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 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 The air mass takes up characteristics from the surface of the lake. Heating from below unstable. Warming + evaporationgains moisture
Fronts Separate different air masses Cold, warm, occluded fronts
Sfc weather associated with a cold front Pressure T Wind Clouds & precip
Vertical slice through a cold front
Sfc weather with a warm front
Vertical structure of a warm front
Occluded fronts Cold type Warm type