Transcript pressure gradient force

REVIEW: Understanding Weather and
Climate
Chapter 1 - Chapter 12
What we have learned?
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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
Chapter1
Atmospheric Composition and structure
Permanent Gases
Variable
Gases
• Water vapor
•Carbon Dioxide
• Ozone
Temperature
Layers
1)
Troposphere
2)
Stratosphere
3)
Mesosphere
4)
Thermosphere
Requirement:
Chapter2: Solar Radiation and the
Seasons
Energy and
Methods of
Energy 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 K
I = (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 m
Sun’s max = 0.5 m
Orientation
• rotation - spins on its axis
• determines day length
• axis tilted 23.5o (constant)
• pts toward Polaris
REASON FOR
SEASONS
• w/o tilt, no seasonal change  constant Spring/Fall conditions with equal
days/nights everywhere
Four Cardinal
Dates:
Summer Solstice
(June 21st)
Fall Equinox
(Sept 22nd)
Winter Solstice
(Dec 21st)
Spring Equinox
(March 21st)
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
What happens to solar radiation as it travels
through the atmosphere?
Atmospheric Influences on Insolation
1. Absorption
2. Reflection and Scattering
3. Transmission
Principal Controls on Temperature
1.
2.
3.
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6.
Latitude
Altitude
Atmospheric Circulation
Land-Water Contrasts
Ocean Currents
Local Effects
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 Moisture
Evaporation and Condensation
• evaporation  liberation of water molecules,
requires energy
• Upon saturation, condensation will begin
• saturation: equilibrium between evaporation and
condensation
Methods of Achieving Saturation
(Condensation)
Air may become saturated:
1. through the addition of water vapor to air at a
constant temperature
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e.g.: hot shower
2. by mixing cold air with warm, moist air
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e.g.: Contrails
3. by cooling air to the dew point
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most common way via atmospheric cooling
Indices of Water Vapor Content
• Humidity: amount of water vapor in
air
• Humidity expressed in a number of
ways  Indices:
• Vapor pressure - the amount of pressure exerted on the atmosphere by water vapor
• 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 q 
mv
mv
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m mv  md
•Saturation specific humidity - highest specific humidity for a given
temperature
and pressure
mv
•Mixing Ratio- The amount of water vapor relative only to a mass of dry airm d
r
•Relative Humidity (RH) = (specific humidity) / (saturation specific
humidity)*100%
•Dew-point - Temperature at which saturation occurs. Indicates moisture content.
condensation begins
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
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fog
radiation
advection
upslope
• dew
• frozen dew
• frost
• cloud
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
Examples
of Stability
Cloud types (but not required in the final)
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???
Growth of Cloud Droplets
• 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
1. Growth by Condensation
2. Growth in Warm Clouds -- collision and coalescence
3. Growth in Cool and Cold Clouds --- Bergeron Process
Forms of Precipitation
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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
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:
• 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 Pressure
and Wind
• Pressure Essentials
• Horizontal Pressure
Gradients
• Cyclones and
Anticyclones
Pressure 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:
• 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
Formation of Air Masses
(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
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Fronts
• 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 warm
warm  warm advancing on cold
stationary  air masses not advancing
occluded  does not separate tropical
from polar/arctic, boundary btw two
polar air masses
Types of fronts
Identification 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
Course: Introduction to Atmospheric sciences(ATOC210) by GyuWon LEE
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
The Life Cycle of a Mid-Latitude Cyclone
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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
Mature Cyclones
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Well-developed fronts circulating about a deep low pressure center
characterize a mature mid-latitude cyclone.
• Deep low pressure center;
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Chance of precipitation increases toward the storm center
– cold front: heavy ppt. (cumulus clouds)
– warm front: lighter ppt. (stratus clouds)
– warm sector: unstable conditions
Occlusion
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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
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
WHAT’S THE POINT OF VORTICITY????
• 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
Necessary ingredients for a developing wave cyclone
1. 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 cyclone
2. Role of the jet stream: upper-level divergence above the surface low
Requirements:
Chapter 10: Atmospheric
Dynamics
Forces 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 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 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 Balance
• Balance between PGF, Coriolis force, and
centrifugal force
Supergeostrophic flow
(CF > PGF )
PGF + Ce = CF
Subgeostrophic flow
(CF < PGF)
PGF = CF + Ce
Comparison
Requirements:
Pressure gradient force
Surface winds
Coriolis force
Geostrophic winds:
straight-line flow aloft
Centripetal force
Frictional force
Gravitational force
Gradient winds:
Curved winds
around lows and
highs aloft
Hydrostatic balance (equilibrium)
Chapter 11: Atmospheric/Oceanic
Circulation
polar Cell
Farrell
Cell
The pattern of surface wind with the rotation of Earth
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Sea breeze and land breeze
Valley breeze and mountain breeze
Chinook wind
Monsoons
Oceanic 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
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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:
• 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
Variability
ENSO MODE
PNA: 4 centers:
Hawaii(20N,160W);
North Pacific Ocean
(45N 165W);
Alberta (55N
115W); and the Gulf
Coast region of USA
(30N 80W)
AO
AO is the dominant mode
of mean-monthly sea level
pressure variability
over the Northern
Hemisphere with
an out-of-phase relation
between the sea level
pressure over the Arctic
basin and that at the midlatitudes (Thompson and
Wallace 1998).
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.
• 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.