Midterm Test: October 19, 2004

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Midterm Test October 19, 2004

• We will be writing in two rooms according to last
  name of students
• 1B 36 (here) A to Lufty-Lapierre, Justyn
• 1B 934 MacDonald, Jessica to Z
• Closed book, no aids allowed
• 10 multiple choice questions
• 7 out of 10 short answer questions do not
  answer all questions, be concise in your answer
• No calculators needed or allowed
• Midterm will cover material up to and including
  lecture of October 14

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ATOC 210 Introduction to Atmospheric Sciences

• October 19 A review of Chapters 1-3, 5, 9-14

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Chapter 1

• The Earth and its Atmosphere

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Air Pressure and Air Density

• Pressure decreases as we ascend in the vertical
  direction

5
Vertical Structure of Atmosphere

• Vertical temperature profile shows different
  layers in the lower atmosphere

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Vertical Structure of Atmosphere

• Boundaries between layers are tropopause,
  stratopause and mesopause
• There are thus 3 regions of relative warmth
  Earth's surface, stratopause and above 80 km

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Chapter 2

• Energy Warming the Earth and the Atmosphere

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Heat Transport in Atmosphere

• Transport of heat is accomplished by conduction,
  convection and radiation 
• conduction transfer of kinetic energy of
  molecules by collisions of molecules
• convection transport of heat within a fluid by
  bulk motion of fluid itself 
• heat transported by conduction and convection is
  "sensible heating" 
• radiation energy transfer which can take place
  in a vacuum, with no intervening physical medium
  (unlike conduction and convection)

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A rising air parcel cools Why?

• Rising air parcel enters a region of lower
  pressure, as pressure decreases with height
• Air molecules push against the surrounding ones
  to equalize pressure, thus doing work
• Energy source for work comes from the parcel
  itself, from its internal energy
• As internal energy depends on temperature, the
  parcel cools
• Conversely, sinking parcels warm
• Almost all clouds are due to rising motion of air
  parcels

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Rising and Sinking Air
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Weather Maps Revisited

• Surface highs (H) are fair weather systems, winds
  around a high blow clockwise and spiral outward
• Surface lows (L) are stormy weather systems,
  winds around a low blow counterclockwise and
  spiral inward
• Inward motion around low leads to rising motion,
  due to conservation of mass
• This in turn results in cooling, condensation and
  precipitation, thus lows are associated with
  stormy weather!

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Sun and Earth Radiation
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Surface Temperature

• Earths surface temperature depends on three
  factors
• Radiation from the sun (known)
• Earths reflectivity (albedo, 30)
• Amount of warming provided by atmosphere
  (greenhouse effect)

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In addition, clouds (droplets) can absorb
radiation in the atmospheric window but are
poor absorbers of solar radiation
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Annual Heat Imbalances

• imbalance is difference between input of
  absorbed solar radiation and output of emitted
  infrared radiation 
• excess of heat input between 37oN and 37oS, and
  a deficit poleward of these latitudes

Figure 3.8
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Chapter 3

• Seasonal and Daily Temperatures

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Figure 3 As the earth revolves about the sun, it
is tilted on its axis by an angle of 23.5. The
earth's axis always points to the same area in
space (as viewed from a distant star). Thus in
June, when the Northern Hemisphere is tipped
toward the sun, more direct sunlight and long
hours of daylight cause warmer weather than in
December, when the Northern Hemisphere is tipped
away from the sun.
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Daily Temperature Variation
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Chapter 5

• Atmospheric Moisture

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Determining Vapor Pressure
Average atmospheric pressure of 1013 mb is
comprised in part by the weight of vapor
molecules. Warmer air can absorb more vapor than
cooler air before it saturates, and can have
higher saturated vapor pressures.
Figure 5.10
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Measures of humidity

• absolute humidity
• specific humidity
• mixing ratio
• vapour pressure
• relative humidity
• dew point

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Dew Point vs. Relative Humidity
While relative humidity may be higher in polar
air compared to desert air, there could be more
water vapour in desert air. Typical desert
air temperature 35C dew point 10C relative
humidity 21 Typical polar region temperature
-2C dew point -2C relative humidity 100
Figure 5.13A
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Chapter 9

• The Atmosphere in Motion Air Pressure, Forces
  and Winds

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Recall our tale of two cities .
Figure 9.2c
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Types of Forces in Atmosphere

• Pressure gradient force
• Coriolis force
• Centripetal force
• Frictional force
• Gravitational force

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Chapter 10

• Wind Small-Scale and Local Systems

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Figure 9.20
Figure 9.21
Non-rotating Platform A Ball moves in a straight
line. Rotating (counter-clockwise) Platform B
Ball appears to deflect to the right of its
intended path. Observer thus concludes there is
a force acting on the ball the Coriolis force.
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Vertical Air Motion
Figure 9.33
Winds converging into a low pressure center
generate upward motion, because of conservation
of mass. These updrafts may cause cloud
formation. Likewise, diverging air molecules
from a high pressure center induce downward
motion.
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Weather Maps Revisited from second lecture

• Surface highs (H) are fair weather systems, winds
  around a high blow clockwise and spiral outward
• Surface lows (L) are stormy weather systems,
  winds around a low blow counterclockwise and
  spiral inward
• Inward motion around low leads to rising motion,
  due to conservation of mass
• This in turn results in cooling, condensation and
  precipitation, thus lows are associated with
  stormy weather!

