Download Week 7 Class PPT Notes

WEEK 7: 8 MAR 2017
• Atmospheric Stability
• Climate Change;
Midterm #1
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Midterm #1
AVERAGE
ATMOSPHERIC
STABILITY
ATMOSPHERIC VERTICAL MOTION
• Critical to both energy and moisture transport
• Without vertical motion
– No clouds
– No precipitation
– No mixing of pollutants away from ground
level
ATMOSPHERIC VERTICAL MOTION
• Two types of vertical motion:
– FORCED (DYNAMIC)
• Up over a hill
• Warmer air over colder air
• Horizontal convergence
– BUOYANT (THERMAL)
• Air rises because it is less dense than its
surroundings
• Stability is especially important here
VERTICAL TEMPERATURE STRUCTURE
LAPSE RATES
• Change of temperature with height
• Two kinds:
– Environmental Lapse Rate (ELR)
• What you would measure with a weather
balloon
– Parcel Lapse Rate
• Temperature changes an air parcel
experiences when displaced vertically
• Assume adiabatic process (i.e., no heat
exchange occurs across parcel boundary)
FACTORS INFLUENCING ELR
 Heating or cooling of the lower atmosphere
 Advection of cold and warm air at different
levels
 New
airmass
ADIABATIC CONCEPTS
• Air Parcels
• Rises and expands => cooling
• Air molecules further apart, few collisions
-> lowers the parcel temperature
• Sinks and compresses, closer together, more
collisions -> warms the parcel
• Adiabatic if no heat exchange between the
parcel and outside air
ADIABATIC LAPSE RATES
•Dry Adiabatic Rate if not saturated
• Lapse rate = 5.5 °F / 1000 feet (constant)
•10°C / 1000m
•Moist Adiabatic Rate if saturated (within cloud)
• Heat from condensation offsets cooling
• Lapse rate = ~3.3 °F / 1000 feet (variable)
•~6°C /1000m
STABILITY
STABILITY IN THE ATMOSPHERE
• If an air parcel is displaced from its original height
• STABLE - Returns to its original height
• UNSTABLE - Accelerates upward because
buoyant
• NEUTRAL - Stays at the level to which it was
displaced
STABILITY IN THE ATMOSPHERE
• IS A PARCEL WARMER/COLDER THAN IT’S
ENVIRONMENT?
• IF COLDER (HEAVIER) THEN STABLE
• IF WARMER (LIGHTER) THEN UNSTABLE
An Initial
Perturbation
Stable
Unstable
Neutral
ABSOLUTE STABLITY
• BOTH DRY AND MOIST PARCELS ARE COLDER
(HEAVIER) THAN ENVIRONMENT
• RETURN TO ORIGINAL POSITIONS
• REASONS
• Radiative cooling of surface at night
• Advection of cold air near the surface, warmer
aloft
• Air moving over a cold surface
•Typically at night, or cold winter days
ABSOLUTE INSTABILITY
• BOTH DRY AND MOIST PARCELS ARE WARMER
(LIGHTER) THAN ENVIRONMENT
• CONTINUE TO RISE
• REASONS
•Surface heating
•Warm Advection
•Cooling aloft
• Typically during the daytime, or hot summer days
CONDITIONAL INSTABILITY
• Dry (Unsaturated) Stable Air is Lifted
• Saturation Occurs – releases Latent heat and
become warmer than surroundings
• Continues to rise as Unstable Air
ATMOSPHERIC STABILITY
• Stability is a state of equilibrium in terms
atmospheric movement; no vertical
movement occurs when the atmosphere is
stable!
• A rising parcel of air expands and cools,
while a sinking parcel is compressed and
warms
• Clouds form as air rises and cools
CLOUD DEVELOPMENT
• Clouds develop as an air parcel rises and
cools below the dew point
• Usually a trigger or process is needed to
initiate the rise of an air parcel
CLOUD DEVELOPMENT
• Convection
– Differential land surface heating creates areas
of high surface temperature
– Air above warm land surface heats, forming a
‘bubble’ of warm air that rises (convection)
– Cloud base forms at Level of Free Convection
or Cloud Condensation Level (CCL)
CLOUD DEVELOPMENT
• Topography
– Orographic uplift
– Orographic clouds
– Windward, leeward, rain shadow
– Lenticular clouds
Stepped Art
Fig. 5.16, p. 134
THE EARTH’S CHANGING
CLIMATE
Climate Change vs. Global Warming
• Climate Change:
– Changes in climate of the past,
present or future associated with
natural or anthropogenic (human)
factors
• Global Warming:
– Warming of the 20th and 21st
century associated with
anthropogenic activities.
