REVIEW: Understanding Weather and Climate

Chapter 1 - Chapter 12

What we have learned?What we have learned?• Chapter 1: Composition and Structure of the Atmosphere • Chapter 2: Solar Radiation and the Seasons • Chapter 3: Energy Balance and Temperature • Chapter 4: Atmospheric moisture • Chapter 5: Cloud Development and Forms • Chapter 6: Precipitation Processes • Chapter 7: Atmospheric Pressure and Wind • Chapter 8: Air Masses and Fronts • Chapter 9: Middle Latitude Cyclones • Chapter 10: Atmospheric Dynamics • Chapter 11: Atmospheric and Oceanic Circulation • Chapter 12: ENSO and Present-Day Climate Variability • Chapter 13: Climate Change and the causes of climate change

Atmospheric Composition and structureAtmospheric Composition and structure

Permanent Gases

Chapter1Chapter1

• Water vapor •Carbon Dioxide • Ozone

Variable Gases

Temperature Layers

1) Troposphere

2) Stratosphere

3) Mesosphere

4) Thermosphere

Requirement:Requirement:

Chapter2: Solar Radiation and the Seasons

Energy and Energy and Methods of Methods of

Energy TransferEnergy Transfer

• Conduction– Molecule to molecule transfer

• Convection– transferred by vertical movement

• Radiation– propagated without medium (i.e. vacuum)– solar radiation provides nearly all energy

Stefan-Boltzmann Law• the total amount of Energy emitted is f(x) of temperature• described as:

I = σ T4

where I = intensity (W/m2) σ = Stefan-Boltzmann constant (5.67x10-8 W/m2/K4)

T = temperature in K

e.g. Earth: avg. temp = 15oC ~290 KI = (5.67x10-8 W/m2/K4)(290 K)4

= 400 W/m2

Earth = 400 W/m2

Sun = 73,000 W/m2

Wien’s Law

• identifies peak wavelength of emission, based on T

max = 2900 / T

where max = wavelength of max. emission (m) T = temperature in K

• hotter objects (Sun) have smaller peak wavelengths (max) than cooler objects (Earth)

Earth’s max = 10 mSun’s max = 0.5 m

Orientation

• rotation - spins on its axis• determines day length• axis tilted 23.5o (constant)• pts toward Polaris

• w/o tilt, no seasonal change constant Spring/Fall conditions with equal days/nights everywhere

REASON FOR SEASONS

Four Cardinal Dates:

Summer Solstice (June 21st)

Fall Equinox (Sept 22nd)

Winter Solstice (Dec 21st)

Spring Equinox (March 21st)

Requirement:Requirement:

• Understand the differences and characteristics of the three energy transfer mechanisms.

• Understand the concept of the two radiation laws discussed in lecture.

• Know the reasons for seasons. You should understand:

1 Perihelion & aphelion; 2. June and December solstices;

3 variations in daylength; 4 March and September equinoxes

Chapter3: Energy Balance and Temperature

Atmospheric Influences on InsolationAtmospheric Influences on Insolation

1. Absorption2. Reflection and Scattering3. Transmission

What happens to solar radiation as it travels through the atmosphere?

Principal Controls on Temperature   Principal Controls on Temperature   

1. Latitude

2. Altitude

3. Atmospheric Circulation

4. Land-Water Contrasts

5. Ocean Currents

6. Local Effects

Requirement:Requirement:

• What are the atmospheric influences on radiation? Be able to list and describe characteristics of these.

• What is albedo? Understand the basics of what happens to solar and terrestrial radiation as it is in, the global energy budget.

• What are the influences on temperature? • Understand the greenhouse effect and know a

few key greenhouse gases.

