Global Warming, Understanding the Forecast

1. Module Six Global Warming, Understanding the Forecast Chapter 5, What Holds the Atmosphere Up?

2. How the greenhouse effect works within the temperature structure of Earth’s atmosphere • The greenhouse effect is powered by the lapse rate • Atmospheric scientists call the change in temperature of the air with altitude the lapse rate • It is about 6°C colder per kilometer of altitude • The lower part of the atmosphere is called the troposphere

3. Atmosphere • The troposphere is the lower part of the atmosphere • It contains about 90% of the air • It contains all of the weather • The boundary of low temperature is about 17 km high on average • The boundary where the air temperature reaches its coldest point is the tropopause • Commercial airplanes fly in the tropopause

4. Atmosphere with altitude

5. Atmospheric layers • Troposphere – about 10 km high, contains 90% of air and all of the weather • Tropopause – boundary where air is the coldest, commercial aircraft area • Stratosphere – air begins to warm up because of ozone content • Mesosphere – not much effect on the weather • Exosphere – ditto

6. No temperature contrast, no greenhouse effect • Remember the layer model with a skin temperature • Think of the skin altitude for the air column as some kind of average altitude from which the IR escapes to space • The idea of a skin layer in the atmosphere is fuzzier than using a glass pane in the layer model but it still a useful concept

7. Lapse rate vs. strength of GH effect • If we increase the GHG concentration of the atmosphere, the IR radiation to space will originate from a higher altitude (skin altitude). • The increase in skin altitude increases the ground temperature. • If the temperature of the atmosphere was the same at all altitudes, then raising the skin temperature would have no impact on the ground temperature. More CO2 → higher skin altitude → warmer ground

8. Pressure as a function of altitude • The pressure in the atmosphere depends primarily on the weight of the air over your head • The weight of the overhead air at sea level is more than • The weight of the overhead air at the top of a mountain • The pressure of the air is non-linear with altitude (unlike scuba diving, where the pressure is linear with depth)

9. Pressure is a non-linear (exponential) function

10. What to remember… • When a gas is depressurized (less pressure) the gas expands • When a gas expands, it cools • When you pressurize a gas it heats up

11. Expansion, Compression and Heat • If we had a gas inside a container with a piston, and pressurized the gas, it would heat up, even in an insulated container with no heat entering or leaving • A closed system with no heat coming in or out is called adiabatic • If gas is compressed adiabatically, it warms up. It takes work to compress a gas, the work energy is transferred to heat • When it expands, it cools, reversing the process and the gas cools down

12. Water vapor and latent heat • Remember chemistry and the phase change diagram, where energy is added, the substance stayed at the same temperature until it completely changed phase, solid to liquid, or liquid to gas. • The energy that was added is called latent heat • Latent heat of fusion between solid and liquid • Latent heat of vaporization between liquid and gas In one direction the heat is added, in the other direction the heat is released.

13. Phase changes Solid + heat → liquid (latent heat of fusion) melting Liquid + heat → gas (latent heat of vaporization) boiling When the phase change goes in the other direction, the same amount of energy is released during condensation or freezing Vapor → liquid + heat released Liquid → solid + heat released

14. Latent heat • You charge up an air parcel with latent heat when you evaporate water into it (vapor contains the latent heat-not sensible heat) • You get the heat back when the water condenses and the latent heat is released • A thermometer does not measure latent heat • A thermometer measures sensible heat (what you can sense)

15. Equilibrium conditions When water is in equilibrium between liquid and vapor, it’s called saturated, or 100% relative humidity, and the equilibrium vapor pressure of water will be high. • Undersaturated occurs when it is cold, the amount of water vapor is lower than the equilibrium value • Supersaturated occurs when vapor pressure is higher than equilibrium, and the vapor tends to condense into precipitation

16. Convection Convection occurs when you heat a fluid from below or cool it from above (either a liquid or a gas) • Fluid expands as temperature increases, density decreases • Unstable condition causes the fluid column to turn over • Warm fluid rises to the top • The Atmosphere tends to mix when it convects