Climate Change
• Climate change is
complicated
• Change impacts some areas
more than others
• Human perspective is relative
- short lifetimes
• GCMs (Global Climate Models)
produce multiple outcomes
Looking into the Past
• Use caution –
• Alternative explanations other than climate
change!
• For example:
– In 17th & 18th centuries, the river Thames in
London usually froze in winter
• Were winters much colder?
• Or:
– Did Old London bridge slow river flow
down?
– Lack of embankments make river wider?
– Lack of waste heat from industrial plants?
Reconstructing Past Climates
• Instruments records
• Proxy measurements
• Tree rings
• Ice cores
• Plate tectonics (folding, faulting) and erosion
complicates the picture
– Record confused or destroyed (90-99%
eroded)
– Little remains from first 90% of Earth’s
lifetime
– Changes in arrangement of continents and
oceans
Last 18,000 Years
•
•
•
•
Cooler and Drier conditions 18K-10K years ago
Warmer and wetter 8K-4K years ago
Historical records can be used now
Mountain glaciers growing 4K-100 years ago
Fig. 15.5, p. 433
16th to 19th Centuries
• Cooler period –”Little Ice Age”
– Glaciers expanded
– Cool summers, severe winters
– Not a period of sustained cold
Fig. 15.6, p. 436
Since Late-19th Century
• Global warming
• Evidence comes primarily from
instruments
• Annual global temperatures have
risen ~ 1º F
• Warmest years in the record have
mostly occurred > 2000
• Warmest 20-year periods: 19251944, 1978-1997
• Slight cooling in-between these
years
Climate System Components
CLIMATE CHANGE / VARIABILITY CAUSED
BY NATURAL EVENTS
– Change in Earth’s tilt;
– Change in composition of the atmosphere;
– Change in Earth’s surface;
– Change in Earth’s orbit around the sun
(causes change in incoming radiation);
CLIMATE CHANGE CAUSED BY HUMAN
(ANTHROPOGENIC) ACTIVITIES
• Aerosols in the Lower Atmosphere
– Sources: factories, autos, trucks, aircraft,
power plants, home furnaces and fireplaces
– Not injected directly into the atmosphere,
but form when gases convert to particles
– Highly reflective sulfate aerosols
– May be a net cooling effect
CLIMATE CHANGE CAUSED BY HUMAN
(ANTHROPOGENIC) ACTIVITIES
• Increasing Levels of Greenhouse Gases
– Carbon dioxide
– Burning fossil fuels
– Deforestation
– Methane, nitrous oxides, CFCs
The Greenhouse Effect
• Greenhouse gases:
– Transparent to incoming solar radiation
– Efficiently absorb outgoing long-wave
radiation
– Lead to a warmer atmosphere
Greenhouse Gases
Current CO2: ~380 ppm
Greenhouse Gases
Greenhouse Gases
• Effectiveness at atmospheric warming
Gas
Concentration (ppm) Greenhouse
Warming
Strength (W/m-2)
Water Vapor
3000
~100
Carbon Dioxide 353
~50
Methane
1.72
1.7
Nitrous oxide 0.31
1.3
Recent Warming
•
•
•
•
Decrease in exceptionally cold temperatures
Increase in exceptionally warm temperatures
Minima have increased more rapidly than maxima
Diurnal temperature range decreased 0.08º C per
decade
• Most pronounced warming – northern continents
• Marked cooling
– NW Atlantic Ocean
– Less in north central Pacific
• Changes appear most clearly in winter
Recent Warming
Mt. Pinatubo - Global Cooling
1991 - Changing
forcing changes
the temperature
(and water vapor,
etc.)
Warming Measurements
IPCC
• Intergovernmental Panel on
Climate Change
• Established 1988 by WMO
• 194 nations
• 3000+ climate scientists
• 2014 Issued 5th Assessment Report
Summary
IPCC 5th Assessment Report
• Warming of the climate system is unequivocal
• Most of global temperature is very likely (>95%) anthropogenic
(human)
• It is likely (with medium confidence) that 1983—2013 was the
warmest 30-year period for 1400 years.
• There is high confidence that the sea level rise since the middle of
the 19th century has been larger than the mean sea level rise of
the prior two millennia.