Chapter4: Atmospheric MoistureChapter4: Atmospheric Moisture

Evaporation and Condensation

• evaporation liberation of water molecules, requires energy

• Upon saturation, condensation will begin

• saturation: equilibrium between evaporation and condensation

Air may become saturated:

1. through the addition of water vapor to air at a constant temperature

• e.g.: hot shower

2. by mixing cold air with warm, moist air• e.g.: Contrails

3. by cooling air to the dew point• most common way via atmospheric cooling

Methods of Achieving Saturation (Condensation)

• Humidity: amount of water vapor in air

• Humidity expressed in a number of ways Indices:

Indices of Water Vapor Content

• Vapor pressure - the amount of pressure exerted on the atmosphere by water the amount of pressure exerted on the atmosphere by water vaporvapor

• Saturation vapor pressure (SVP) – maximum vapor pressure • maximum amt of vapor that can exist at a given temperature

•Absolute Humidity - density of water vapor, expressed in g/m3

•Specific Humidity - mass of water vapor, expressed in g/kg

•Saturation specific humidity - highest specific humidity for a given temperature and pressure

•Mixing Ratio- The amount of water vapor relative only to a mass of dry air

•Relative Humidity (RH) = (specific humidity) / (saturation specific humidity)*100%

•Dew-point - Temperature at which saturation occurs. Indicates moisture content. condensation begins

dv

vv

mm

m

m

mq

d

v

m

mr

Diabatic Processes

• involves the addition/removal of heat energy

• energy is transferred from areas of high temperature toward those of lower temperature

Adiabatic Process

• when temperature changes w/o addition/removal of heat • Cloud formation: primarily due to temperature changes with no heat exchange with surrounding environment

Forms of Condensation• fog radiation

advection upslope

• dew• frozen dew• frost• cloud

Requirements:Requirements:

• Understand the meaning behind the processes in the water cycle, especially: condensation, evaporation; what is saturation?

• What are the indices of water vapor content?

know drawback and advantages of each index.• Distinguish between adiabatic and diabatic

processes• Know characteristics of the forms of

condensation: dew, frost, frozen dew, fog.

Chapter 5: Development and Forms of clouds

Lifting Mechanisms (initial uplift push)

1. Orographic Lifting

2. Frontal Lifting

3. Convergence

4. Localized Convection

  Three Three Examples Examples of Stabilityof Stability

Cloud types (but not required in the final)

Requirements:Requirements:

• What are the four mechanisms that lift air? What happens in each mechanism?

• Understand the differences between absolutely stable, absolutely unstable, and conditionally unstable air.

• Know why the saturated adiabatic lapse rate is less than the dry adiabatic lapse rate.

• What factors influence the ELR?

Chapter 6: Precipitation Processes:

Why does it rain on us???

• Gravity and frictional drag balance to achieve terminal velocity

• terminal velocities for cloud drops, due to their small size, cannot exceed even weak updrafts

• volume of cloud drop must be 1,000,000 times greater than average drop to overcome updrafts

Growth of Cloud Droplets

1. Growth by Condensation

2. Growth in Warm Clouds -- collision and coalescence

3. Growth in Cool and Cold Clouds --- Bergeron Process

• Snow results from the Bergeron process, riming, and aggregation

• Rain: always associated with warm clouds and sometimes cool clouds (T > 0C)

• Rain showers – episodic precipitation events associated with convective activity and cumulus clouds

•Sleet begins as ice crystals which melt into rain through a mid-level inversion refreeze near surface .

•Freezing Rain forms similarly to sleet, however, the drop does not completely solidify before striking the surface

•Graupel – ice crystals that undergo extensive riming

Forms of Precipitation

Lake effect snows develop on leeside of water bodies, e.g. Great Lakes

• As cold air from the north or northwest flows over the lake, heat and water vapor are transferred upward and make air moist and unstable. As the air passes over the shore, the wind slow down due to large friction => convergence = > air rising => clouds = > heavy snows.

(a) an initial mechanism for uplift

(b) unstable air

(c ) sufficient moisture.

Requirements:Requirements:

• What is the terminal velocity?

• Distinguish between the collision-coalescence and Bergeron process. Which occurs in warm clouds, and cold clouds? Which occurs mostly in the Tropics?

• Know the types of precipitation and the processes that form them.

• Know the mechanism of lake effect snows.

