Midterm I Review

1. Midterm I Review

2. The most common atmospheric circulation structure H L Cooling or No Heating Heating H L • Imbalance of heating • Imbalance of temperature • Imbalance of pressure •  Wind

3. Topics we have discussed • Overviews I: Extreme weather and climate • Overview II: Success and failure of weather and climate prediction • Overview III: Why is it so difficult to predict weather and climate? • Evolution of the atmosphere • The incoming solar energy • What cause the four seasons? • What is the greenhouse effect? • Vertical structure of the atmosphere • What set the atmosphere in motion? • How does air move around the globe?

4. The mission of the atmospheric sciences is to understand and predict weather, climate, and related disasters

5. Overview I: Extreme weather and climate • Atmosphere: A mixture of gas molecules, microscopically small particles of solid and liquid, and falling precipitation • Meteorology: The study of the atmosphere and the processes that form weather • Weather: The state of the atmosphere at a given time and place • Climate: The statistical properties of the atmosphere. (i.e. averages and variability) • Weather- and climate-related disasters: tropical cyclones, tornados, floods, droughts, winter storms, extreme heat, extreme cold, lightning, El Nino, global warming • Impacts of weather/climate on agriculture, business, international relationships, history, science, philosophy, public health, psychology, social work, education, …

6. Weather- and Climate-related Disasters

7. Weather/climate and Psychology, Public Health, Economy, Transportation, Education, …

8. Overview II: Success and failure of weather and climate prediction • The modern climatology (meteorology) was born in the 1940s (a very young science!), but has been growing very fast! Now we have a global observational network with many satellites, ships, radars and surface stations, as well as very comprehensive prediction models running on the world’s fastest supercomputers. • The current status of weather and climate predictions: (1) weather prediction good to 10 days, (2) tropical cyclone prediction good in track but not in intensity, (3) climate prediction good to two seasons, (4) climate change projections have a 3-fold difference in magnitude.

9. Observing the atmosphere

10. World’s major climate modeling centers

11. Overview III: Why is it so difficult to predict weather and climate? • The main reasons of the difficulties: (1) Teleconnection problem, (2) Feedback problem, and (3) Subgrid-scale problem, (4) Limitation of concept/theory/model.

12. Problem I: Different parts of the world are strongly connected to each other (The “Teleconnection Problem”) Global atmospheric flow

13. Factors affecting US weather and climate Arctic N. Atlantic Atlantic/ Sahel Madden-Julian Oscillation El Nino Amazon

14. Any location is affected by all the other locations, and in turn is affecting all the other locations

15. Problem II: Different components of the earth system (atmosphere, land, ocean, ice, clouds, etc) are strongly interacting with each other (The “Feedback Problem”)

16. Problem III: The global climate models divide the earth into many small pixels (called grids), but the earth system composes of both very big objects (such as the whole Pacific Ocean) and very small objects (such as the cloud droplets), making it very difficult to draw them on the same page (The “Subgrid-Scale Problem”)

17. Evolution of the atmosphere • The standard units of measurements (SI) • Earth’s three atmospheres: 1st: 4.6 billion years ago, H, He Transition: formation of magnetic field, volcano activities 2nd: 4 billion years ago, CO2, H2O, N2 Transition: emergence of life, formation of ocean 3rd: 400 million years ago, O2 Important event: formation of seven continents • What is the residence time? What is the difference between the permanent and variable gases? Name 3 of each. What are the most and second most abundant gases? • Given that variable gases are so rare, why are they considered at all? How are CO2 and O3 changing? • Earth’s climate history: ice ages (at least 5 have occurred so far. We’re in an ice age!), 100,000-year cycle, little ice age (1350-1850AD)

18. Standard units of measurementSI (System International) Quantity Name Units Symbol Length meter m m Mass kilogram kg kg Time second s s Temperature Kelvin K K Density kilogram kg/m3 kg/m3 per cubic meter Speed meter per m/s m/s second Force newton m.kg/s2 N Pressure pascal N/m2 Pa Energy joule N.m J Power watt J/s W

19. Evolution of the Sun and the Earth The Earth was born 4.6 billion years ago.

20. Permanent gases and variable gases • Residence time: The amount of time a gas is in the atmosphere • The permanent gases: gases having long residence times (N2=42,000,000 y, O2=5,000 y), 99.999% of total atmosphere mass • The variable gases: Gases generally having shorter residence times (H2O=10 days, CO2=150 y).

