? How are CO2 and O3 changing?

– Earth’s climate history: ice ages (at least 5 have occurred so far. We’re in an ice age!), 100,000-year cycle, little ice age (1350-1850AD)

Standard units of measurement

SI (System International)Quantity Name Units SymbolLength meter m mMass kilogram kg kgTime second s sTemperature Kelvin K KDensity kilogram kg/m3 kg/m3

per cubic meterSpeed meter per m/s m/s

secondForce newton m.kg/s2

NPressure pascal N/m2 PaEnergy joule N.m JPower watt J/s W

Evolution of the Sun and the Earth

The Earth was born 4.6 billion years ago.

Permanent gases and variable gases

• Residence time: The amount of time a gas is in the atmosphere

• The permanent gases: gases having long residence times (N2=42,000,000 y, O2=5,000 y), 99.999% of total atmosphere mass

• The variable gases: Gases generally having shorter residence times (H2O=10 days, CO2=150 y).

Importance of the Variable Gases

• CO2 and water vapor are the major greenhouse gases

• Water can exist in all three states on Earth. Global water cycle is the process of water being cycled from the planet to the atmosphere and back again.

• O3 protects us against harmful ultraviolet radiation

Montreal Protocol in 1987 to ban freons

IPCC (2001)

Change of CO2 Change of O3

The incoming solar energy

– What is energy? 3 methods of energy transfer

– The names of the 6 wavelength categories in the electromagnetic radiation spectrum

– Intensity of radiation (Stefan-Boltzman law): I=T4

– Wavelength of radiation (Wein’s law): max = b/T

– The wavelength range of Sun (shortwave) and Earth (longwave) radition

– The 11-year solar cycle

Methods of Energy Transfer

• Conduction– Molecule to molecule transfer– Heat flow: warm to cold – e.g. leather seats in a car

• Convection– transferred by vertical movement– physical mixing – e.g. boiling water

• Radiation– propagated without medium (i.e. vacuum)– solar radiation provides nearly all energy – The rest of this chapter deals with radiation

The Electromagnetic Spectrum

The limitations of the human eye!

Sun = “shortwave” (0.4-0.7 μm) Peak 0.5 μm (green)

Earth = “longwave” (4-100 μm) Peak 10 μm (infrared)

What cause the four seasons?

– The two basic motions of the Earth– What causes the four seasons: the

Earth’s tilt and the 3 ways it affects the solar insolation (change of length of the day, beam spreading, beam depletion)

– Change of the Earth’s orbit at longer time scales (Milankovitch cycles): eccentricity, axial tilt, and precession

The Earth’s two basic motions: revolution with a period of 1 year, and rotation with a period of 1 day.

The change of seasons is caused by the Earth’s 23.5o tilt from the line perpendicular to its orbit plane (toward the sun during summer), which affects the receipt of solar insolation in three ways:

1. Length of Daylight period

2. Angle at which sunlight hits the surface (“Beam Spreading”)

3. Thickness of atmosphere through which sunlight must travel (“Beam Depletion)

Video: The Bare Necessities (from The Jungle Book)

• http://www.cornel1801.com/disney/Jungle-Book-Bare-Necessities/videosong.html

What is the greenhouse effect?

• Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy is absorbed by the surface?

• Atmospheric influences on radiation (3 ways)

• The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange sunsets?

• What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important?

• Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions)

Earth’s energy budget (averaged over the whole globe and over a long time

• At the top of the atmosphere (3-way balance):

Incoming shortwave = Reflected Shortwave + Emitted longwave

• At the surface (5-way balance):

Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming)

+ Latent heat flux + sensible heat flux

Sensible heat 7% Latent heat

23%

Net Longwave 21%

Yellow: shortwave

Red: longwave

The greenhouse effect helps to keep the earth surface at a comfortable The greenhouse effect helps to keep the earth surface at a comfortable temperature. But when it’s too strong, the temperature becomes too warm.temperature. But when it’s too strong, the temperature becomes too warm.

Transparent Transparent to solar to solar (shortwave) (shortwave) radiationradiation

Opaque to Opaque to earth’s earth’s (longwave) (longwave) radiationradiation

Major GH gases: CO2, H20(v), CH4

Atmospheric absorption - The Greenhouse Effect

The importance of methane (CH4)

• 23 times more powerful as a greenhouse gas than CO2

• The livestock sector is a major player, which accounts for 35-40% global anthropogenic emissions of methane (their burps!)

• The livestock sector is responsible for 18% of total greenhouse gas emissions, which is higher than transportation (cars, airplanes, etc)

• Therefore, consuming less meat is more efficient in reducing global warming than not driving cars.