Chapter 7 Atmospheric PressureChapter 7 Atmospheric Pressure

and Wind and Wind • Pressure Essentials  • Horizontal Pressure

Gradients • Cyclones and

Anticyclones  

Pressure GradientsPressure Gradients• The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher

to areas of lower pressure.• Hydrostatic Equilibrium

• State equation of ideal gas

• High pressure areas (anticyclones) clockwise airflow in the Northern Hemisphere (opposite flow direction in S. Hemisphere)– Characterized by descending air which warms creating

clear skies

• Low pressure areas (cyclones) counterclockwise airflow in N. Hemisphere (opposite flow in S. Hemisphere) – Air converges toward low pressure centers, cyclones are

characterized by ascending air which cools to form clouds and possibly precipitation

• In the upper atmosphere, ridges correspond to surface anticyclones while troughs correspond to surface cyclones

Requirements:Requirements:

• What does the equation of state relate? • What is the pressure gradient? How does

density affect this? ** remember! The pressure gradient force is

the sole generator of the winds. The Coriolis force changes wind direction; friction changes wind speed.

• What are cyclones, anticyclones, trough, and ridges?

Chapter 8: Air Masses and Fronts

(1) air gains temperature and humidity characteristics of the surface. (2) Topographically uniform areas. (3) It requires days for temp/moisture imprinting to form air masses.

(4) air masses classified by temp/moisture characteristics of source region

– moisture: continental (dry) v. maritime (marine) – c or m– temp: tropical (warm), polar (cold), arctic (very cold) – T, P or A

•

Formation of Air Masses

• separate air masses leads to changes in temperature and humidity as one air mass is replaced by another

• changes in temp lead to uplift and ppt

• four types of fronts: cold cold advancing on warmwarm warm advancing on coldstationary air masses not advancing

occluded does not separate tropical from polar/arctic, boundary btw twopolar air masses

Fronts

Course: Introduction to Atmospheric sciences(ATOC210) by GyuWon LEE

Types of frontsIdentification of fronts

1. Sharp temperature changes

2. Change in the air’s moisture

3. Shifts in wind direction

4. Pressure and pressure change

5. Clouds and precipitation patterns

Requirements:Requirements:

• Know the types of fronts, and the characteristics of each

• Know the type of air masses, and the characteristics of each (which are moist, warm, etc).

• How to identify fronts in weather maps?

Chapter 9: Mid-Latitude Cyclones

• cyclogenesis – formation of mid-latitude cyclones along the polar front • • boundary separating polar easterlies from westerlies• • low pressure area forms counterclockwise flow (N.H.)• • cold air migrates equatorward• • Warmer air moves poleward

The Life Cycle of a Mid-Latitude Cyclone

• Well-developed fronts circulating about a deep low pressure center characterize a mature mid-latitude cyclone.

• Deep low pressure center;• Chance of precipitation increases toward the storm center

– cold front: heavy ppt. (cumulus clouds)– warm front: lighter ppt. (stratus clouds)– warm sector: unstable conditions

Mature Cyclones

• when the cold front joins the warm front, closing off the warm sector, surface temperature differences are minimized

• effectively the warm air is cut-off from the surface• The system is in occlusion, the end of the system’s life cycle • evolution eastward migration

Occlusion

Rossby Waves and Vorticity• vorticity rotation of a fluid (air)

• Absolute vorticity: - relative vorticity motion of air relative to Earth’s surface- Earth vorticity rotation of Earth around axis

• Air rotating in same direction as Earth rotation counterclockwise +ive vorticity• Air rotating in opposite direction as Earth rotation clockwise -ive vorticity• maximum and minimum vorticity associated with troughs and ridges, respectively

• changes in vorticity in upper troposphere leads to surface pressure changes • Increase in absolute vorticity convergence• decrease in absolute vorticity divergence • decrease vorticity divergence draws air upward from surface surface LP• referred to as dynamic lows (v. thermal lows)• dynamic lows (surface) exist downwind of trough axis

• increase vorticity convergence air piles up, sinks downward surface High

WHAT’S THE POINT OF VORTICITY????

Necessary ingredients for a developing wave cyclone1. Upper-air support

- A shortwave moves through this region, disturbing the flow.- Diverging air aloft causes the sfc pressure to decreases beneath position 2 rising air motion.- Cold air sinks and warm air rises: potential energy is transformed into kinetic energy- Cut-off low

Necessary ingredients for a developing wave cyclone2. Role of the jet stream: upper-level divergence above the surface low

Requirements:Requirements:

Chapter 10: Atmospheric Chapter 10: Atmospheric DynamicsDynamics

Forces We Will ConsiderForces We Will Consider

• Gravity

• Pressure Gradient Force

• Coriolis Force• Centrifugal Force / Centripetal

Acceleration

• Friction

Coriolis force (CF)

- The Coriolis force causes the wind to deflect to the right of its intended path in the Northern Hemisphere and to the left of its intended path in the Southern Hemisphere. It acts at a right angle to the wind.