21. IPCC (2001) Importance of the Variable Gases • CO2 and water vapor are the major greenhouse gases • Water can exist in all three states on Earth. Global water cycle is the process of water being cycled from the planet to the atmosphere and back again. • O3 protects us against harmful ultraviolet radiation Change of CO2 Montreal Protocol in 1987 to ban freons Change of O3

22. The incoming solar energy • What is energy? 3 methods of energy transfer • The names of the 6 wavelength categories in the electromagnetic radiation spectrum • Intensity of radiation (Stefan-Boltzman law): I=T4 • Wavelength of radiation (Wein’s law): max = b/T • The wavelength range of Sun (shortwave) and Earth (longwave) radition • The 11-year solar cycle

23. Methods of Energy Transfer • Conduction • Molecule to molecule transfer • Heat flow: warm to cold • e.g. leather seats in a car • Convection • transferred by vertical movement • physical mixing • e.g. boiling water • Radiation • propagated without medium (i.e. vacuum) • solar radiation provides nearly all energy • The rest of this chapter deals with radiation

24. The Electromagnetic Spectrum Sun = “shortwave” (0.4-0.7 μm) Peak 0.5 μm (green) The limitations of the human eye! Earth = “longwave” (4-100 μm) Peak 10 μm (infrared)

25. What cause the four seasons? • The two basic motions of the Earth • What causes the four seasons: the Earth’s tilt and the 3 ways it affects the solar insolation (change of length of the day, beam spreading, beam depletion) • Change of the Earth’s orbit at longer time scales (Milankovitch cycles): eccentricity, axial tilt, and precession

26. Length of Daylight period • Angle at which sunlight hits the surface (“Beam Spreading”) • Thickness of atmosphere through which sunlight must travel (“Beam Depletion) The Earth’s two basic motions: revolution with a period of 1 year, and rotation with a period of 1 day.The change of seasons is caused by the Earth’s 23.5o tilt from the line perpendicular to its orbit plane (toward the sun during summer), which affects the receipt of solar insolation in three ways:

27. What is the greenhouse effect? • Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy is absorbed by the surface? • Atmospheric influences on radiation (3 ways) • The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange sunsets? • What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? • Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions)

28. Earth’s energy budget (averaged over the whole globe and over a long time Yellow: shortwave Red: longwave • At the top of the atmosphere (3-way balance): Incoming shortwave = Reflected Shortwave+ Emitted longwave • At the surface (5-way balance): Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming) + Latent heat flux + sensible heat flux Net Longwave 21% Sensible heat 7% Latent heat 23%

29. Atmospheric absorption - The Greenhouse Effect Transparent to solar (shortwave) radiation Opaque to earth’s (longwave) radiation Major GH gases: CO2, H20(v), CH4 The greenhouse effect helps to keep the earth surface at a comfortable temperature. But when it’s too strong, the temperature becomes too warm.

30. The importance of methane (CH4) • 23 times more powerful as a greenhouse gas than CO2 • The livestock sector is a major player, which accounts for 35-40% global anthropogenic emissions of methane (their burps!) • The livestock sector is responsible for 18% of total greenhouse gas emissions, which is higher than transportation (cars, airplanes, etc) • Therefore, consuming less meat is more efficient in reducing global warming than not driving cars.

31. Vertical structure of the atmosphere • Thickness of the atmosphere: less than 2% of Earth’s thickness • Definition of temperature. 3 units. • Definition of pressure and its unit. • Definition of pressure gradient. Pressure gradient sets the air in motion. • Equation of state (P=ρTR) • Vertical Pressure Distribution. How does pressure change with height? What is the hydrostatic equilibrium?

