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Lecture 17
Required reading: Climate change report
2008 from the National Academies
• Greenhouse gases and global warming
– Global warming and H2O
– Climate modeling
• Air masses
• Fronts
• Cyclogenesis
Urban heat island effect
Notice the warmth of Pittsburgh compared to
the surrounding rural area
Difference in hourly temps between Chicago
and Aurora, IL. Demonstrates Chicago’s
heat island effect (mT air mass)
Greenhouse gases and climate change
• H2O, CO2, CH4, CFCs are all gh gases
• Humans have changed the amount of gh
gases in the atmosphere over the past
century by > 25%
• Based on radiation arguments of the
greenhouse effect, temperature should
have increased a lot too, and it has
• Skeptics say that it is hard to prove exactly
why because
Greenhouse gases and climate change
(cont)
• The climate system is complex with much
natural variability
• Use process of elimination:
• Striking warming signal since 1978 During
this time
– Has the output of the sun decreased? NO
– Has the frequency of volcanic eruptions
decreased? NO
• Has to be due to anthropogenic effects
Climate change and atmospheric water
• Human activities add little H2O to the
atmosphere directly
• Saturation water pressure increases very
rapidly with temperature
• Since H2O is a strong gh gas this will lead
to still warmer temperatures, still more
water vapor etc
• On the other hand cloudiness may also
increase, which would be a cooling effect
Global warming and atmospheric H2O,
continued
• More aerosols can mean more CCN, more
droplets can form in clouds and the clouds
may reflect more
• Indirect aerosol effect on climate
• Contrails are airplane-induced clouds
Stratus clouds off California, ship tracks can
be seen in the clouds since their reflectance
is enhanced by increased aerosols
Climate modeling
• GCMs, global climate models consist of an
atmospheric model, coupled to an ocean
model, coupled to a sea ice model, land
surface model. Latest versions include
chemistry in atm & ocean
• Give statistical estimates of future
conditions
• Sensitivity studies to understand
processes
Schematic of atmospheric processes
included in a GCM
Let’s go to the NAS report
• Climate model projections for the century
(until 2100)
• Depend on “scenarios” for climate change.
Each scenario represents plausible and
internally consistent prediction of future
human activites.
• Predict population growth, economic
development, energy and land use– social
science
Weather – Air Mass
• extremely large body of air with similar
characteristics of temperature and
moisture. Forms when air stagnates for
long periods of time over a uniform surface
• Characteristic weather of an air mass is
determined by the surface above which it
forms
Air mass types (classified based on
temperature and humidity)
• Polar (P): polew of 60N or S - cold
• Tropical (T): equatorw of 30N or S – warm
• Arctic (A): over Arctic – very cold
• Continental (c): over land – dry
• Maritime (m): over oceans – moist
Air mass source regions: must have light
or no winds so the air has time to acquire
the characteristics of the surface, uniform
surface characteristics needed
Major air mass sources of the world
Air masses affecting North America
Hourly temperature at Madison WI during an
Arctic outbreak (A air mass)
mT airmass from Gulf of Mexico
Air mass modification
• An air mass may be modified when
advected away from the source region.
The modified air mass may be
warmed/cooled from below --- remember
atmospheric stability! It may also take up
water vapor or be dried out by passing
over certain terrain etc.
cP-w
IR image: after passage of cold front, cP air mass
passes over the relatively warm Gulf stream- goes
unstable
Winter cP air moving over warm body of
water becomes unstable
Lake effect snows (contours show annual
snowfall)
The air mass takes up characteristics
from the surface of the lake. Heating
from below  unstable. Warming +
evaporationgains moisture
Fronts
• Separate different air masses
• Cold, warm, occluded fronts
Sfc weather associated with a cold front
Pressure
T
Clouds &
precip
Wind
Vertical slice through a cold front
Sfc weather with a warm front
Vertical structure of a warm front
Occluded fronts
Cold
type
Warm
type