Vertical structure of the atmosphere

• Thickness of the atmosphere: less than 2% of Earth’s thickness

• Definition of temperature. 3 units. • Definition of pressure and its unit. • Definition of pressure gradient. Pressure gradient sets

the air in motion.• Equation of state (P=ρTR)• Vertical Pressure Distribution. How does pressure

change with height? What is the hydrostatic equilibrium?

Temperature, pressure, winds• Temperature – measure of average kinetic energy

(motion) of individual molecules in matter. 3 units: Kelvin (K), Celsius (C), Fahrenheit (F)

• Pressure – force exerted/unit area (weight above you). units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa)

• Pressure gradient – pressure difference between two locations divided by the distance between those two locations

• Winds– Zonal winds (east-west): Eastward is called

westerly– Meridional winds (north-south): Northward is

called southerly

Temperature Layers

The names of the 4 layers

What separate them?

The approximate height of tropopause, stratopause and mesopause

Pressure decreases with height

Because downward gravity force is balanced by vertical pressure gradient force (called hydrostatic equilibrium)

ΔΔp/p/ΔΔz = z = ρρgg

Pressure decreases non-linearly w/ height (Because air is compressible, so denser near the surface)

Vertical pressure distribution: Hydrostatic equilibrium

ρρgg

ΔΔp/p/ΔΔzz

What set the atmosphere in motion?

• Know 3 Forces that affect wind speed /direction• Especially work on Coriolis force, as this is the hardest to

understand. Which direction is air deflected to by Coriolis force?

• What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds

• Troughs, ridges, cyclones and anticyclones. Do they correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them?

Forces affecting the horizontal winds

• Horizontal pressure gradients responsible for wind generation

• Three forces affecting horizontal winds:

1. Pressure Gradient Force  (PGF)

2. Coriolis Effect  (CE)

3. Friction Force (FF)

CE:

• The Earth’s rotation deflects any moving object to the right of its moving direction in NH (left in SH). Like walking in a turning bus.

• CE increases poleward (greatest at the poles, 0 at the equator), and increases with the speed of moving object

Geostrophic Balance (Geostrophic flow)

PGF = - CE

• When the effects of friction can be neglected (such as in the upper air away from surface roughness), the wind speed/direction is simply a balance between the PGF and CE.

• Air motion is deflected by the Coriolis force to be perpendicular to PGF

PGF

Upper air: isobars usually not closed off•Troughs (low pressure areas)•Ridges (high pressure areas)

Near surface: isobars usually closed off due to surface friction•Cyclones (Low pressure areas)•Anticyclones (High pressure areas)

Cyclones, Anticyclones, Troughs and Ridges

How does air move around the globe?

• Three precipitation (heating) belts. Primary high and lows

• Three-cell model. Mechanism for each cell• Two characteristics of zonal mean temperature

structure• Two characteristics of zonal mean wind structure.

Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams?

• Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert.

• What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?

Vertical structure and mechanisms

Hadley Cell (thermal): Heating in tropics forms surface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft descends in the subtropics

Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells

Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles

HadleyPolar

Vertical structure of temperature

Two characteristics:

• Horizontally uniform in the tropics

• Steep gradient in the extratropics

Vertical structure of zonal wind

Two characteristics:

• Westerly winds in the extratropical troposphere (caused by the Coriolis force)

• Jet streams: local maximum of winds (caused by sharp pressure gradient across the boundary between warm tropical air and cold polar air)

• Ocean surface currents – horizontal water motions

• Transfer energy and influence overlying atmosphere

• Surface currents result from frictional drag caused by wind - Ekman Spiral

General circulation of the oceans

• Water moves at a 45o angle (right) in N.H. to prevailing wind direction

• Due to influence of Coriolis effect

• Greater angle at depth

About the midterm

• There will be ~50 multiple-choice questions

• Please pay special attention to the key points highlighted in red on the review slides (some easy grades)

• Sample questions

1) In the SI system, the standard unit of length is:

A) yard, B) meter, C) gram, D) pound.

2) Which of the following is NOT a variable gas?

A) water vapor. B) nitrogen. C) carbon dioxide. D) ozone.

3) In the northern hemisphere, when the surface wind blows toward the east, the underlying ocean current flows toward:

A) the west. B) the north. C) the southeast. D) the northwest.

4) Anticyclones:

A) are associated with low-pressure systems in the northern hemisphere.

B) experience Coriolis effects that deflect air to the left in the Northern Hemisphere.

C) are associated with supersonic winds.

D) are associated with counter-clockwise flow in the southern hemisphere.