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Planetary Boundary Layer
The layer of air influenced by surface friction
is called the planetary boundary layer
(PBL). The mixing depth of the PBL can increase
as a) air becomes more unstable (e.g. surface
heats) b) terrain roughens c) wind speeds
increase
Figure 10.3
(a) Atmosphere is stable (b) Atmosphere is
unstable
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Thermal Circulations
Figure 10.19A
Figure 10.19B
Warm air is less dense than cold air. Pressure
decreases more slowly with height in warm air.
This leads to higher pressure above the surface
in the warm air compared to the pressure in the
cold air at the same height.
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Thermal Circulations
Figure 10.19c

• If mass is removed from the column in the warm
  air the surface pressure decreases in the warm
  air. So air flows from cold air to warm air at
  the surface. The cycle is completed by air rising
  in the warm region and sinking in the cool region.

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Chapter 11

• Wind Global Systems

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Three Cell Circulation Model
A rotating earth breaks the single cell into
three cells. The Hadley cell extends to the
subtropics, the thermally indirect Ferrel cell
extends over the mid latitudes, and the Polar
cell extends over the poles. The Coriolis force
generates mid-latitude westerlies, the NE and SE
trade winds, and the polar easterlies.
Figure 11.2A
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Note surface winds are no longer easterly
everywhere
Figure 11.2b
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JANUARY
Figure 11.3A
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Figure 11.10
Jet streams are into the page, from west to east
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Non-El Nino (or La Nina) conditions
Trade Winds transport surface water from east to
west Pacific. Upwelling brings nutrient rich cold
water to surface near Peru. Cold water is warmed
by sunlight and atmosphere as it moves west. The
warmed water piles up in the western
Pacific. Surface pressures over the warm water in
the west tend to be lower than over the cold east
Pacific water.
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El Nino conditions
Surface pressure decreases over east Pacific and
rises over west Pacific, due to see-saw variation
of pressure in tropical Pacific (Southern
Oscillation). Trade winds weaken. Warm water
from the west Pacific flows eastward, causing
warm surface temperatures right across the
tropical Pacific.
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Chapter 12

• Air Masses and Fronts

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Fronts
Figure 12.12
A front, or transition zone, separates air masses
with different properties. Four kinds of fronts
cold, warm, stationary, occluded
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Weather across cold front (along line X X)
Figure 12.14
The front rises steeply, about1 km over 50 km
distance, with showers and thunderstorms at the
front.
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Across a Warm Front (along P P)
Figure 12.18A
The cross-sectional view shows the gentle slope
of overrunning warm air (shallow slope 1300) , a
typical temperature inversion (32oF isotherm
bends back), and the shifting winds.
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Figure 12.19 Formation of a cold occluded front
The faster moving cold front (a) catches up to
the slower moving warm front (b), and forces it
and the warm air mass to rise off the ground
(c). Green shading represents precipitation
(c)
(b)
(a)
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Figure 12.20a Formation of a warm occluded front
The faster moving cold front in (a) overtakes the
slower moving warm front in (b). The lighter air
behind the cold front rises up and over the
denser air ahead of the warm front.
(b)
(a)
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Chapter 13

• Middle-Latitude Cyclones

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Idealized life cycle of a wave cyclone (Figure
13.1)
(a)
(b)
(c)
(d)
(e)
(f)
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Shortwave Disturbance
Shortwave ripples within the Rossby waves move
faster and propagate downwind into the Rossby
troughs and cause them to deepen. Barotropic
conditions, where isobars and isotherms are
parallel, then degenerate into a baroclinic
state where the contours cross, and cold or warm
air is advected downwind.
Figure 13.7A
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Summary of a developing wave cyclone
Upper and surface maps illustrate the role of
convergence and divergence aloft, and the
patterns of clouds, precipitation, and
temperatures on the ground.
Figure 13.11
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Chapter 14

• Weather Forecasting

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Three steps in generating a numerical forecast
from a weather prediction model 1) Analysis
Determine the present state or initial
conditions, by incorporating observational data
such as 6-hourly surface and 12-hourly sounding
data onto grid points of a computer model 2)
Prediction Run the model starting from the
present state for a period of lead time to
generate a forecast for that period 3)
Prognosis Interpret the model results by
experienced forecasters
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Why are weather forecasts inaccurate?

• In spite of the power of the supercomputers,
  computing capability limits the number of grid
  points in forecast models.
• Due to a lack of understanding, many of the
  physical processes can only be represented
  approximately in the models.
• There are gaps in the observational data that are
  used to initialize the models (e.g. over the
  oceans).
• The atmosphere is inherently chaotic. The
  influence of errors in the initial measurements,
  or variations in atmospheric properties on scales
  smaller than the measurements, amplify quickly
  with time and contaminate the forecast.

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Chaos The Butterfly Effect

• A butterfly flapping it's wings in one area of
  the world (e.g., the Amazon) can cause a tornado
  to occur in another remote area of the world
  (e.g., midwest US)!
• The extreme sensitivity to initial conditions
  limits the predictability of weather.
• This sensitivity leads to rapid growth of
  initially small errors that are inevitable, which
  eventually contaminate forecast (chaos)
• From theory and model studies, the predictability
  limit of daily weather is about 2 weeks.