32. Temperature, pressure, winds • Temperature – measure of average kinetic energy (motion) of individual molecules in matter. 3 units: Kelvin (K), Celsius (C), Fahrenheit (F) • Pressure – force exerted/unit area (weight above you). units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa) • Pressure gradient – pressure difference between two locations divided by the distance between those two locations • Winds • Zonal winds (east-west): Eastward is called westerly • Meridional winds (north-south): Northward is called southerly

33. Temperature Layers The names of the 4 layers What separate them? The approximate height of tropopause, stratopause and mesopause

34. Vertical pressure distribution: Hydrostatic equilibrium • Pressure decreases with height Becausedownward gravity force is balanced by vertical pressure gradient force (called hydrostatic equilibrium) Δp/Δz = ρg • Pressure decreases non-linearly w/ height (Because air is compressible, so denser near the surface) Δp/Δz ρg

35. What set the atmosphere in motion? • Know 3 Forces that affect wind speed /direction • Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force? • What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds • Troughs, ridges, cyclones and anticyclones. Do they correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them?

36. Forces affecting the horizontal winds • Horizontal pressure gradients responsible for wind generation • Three forces affecting horizontal winds: • Pressure Gradient Force  (PGF) • Coriolis Effect  (CE) • Friction Force (FF) CE: • The Earth’s rotation deflects any moving object to the right of its moving direction in NH (left in SH). Like walking in a turning bus. • CE increases poleward (greatest at the poles, 0 at the equator), and increases with the speed of moving object

37. Geostrophic Balance (Geostrophic flow) • PGF = - CE • When the effects of friction can be neglected (such as in the upper air away from surface roughness), the wind speed/direction is simply a balance between the PGF and CE. • Air motion is deflected by the Coriolis force to be perpendicular to PGF PGF

38. Cyclones, Anticyclones, Troughs and Ridges Upper air: isobars usually not closed off • Troughs (low pressure areas) • Ridges (high pressure areas) Near surface: isobars usually closed off due to surface friction • Cyclones(Low pressure areas) • Anticyclones(High pressure areas)

39. How does air move around the globe? • Three precipitation (heating) belts. Primary high and lows • Three-cell model. Mechanism for each cell • Two characteristics of zonal mean temperature structure • Two characteristics of zonal mean wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams? • Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert. • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?

40. Vertical structure and mechanisms • Polar Cell (thermal): • Driven by heating at 50 degree latitude and cooling at the poles • Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells • Hadley Cell (thermal): Heating in tropics  formssurface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft  descends in the subtropics Polar Hadley

41. Vertical structure of temperature Two characteristics: • Horizontally uniform in the tropics • Steep gradient in the extratropics

42. Vertical structure of zonal wind Two characteristics: • Westerly winds in the extratropical troposphere (caused by the Coriolis force) • Jet streams: local maximum of winds (caused by sharp pressure gradient across the boundary between warm tropical air and cold polar air)

43. Ocean surface currents – horizontal water motions Transfer energy and influence overlying atmosphere Surface currents result from frictional drag caused by wind - Ekman Spiral General circulation of the oceans • Water moves at a 45o angle (right) • in N.H. to prevailing wind direction • Due to influence of Coriolis effect • Greater angle at depth

44. About the midterm • There will be ~50 multiple-choice questions • Please pay special attention to the key points highlighted in red on the review slides (some easy grades) • Sample questions

45. In the SI system, the standard unit of length is: A) yard, B) meter, C) gram, D) pound. 2) Which of the following is NOT a variable gas? A) water vapor. B) nitrogen. C) carbon dioxide. D) ozone. • In the northern hemisphere, when the surface wind blows toward the east, the underlying ocean current flows toward: A) the west. B) the north. C) the southeast. D) the northwest. 4) Anticyclones: A) are associated with low-pressure systems in the northern hemisphere. B) experience Coriolis effects that deflect air to the left in the Northern Hemisphere. C) are associated with supersonic winds. D) are associated with counter-clockwise flow in the southern hemisphere.