AOSS 321, Winter 2009 Earth System Dynamics Lecture 2 1/13/2009

1. AOSS 321, Winter 2009Earth System DynamicsLecture 21/13/2009 Christiane Jablonowski Eric Hetland cjablono@umich.eduehetland@umich.edu 734-763-6238 734-615-3177

2. Characteristics of the Atmosphere • Composition of the air, greenhouse gases • Temperature, atmospheric layers • Pressure, pressure systems • Wind • General circulation of the atmosphere, seasons, ocean circulation, albedo, heat & heat transport • Humidity, clouds, precipitation • Sun, electromagnetic waves, Earth’s energy balanceLet’s form groups and discuss the elements, e.g. • physical characteristics and units, how measured or observed, graphical representation, characteristics of the global circulation & distribution, typical values

3. Composition of the air 1 • Nitrogen (N2) 78.08% • Oxygen (O2) 20.95% • Argon (Ar) 0.93% • CO2 383 ppm • Neon (Ne) 18.2 ppm • Helium (He) 5.2 ppm • Methane (CH4) 1.7 ppm • … • Water vapor (H2O) (1%-4%)highly variable

4. Greenhouse gases: CO2 • CO2 measurements in Mauna Loa, Hawaii • Well-mixed trace gas (greenhouse gas) in the lower atmosphere • Barren lava field of an active volcano19º32' N, 155º 35' W, 3397 m above mean sea level (MSL)

5. Greenhouse gases: CO2 • Carbon dioxide (CO2) concentrations in ppm (parts per million) in Mauna Loa, Hawaii 380 ppm ‘Keeling’curve Year May 2005

6. Greenhouse gases (GHG) • Earth's most abundant greenhouse gases • water vapor (H2O) • carbon dioxide (CO2) • methane (CH4) • nitrous oxide (NO2), commonly known as "laughing gas" • ozone (O3) • chlorofluorocarbons (CFCs) • Ranked by their contribution to the greenhouse effect, the most important ones are: • water vapor, which contributes 36–70% • carbon dioxide, which contributes 9–26% • methane, which contributes 4–9% • ozone, which contributes 3–7% • What are the atmospheric lifetimes of the GHGs?

7. CO2 2100 460 ppm Global CO2 and T Trends CO2 2005 380 ppm • Carbon dioxide (CO2) is, because of our emissions, much higher than ever experienced by human kind • Temperature is expected to follow • Do we need to be worried? 350,000 years of Surface Temperature and Carbon Dioxide (CO2) at Vostok, Antarctica ice cores

8. Atmospheric Layers 2

9. Temperature • Weather maps: http://www.rap.ucar.edu/weather/ • Station data, Boulder, CO • Lines of equal temperature are called isotherms • Typical plots: • Time series at a fixed location (station) • 1D vertical temperature profiles at a station (radiosonde sounding, skew-T diagrams) • 2D horizontal cross section near the surface or at a specific pressure level, e.g. 500 hPa • 2D meridional-vertical cross section (zonal-mean) • Pay attention to the horizontal and vertical axis labels

10. Temperature: Typical Vertical Profiles • Weather: takes place in the troposphere, the lower 10-15 km of the atmosphere

11. Temperature:Latitude-pressure cross section • Annual mean, zonal-mean temperature T in oC,linear pressure scale (vertical direction) 200 -55 10 km 0 -15 20 1000 SP -60º -30º 30º 60º NP

12. Temperature:Latitude-pressure cross section • Temperature T (annual mean, zonal-mean) in K, logarithmic pressure scale, includes the stratosphere 260 Pressure (hPa) 230 210 100 300 1000 NP 40ºN 40ºS SP Eq

13. Surface Pressure 3 • Surface pressure map (at mean sea level) • Isobars: Contour lines that connect equal pressure values Weather maps: http://www.rap.ucar.edu/weather/ NCAR weather station:http://www.eol.ucar.edu/cgi-bin/weather.cgi?site=fl

14. Mean sea level pressure (MSLP) • Extrapolate the surface pressure to mean sea level • Effects of surface pressure differences due to mountains (orography) are eliminated • Plot of annual-mean MSLP in hPa NP Latitude Eq SP Longitude

15. Mean Sea Level Pressure • Annual-mean zonal-mean MSLP in hPa SP NP Equator

16. Pressure: Units • Conversion of pressure units • 29.92”Hg = 1.0 atm = 101.325 kPa = 1013.25 mb101.325 kPa = 1013.25 hPa • We use the unit: hPa (= 100 Pa)

17. Zonal (W-E) wind 4 • Zonal-mean, annual-mean zonal wind u in m/s • Typical scale: 10 m/s, jet streams over 30 m/s Pressure (hPa) NP Eq SP

18. Jet stream • Narrow band with high wind speeds • Wind direction is mostly from west to east (west wind) • Approximate height: 10-12 km (just under the tropopause around 200 hPa) • Aircraft pay attention to the position of the jet stream, why?