• Concentration of greenhouse gases in the atmosphere has
increased to levels unprecedented on earth in 800,000 years.
• Anthropogenic warming and sea level rise would continue for
centuries
• Probability warming is only natural variation is less than 5%
• World temperatures could rise between 2 and 15°F
• Sea levels will probably rise by 7 to 23”
Future projections of CO2 Concentrations
• What happens in the future
depends on how much more CO2 we
release into the atmosphere
• Even the low-emission scenarios
result in greatly increased CO2
concentrations by the year 2100
– Current Concentration: 380 ppm
– Max scenario: 935 ppm
– Min scenario: 420 ppm
Climate Change Summary
• Higher temperatures –
• Especially on land
• Polar Regions
• Hydrological cycle more intense
• Storms have more “fuel,” so they can be more
powerful, thus more intense rainfall events
• Sea levels rise
• Oceans expand with extra heat
• Melting of polar ice
Future Temperature Projections
Temperature Projections
• Projected Warming (2000 – 2100)
– From ~2.5°F to ~12.5°F
– Not all places will warm at the same rate
• Curves represent seven independent scenarios
• Each bar on right represent range of warming
produced by models of differing sensitivities for
a specific scenario.
GCM Forecasts
“Median” Scenario Temperature Projections
• Land areas projected to warm more than oceans
• Greater warming at high latitudes
Annual mean temperature change: 2071 to 2100 Relative to 1990
Other Possible Changes
• As Poles warm more quickly than tropics…
– The jet stream will weaken and move north
• The storm track will also move north
• Latitude bands 30-40 degrees should get drier
• Rain events begin to replace snow events
• Reduction in the # of strong tornadoes in U.S.
• Tornado Alley migrates north
• Atlantic hurricanes will more easily form and be
generally stronger
“Median” Scenario Precipitation Projections
Some areas are projected to become wetter,
others drier with an overall increase projected
Annual mean precipitation change: 2071 to 2100 Relative to 1990
Sea Level Projections
Potential Climate Change Impacts
Predictions For the Bay Area
• Decreased winter precipitation as jet stream
moves north
• Increased summer precipitation as water is
warmer/more subtropical moisture
• Weaker sea breezes due to warmer ocean
temperatures results in hotter summers
• Less snowpack in Sierra Nevada, leading to
water shortages
Feedback Mechanisms
• Climate is linked with many physical processes
– A change in part of the climate system may
cause subsequent changes in other parts
– Subsequent changes could support or act
against the original change
Positive Feedback
• When the response in a second variable reinforces
the change in the initial variable
• Example of positive feedback:
– Global temperatures increase
– Increase in temperature melts the ice and snow in
the upper latitudes
– Loss of ice and snow results in a lower albedo at
the surface in the upper latitudes
– Lower albedo leads to less reflection and more
insolation
– More insolation results in warmer temperatures
Negative Feedback
• When the response in a second variable lessens the
change caused by the initial variable
• Example of negative feedback:
– Global warming leads to more atmospheric water
vapor
– Increased water vapor leads to increased cloud
cover
– Increased cloud cover leads to a higher albedo
– Higher albedo results in less insolation at the
surface
– Reduced insolation at the surface leads to cooling
Deniers
Counter Argument to Change in Solar Output
Counter Argument
Counter Argument
Counter Argument
Counter Argument
Counter Argument to Warming Due to Waste Heat
More Information
• New Scientist
– Climate Change: A Guide for the Perplexed
– http://www.newscientist.com/article/dn11462climate-change-a-guide-for-the-perplexed.html
• American Meteorological Society
– http://www.ametsoc.org/policy/2007climatechang
e.pdf
• IPCC
– http://www.ipcc.ch/
READING ASSIGNMENT
• Wed, Mar. 15th & 29th
1. Pielke and Pielke; Chapter 6, pp. 139-147; 150-153.
(Responding to warnings...getting the message out...what
happens when people are skeptical)
2. Simmons and Sutter: Chapter 4 (Responding to
warnings...getting the message out...what happens when
people are skeptical)
3. Williams : Scientist Stories. For writing assignment; Chapter 9;
Hurricanes
4. Zebrowski, Chapter 3, pp. 53-62; 77-83; 96-101; Chapter 5, pp.
143-145; 157-163 Chapter 8, 229-251 (The interrelationship of
human settlement patterns and natural disaster, how winds
affect the surface of the sea, hurricane-related flood and
wind disasters )