1. Atmospheric Science Miriam Marlier 2/25/2011

2. Outline • Composition • Circulation patterns • Hadley • Effects on latitudinal distribution of vegetation. • El Niño • Monsoons • Atmospheric chemistry and radiation • Measurement

3. Present-Day Composition • Archean atmosphere was strongly reducing. • Great oxidation event: 2.1-2.4 Ga. • O2 maintained by biology, which needs O3 to filter out shortwave radiation.

4. Increase in temperature due to ozone  Where weather happens 

5. What Causes this Pattern?

6. Tropics • Incoming solar radiation peaks in the tropics, which warms the surface and drives atmospheric circulation. • Location of maximum heating varies seasonally.

7. Instability Drives Convection • Warm air is underlying cold air = instability. • Adiabatic lapse rate (no heat exchange) defines the temperature decrease with height. • Γd=-dT/dz=9.8 K/km • From the first law of thermodynamics. • Convection occurs when an air parcel is unstable compared to the environment. Happens ~1 hour.

8. What About Water? • A rising air parcel will cool and then water condenses and releases latent heat. • This offsets adiabatic cooling, so the lapse rate is less. • Γm=5 K/km

10. What happens outside of the tropics? • Air cools at it travels aloft towards subtropics, sinks over broad region in subtropics (~30°). • Poleward of 30° in radiative-dynamic equilibrium. • The temperature gradient between tropics and poles is at a maximum in mid-latitudes. • But since this is happening over a large scale, the Coriolis force must be considered.

11. Hydrostatic Equilibrium • Pressure changes with height based on density differences, which depends on temperature. • Warmer air will have surfaces further apart. • dP/dz=-ρg • Important implications for atmospheric flow because induces horizontal pressure gradients.

12. Geostrophic Wind • Wind that occurs because the pressure gradient force balances Coriolis force. • Rossby number defines scale at which Coriolis force is important.

13. Thermal Wind + Jet Stream • Vertical shear of the geostrophic wind. • Jet stream redistributes heat at mid-latitudes. • Brings warm air poleward and cold air equatorward. • Important for the mid-latitudes.

14. Precipitation where air is rising. • Evaporation where air is sinking. • This influences broad vegetation patterns…

15. What Does This Mean for the Biosphere?

16. What Does This Mean for the Biosphere? Photosynthesis: CO2 + H20 + light  carbohydrates + O2 • Keep in mind that there are other influences on precipitation + temperature beyond the general circulation. Land surfaces are especially important. • But lapse rates, moisture sources, and condensation can also explain patterns found at high altitudes or the interior of continents. • Elevation, soils, disturbance, and nutrient availability are locally important.

17. What about Longitudinal Circulation?

18. El Nino-Southern Oscillation • Deep convection in W. Pacific during normal conditions sets up westerly flow aloft and easterly flow at the surface. • Easterly surface winds  easterly surface ocean currents  water piles up in W. Pacific  upwelling in E. Pacific to conserve mass. • La Nina is an enhanced normal state, El Nino when it weakens. • Oscillation because two regions are out of phase. • Important for changes in upwelling and teleconnections around the world.

19. Monsoons • Seasonal reversal of winds. • Difference in thermal capacity of land vs. ocean.

20. Atmospheric Radiation • Atmosphere is relatively transparent to solar radiation, but not to outgoing wavelengths. Energy is proportional to T^4 (Stefan-Boltzmann). • Absorption is especially important if within a window. • Particles must be charged in order to interact with EM radiation. • Scattering depends on size and wavelength. 6000K 255K Cooling! Warming!

21. More on Clouds • Must consider height, reflectivity, and absorption to understand climate impacts. • If absorb surface radiation at high altitude=warming effect. • If primarily reflect solar radiation=cooling effect. • Water droplets form with aerosols because more energetically favorable.

22. How are we Changing Radiation?

23. What Might Change in the Future? • Water vapor has an exponential relationship with temperature (Clausius-Clapeyron). Most important GHG! • Aerosols act as cloud condensation nuclei, which alters radiative and hydrological properties of clouds. • Circulation might change if we alter atmospheric stability.

24. How Do We Measure This? • Remote sensing from space must operate between atmospheric windows. • Passive sensors record radiation coming from the surface (reflected or emitted). • Active sensors emit their own radiation. • Extremely important over last few decades.

25. Remote Sensing • Atmospheric properties: clouds, precipitation, temperature. • Land surface properties: based on reflectance, includes geology, vegetation, water.