- The Coriolis force is largest at the pole and zero at the equator

- The stronger the wind speed, the greater the deflection

- The Coriolis force changes only wind direction, not wind speed.

- We measure motion on the rotating Earth. Thus, we need to be concerned with the Coriolis force

Atmospheric Force BalancesAtmospheric Force Balances

• First, MUST have a pressure gradient force (PGF) for the wind to blow.

• Otherwise, all other forces are irrelevant.

• Already discussed hydrostatic balance, a balance between the vertical PGF and gravity. There are many others that describe atmospheric flow…

Geostrophic BalanceGeostrophic Balance• Balance between PGF and Coriolis force

Fig. 6-15, p. 172

• Therefore, wind blows parallel to isobars, which is useful to consider when looking at weather map.

• Buy-Ballot’s “law”: If you stand with your back to the wind in the N.H, low pressure will be on your left and high pressure on your right.

Gradient Wind BalanceGradient Wind Balance• Balance between PGF, Coriolis force, and

centrifugal force

Supergeostrophic flow(CF > PGF )

PGF + Ce = CF

Subgeostrophic flow

(CF < PGF)

PGF = CF + Ce

ComparisonComparison

Pressure gradient force

Coriolis force

Centripetal force

Frictional force

Gravitational force

Geostrophic winds: straight-line flow aloft

Hydrostatic balance (equilibrium)

Surface winds

Gradient winds: Curved winds around lows and highs aloft

Requirements:Requirements:

Chapter 11: Atmospheric/Oceanic Chapter 11: Atmospheric/Oceanic Circulation Circulation

Farrell Farrell CellCell

polar Cellpolar Cell

The pattern of surface wind with the rotation of EarthThe pattern of surface wind with the rotation of Earth

• Sea breeze and land breeze

• Valley breeze and mountain breeze

• Chinook wind

• Monsoons

Oceanic CirculationOceanic Circulation

• Surface current: Ekman current Coriolis Force = Wind stress. The surface current is 45°

to the right of the wind in the northern hemisphere.

• subsurface current: geostrophic current Coriolis Force = Pressure gradient force

• Deep circulation: thermohaline circulation

• Ekman mass transport. The transport is perpendicular to the wind stress, and to the right of the wind in the northern hemisphere.

Requirements:Requirements:• What are the three cell models? What are

the characteristics of each cell? • What are the ITCZ, and what are the

pressure and wind distributions corresponding with the three cells?

• Know local circulations including monsoon, land and see breeze etc.,

• What are the oceanic circulation? What are the mechanisms responsible for different oceanic currents?

Chapter 12: Present-Day Climate Chapter 12: Present-Day Climate Variability Variability

ENSO MODEENSO MODE

PNA: 4 centers: PNA: 4 centers: HawaiiHawaii(20N,160W);(20N,160W);North Pacific OceanNorth Pacific Ocean (45N 165W); (45N 165W); Alberta Alberta (55N (55N 115W); and the 115W); and the Gulf Gulf CoastCoast region of region of USA (30N 80W)USA (30N 80W)

AO is the dominant mode AO is the dominant mode of mean-monthly sea level of mean-monthly sea level pressure variability pressure variability over the Northern over the Northern Hemisphere withHemisphere withan out-of-phase relation an out-of-phase relation between the sea level between the sea level pressure over the Arctic pressure over the Arctic basin and that at the mid-basin and that at the mid-latitudes (Thompson and latitudes (Thompson and Wallace 1998).Wallace 1998).

AO AO

Requirements:Requirements:

• What are ENSO, AO, NAO and PNA?

• How do these climate variability modes impact the climate over the northern America?

• Know the simple hypothesis of EL Nino mechanism.

Final Exam.Final Exam.

• Part A: Answer all 60 multiple-choice questions. This part is 60%.

• Part B: Answer questions. This part is worth 40%.

• Electronic calculators are not allowed.

• Final Exam. covers chapter 1 to 12 of lecture notes.