17. Air is compressible • The air is not all the same temperature • Pressure is higher at the bottom because o f the weight of the air column • Compressed air at the bottom heats up • Because the air is well mixed, the moving air will always find itself at the same temperature as the rest of the air in the column • This is what static stability looks like in a column of compressible air – the same temperature as the rest of the column

18. Convection in the atmosphere • Driven by sunlight hitting the ground • Warms the air at the bottom of the column • Warm air begins to rise, as it rises, it expands, and cools • While ascending, it remains lighter and warmer than the air around it • If it does not mix on the way up, the air can get all the way to the top of the column • If it mixes on the way up, the whole column warms up uniformly

19. Moist Convection The latent heat in water vapor drives most of the drama in our weather

20. Moist convection • Air at the surface of the Earth with a relative humidity of 100% rises due to convection • As the temperature drops, the equilibrium amount of water vapor decreases • Supersaturation drives water to condense into droplets or ice • The story of cloud formation will continue in chapter 7

21. Water vapor • It changes the temperature of the air • It systematically changes the lapse rate • Dry convection has a lapse rate of about 10°C temperature change per km of altitude • Add the latent heat in moist convection, the lapse rate decreases to about 6°C per km • It is possible that the lapse rate of the atmosphere could be different in a changing climate

22. Take home points, Chapter 5 • Air in the upper troposphere is colder than air at the ground because of the process of moist convection. The process includes the following: • Convection is driven by sunlight heating the air near the ground • The air rises and cools because it expands • Water vapor condenses, releasing heat as the air rises

23. Continued… • The moist convecting air gets colder with altitude, but not as much as if it were dry • If the air did not get colder with altitude at all, there would be no greenhouse effect

24. Revisit the layers of the atmosphere • Troposphere • Stratosphere • Mesosphere Entering outer space: • Ionosphere • Exosphere

25. Global Warming, Understanding the Forecast Chapter 6, Weather and Climate How the Weather Affects the Climate

26. Chaos • 10 days is the limit for predicting weather because weather is chaotic – an extreme sensitivity to initial conditions, so that small differences between two states tend to amplify, and the states diverge from each other • The butterfly effect, a puff of air from a butterfly’s wing eventually resulting in a giant storm somewhere that would not have happened if the butterfly had never existed

27. Butterfly effect • First observed in a weather simulation model • The model stopped running • Edward Lorenz restarted it by typing in the variables like temperature and wind speed • He had small, insignificant changes, such as rounding errors • The model diverged completely from the results of the initial simulation

28. Edward Norton Lorenz • Mathematician • Edward Norton Lorenz was an American mathematician and meteorologist, and a pioneer of chaos theory. He discovered the strange attractor notion and coined the term butterfly effect. Wikipedia • Born: May 23, 1917, West Hartford, CT • Died: April 16, 2008, Cambridge, MA • Books: The essence of chaos • Education: Massachusetts Institute of Technology, Dartmouth College, Harvard University

29. Weather • Forecasts rely on computer models • Small imperfections in the initial conditions and the model cause the model weather to diverge from the real weather • By about 10 days the prediction is worthless • To overcome the error, run the model may times with tiny variation in initial conditions • “an ensemble of model runs”

30. Climate • Defined as some time average of the weather • Climatological January (or any other month) would be the average of many Januaries • The weather is chaotic, but the climate generally is not • The weather would predict rain on a particular day, whereas the climatologist may predict a rainy season

31. Averaging Layer Model Real World • Warm and cold • Summers and winters • Day and night • Completely balanced energy budget • Averaging is valid • Some places much hotter • Some place much colder • Radiative energy budget at some place could be wildly out of balance • Will averaging change the answer to something unreasonable?