19. Meridional (S-N) wind • Monthly-mean, zonal-mean meridional wind v in m/s are relatively weak (a few m/s) • Compare to instantaneous v (typical scale: 10 m/s) Height (km) Pressure (hPa)

20. Vertical velocity • Vertical velocity (Cartesian system) in m/s w = dz/dt • Vertical pressure velocity in Pa/s  = dp/dt • Typical scale of w in midlatitudinal weather systems is on the order of 1 cm/s • Typical vertical velocity in thunderstorms: on the order of 10 m/s (and more) • Upward motion in low pressure systems • Downward motion in high pressure systems

21. Vertical pressure velocity • Monthly-mean, zonal-mean vertical pressure velocity  = dp/dt in Pa/s Height (km) Pressure (hPa)

22. Wind direction, speed & wind shear • Wind direction is the direction from which the wind is blowing • West wind: blows from west to east • South wind: blows from south to north • Eastward: means a west wind • Easterly: means an east wind • Wind speed: Magnitude of the wind vector • Isotach: lines with equal wind speed • Wind shear: difference between the wind directions and speeds at two different heights (subtract the two wind vectors)

23. Wind direction • Cardinal directions (NESW) and azimuth degrees 0º Azimuth:increasescounterclockwise 315º 45º 90º 270º 225º 135º 180º

24. How to draw wind vectors Length of a wind vector indicates the speed Draw: • Wind vector with speed 10 m/s and wind vector with speed 20 m/s (identical direction) • Westerly wind • West wind • Westward wind • SSE wind • Wind from 30° • Wind shear between a NW wind at 10 km and NE wind at 5 km. Both wind speeds are identical.

25. Wind symbols on weather maps Zoom into some surfacemeasurements (stationreports) Wind barbs • International convention • Indicate wind direction and speed in knots • 1 knot = 0.51 m/s

26. 5 General Circulation of the Atmosphere • Global wind and circulation systems

27. Seasons • Due to tilt of the Earth's axis to its orbital plane • Tilt is about 23.5 degrees

28. Seasons • Solstices and equinoxes

29. Heat transport: Ocean & Atmosphere equally important total Northward heat transport (1015 W) atmosphere ocean Equator 90° N Latitude

30. Ocean-Atmosphere Interaction:Ocean surface currents are wind-driven

31. Surface Currents: Ekman transport

32. Ocean circulation: Oceanic conveyor belt

33. The conveyor belt: Heat engine for Europe

34. Albedo • Defined as:Ratio of diffusely reflected to incident electromagnetic radiation • Most often expressed as a percentage • Also depends on the direction of incoming radiation • Highly variable

35. Earth-Ocean-Atmosphere Interactions Gases Particles

36. Heat and Heat transport • Heat is a form of energy that can change the temperature of an object • Measured in energy units, e.g. Joule (J) which is equivalent to Nm • Mechanisms of heat transfer: • Conduction: molecular transfer of energy • Convection: transfer by fluid motion • Radiation: transfer by electromagnetic waves even through vacuum • Check the difference between conduction and convection:http://www.atmos.washington.edu/~durrand/demos/convection_conduction.htm

37. 6 Humidity, Clouds & Precipitation • Annual mean zonal-mean specific humidity (g/kg): ratio of water vapor (in g) to moist air (in kg) Almost all moisture is in the troposphere

38. Water Cycle

39. Clouds • Convective clouds in Intertropical Convergence Zone (ITCZ) • Cloud bands associated with frontal zones in midlatitudes

40. Clouds • What Are Clouds?Clouds are made up of billions of tiny droplets of ice or water. Each cloud droplet is so small and light that it is held up by air currents. • How Are Clouds Made?Clouds are condensation, formed when warm air rises and is cooled to below a certain temperature, called the 'dew point'. • Why Study Clouds?Clouds provide one of the keys to understanding the weather. Modeling clouds and their feedbacks is a key challenge when assessing climate change.

41. Clouds & Precipitation • Cloud classifications according to their vertical position: • Low • Mid-level • High clouds (ice clouds: cirrus) • Cloud photos:http://australiasevereweather.com/photography/ • UCAR’s digital image library:http://www.fin.ucar.edu/ucardil/ • Fun facts: Earth’s annual mean globally-averaged precipitation: Rainiest spot on Earth:

42. Clouds • Clouds influence the planetary albedo • Cloud classification depends primarily on the cloud altitude

43. Vertical distribution of clouds ice clouds water clouds

44. Role of low clouds • Stratus clouds, which are mostly composed of liquid water droplets, reflect most of the incoming shortwave radiation (thin lines), but re-emit large amounts of outgoing longwave radiation (thick lines). • Their overall effect is to cool the Earth.

45. Role of ice clouds • Cirrus clouds, which are mostly composed of ice crystals, transmit most of the incoming shortwave radiation (thin lines), but trap some of the outgoing longwave radiation (thick lines). • Their overall effect is to warm the Earth.

46. Precipitation • Annual-mean precipitation in mm/day NP Eq Latitude SP Longitude

47. The Sun 7 • Wikipedia:http://en.wikipedia.org/wiki/Solar_constant • Solar constant: amount of incoming solar radiation per unit area, measured on the outer surface of Earth’s atmosphere, in a plane perpendicular to the rays. • The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly S0 = 1366 W/m2 • Annually averaged radiant flux received on Earth per unit area: S0/4 = 342 W/m2

48. Radiation Spectrum visible UV (ultraviolet)

49. Radiated Energy: Black-Body • Planck curves (radiative flux)

50. Earth’s energy balance Balance requires: Incoming energy = Outgoing energy