32. Averaging a non-linear system Top panel – averaging radiative energy flux (S-B equation) over a large temperature range introduces a large bias. Bottom panel – over the temperature range of normal Earth conditions, the blackbody radiation energy flux is closer to linear, so averaging over a small range would be less of a problem

33. The Fluctuating Heat Budget • Not stable like a model, but fluctuates widely • Solar energy comes in only during the day • Sunshine varies seasonally and by location • Infrared is radiated day and night • The energy budget is in balance over a 24 hour period • But at any time in any spot on the planet, the energy is usually out of balance

34. Seasonal variations The seasons are caused by the tilt of the Earth relative to its orbit around the sun, the obliquity • Wintertime, days are shorter and the sun is lower • Added over a day – the winter hemisphere has less sunlight

35. Seasons are NOT caused by the Earths distance from the sun • The eccentricity cycle refers to the shape of the Earth’s orbit around the sun • It varies from elliptical, to circular • Currently we are in a near circular orbit • The Earth is actually closer to the sun in January than it is in July • Seasons are not caused by proximity to the sun

36. Earth’s seasons are caused by the tilt of the poles relative to the orbit, and not by its distance to the Sun

37. Incoming flux depends on latitude and day of the year Northern hemisphere summer is in the middle of the plot, which shows flux as a function of latitude and time of the year.

38. Interesting to note from the plot • Highest daily fluxes are at the poles during the summer • Poles get six months of sunlight • Sun whirls around in a circle above the horizon (not overhead) • Why isn’t it a tropical garden in the summer?

39. Thermal Inertia • Damps out the temperature swing between day and night • Damps out the temperature swing as the seasons change • Even damps out the temperature change of global warming

40. Oceans • Has a tremendous capacity to absorb and release heat from the atmosphere • Land – not so much – diffusion through the soil is slow and only affects the first meter or two • Cool water surface turns over and has convective mixing to about 100 meters • Maritime areas have milder seasons • Middle of large continents have more intense seasonal cycles

41. Averaging a seasonal cycle • Out of balance because of the heat distribution from the water and from the wind • The outgoing heat in the tropics can’t keep up with the incoming solar radiation • The heat is carried to cooler, higher latitudes by water and winds • The Earth can vent the excess heat to space from the higher latitudes

42. Heat carried to higher latitudes for venting to space

43. The Coriolis Acceleration • http://www.youtube.com/watch?v=i2mec3vgeaI • http://www.youtube.com/watch?v=aeY9tY9vKgs • http://www.youtube.com/watch?v=iqpV1236_Q0 Two clips on the Coriolis Effect and one shows a Foucault Pendulum, demonstrating the rotation of the Earth.

44. Coriolis Effect • The water and the air feel the most effect at the poles (incredibly high tides in higher latitudes, nearly no tide difference at the equator) • At the equator there is no apparent rotation • The middle latitudes fall somewhere between these two extremes

45. Modeling the Weather • Fluids are governed by Newton’s Laws of Motionbecause fluid has mass and inertia • Inertia is the sluggishness of matter to resist changes in motion • Tendency to keep moving if it’s moving, or remain stationary if it is already stationary • To change speed or direction, motion requires a force such as gravity or a change in pressure (weather)

46. Bathtub vs. Earth • Bathtub flows more quickly than the Earth rotates, so does not feel the Earth’s rotation • Flows in the atmosphere and ocean persist long enough to feel the effect of a rotating Earth • Ocean flows can be driven by friction with the wind • Coriolis acceleration tries to deflect the flow to the right in the northern hemisphere • After a few rotations, a steady state is reached where the fluid flows 90 degrees to the wind

47. The eventual steady state Top – the fluid initially flows in the direction of the wind. Middle – after a while the Coriolis force swings the fluid to the right. Eventually, the fluid itself flows 90 degrees to the wind or pressure force, and the Coriolis force just balances the wind or the pressure force. Bottom – the steady state where the flow stops changing and remains steady.

48. Geostrophic Flow In a rotating world the fluid will eventually end up flowing completely crossways to the direction that It’s pushed. This condition is called geostrophic flow. A geostrophic flow balances the forces on it against each other.

49. Geostrophic cells on weather maps • Cells of high pressure and low pressure with flow going around them • Low pressure, pressure force points inward, 90° to the right of that the winds flow counterclockwise in the N. hemisphere – cyclonic direction of flow • High pressure, pressure force points outward, and the flow is clockwise around the high pressure – anticyclonic direction

50. Surface wind field from a climate model (computer generated)