Atmospheric Structure and Processes

1. Atmospheric Structure and Processes Spring 2012, Lecture 6

2. Tropospheric Properties • As altitude increases within the troposphere, temperature decreases • Heating is from the ground up • Mountain climbers experience cooling at altitude • At the level of the tropopause, a temperature minimum occurs – about -70º C

3. Stratospheric Properties • Above the tropopause, the temperature begins to climb again • The ozone layer within the stratosphere absorbs ultraviolet (UV) radiation, and reradiates it in the infrared • This produces in-situ heating • Since the UV radiation comes from the sun, heating is strongest at the top of the stratosphere

4. Pressure • Pressure is the force per unit area applied perpendicular to the surface of an object

5. Response to Pressure • Compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure change • Objects may be said to be compressible or incompressible, depending on the degree of volume change they experience per unit of pressure

6. Compression of Water • Water is often said to be incompressible • At a depth of 4 km, with pressures are around 40 megapascals, water has a volume decrease of 1.8% • At 0º C, the compressibility is less than one part in a billion per Pascal • (One atmosphere is 101,000 Pascals)

7. Linear Pressure Response • As the figure shows, this means that water shows a linear response to an increase in pressure

8. Non-Linear Pressure Response • The figure is a graph of the actual change in pressure with increasing altitude, and is clearly non-linear • At an altitude of 8 kilometers, pressure is half as much as at sea-level • This is because the atmosphere is compressible Vertical scale is km

9. Compressible vs. Incompressible • The figure shows a response to pressure by a compressible substance (air), and an incompressible substance, water • There is more air per meter at low altitude than at higher altitude • The amount of water per meter does not depend on the depth to a significant extent

10. Exponential Function • The change in pressure with altitude is an example of an exponential function • Q = ekx, where: • Q = quantity in question • k is a constant, which may be positive or negative • x is a variable • e is an irrational number equal to 2.718281828….

11. Exponential Change • Exponential change can be positive, like ex in the diagram • Population growth is an example • Exponential change can be negative, often called decay, like e-x in the diagram • Radioactive decay is an example

12. Change of Pressure with Altitude • Pressure clearly decays (grows smaller) with altitude • We can calculate the change in pressure as follows • P(z) = 1 atm • e-z[km]/8 km • z is the height above the ground, measured in kms

13. Temperature • Temperature is related to the average kinetic energy of the molecules in the volume under consideration • The faster molecules move, the higher the temperature • It does not matter how many molecules there are per unit volume

14. Heat Content (Enthalpy) • The heat content is equal to the energy required to create a system, plus the energy required to displace the surroundings, creating room for the system • If a gas is compressed, it warms up – we did work on the system to compress it, which added energy • If a gas expands, it cools down – the gas expanded, doing work on the universe

15. Adiabatic Change • Adiabatic change refers to change with no change in heat content • Adiabatic expansion – a gas occupies a bigger volume, but the molecules move slower • Adiabatic compression - a gas occupies a smaller volume, but the molecules move faster

16. Lapse Rate • As gas rises in the atmosphere, it expands, because pressure is less • If conditions are adiabatic, the gas will behave as shown in the diagram, depending on how much water it holds

17. Lapse Rate Definition • The lapse rate is defined as the change with height of an atmospheric variable • The variable is usually temperature • The adiabatic lapse rate is the change with constant heat content

18. Phase Changes • Substances, such as water, can exist in any of three phases • Gas (Water vapor) • Liquid • Solid (Water ice) • A change in phase involves heat • Water vapor → Water + heat • Ice + heat → Water

19. Latent Heat • If you stick your hand in an oven at 100º C for a short time, you will not be burned • If steam from a kettle contacts your hand, you probably will be • Steam has extra energy, called latent heat • When the steam hits your hand, some of it condenses, transferring energy to your hand, and burns you

20. Vapor Pressure • Water molecules in the air contribute to the total pressure within a system • The pressure is known as the vapor pressure • Vapor pressure is primarily a function of the temperature • The higher the temperature, the higher the vapor pressure

21. Saturation • At any given temperature, air can hold a certain amount of water vapor at equilibrium • Equilibrium means if one water molecule evaporates, another will condense • If the water vapor content is below the equilibrium value, the air is undersaturated – water will tend to evaporate • If it is above the equilibrium value, it is supersaturated – water will tend to condense

22. Humidity • Relative humidity is the water vapor pressure divided by the saturation pressure • As relative humidity increases, it is harder to evaporate water – sweating as a means of cooling becomes less and less efficient • Absolute humidity is the amount of water the air holds, per unit volume • Usually expressed as grams per m3

23. Convection • Convection is a movement of molecules within a fluid, either liquid or gas • It is sometimes used to mean the heat transfer produced by such motion • As such, it is a third means of heat transfer, along with radiation and conduction

24. Producing Convection • Convection may occur when a fluid is heated from below, which causes the bottom fluid to expand, becoming less dense, and thus rising • Or it may be produced by cooling from above, which causes the top fluid to contract, becoming more dense, and thus falling • Convection is a common process in thunderstorms and hurricanes

25. Convection Diagram - 1 • In A, a fluid has a uniform temperature, and is well-mixed • In this situation, the fluid is stable • In B, the fluid is heated from below, increasing the temperature and decreasing the density • The fluid is now convectively unstable

26. Convection Diagram - 2 • If the fluid consists of two immiscible components, the heated portion will rise to the top, float until it cools, and then sink – the principle of a lava lamp, as shown in C • If the fluid is a single component, it will mix, and the entire fluid will become warmer, as shown in D

27. Lava Lamps Slow heating Rapid heating

28. Convection in Compressible Fluids • Figure a represents a stable situation in the troposphere, with temperature decreasing with altitude • Figure b shows heating from below – the heated air is less dense, so it rises, but along its own adiabat – it can rise to the top of the gas column if mixing does not occur • If mixing occurs, the temperature profile of the whole column is increased

29. Dry vs. Wet Adiabats • If air with relative humidity = 100% rises in the atmosphere, it will expand and cool • Cool air holds less moisture, so the water vapor will start to condense to form droplets • Condensing water releases latent heat, helping to offset the cooling due to expansion • This accounts for the dry and wet adiabats in the diagram

30. Radiative vs. Convective Equilibrium • In the layer model we examined, there is no convection, only blackbody radiation • In reality, convection is important • The radiative equilibrium lapse rate is about 16K/km • The convective lapse rate for a dry adiabat is around 10K/km, and for a wet adiabat around 6K/km • This is called radiative-convective equilibrium

31. Radiation Altitude • Some IR radiation goes directly into space, through IR windows • Other IR wavelengths are absorbed and reradiated from the coldest part of the atmosphere, the tropopause • We can imagine an equilibrium altitude that averages the different wavelengths, and this was the skin altitude encountered earlier • Skin temperature is commonly defined as the temperature of the interface between the earth's surface and its atmosphere

32. Increasing Skin Altitude • As GHG concentration goes up, more radiation is trapped, and more radiation to space comes from the tropopause • This raises the skin altitude, which we can denote as zskin

33. Calculating Ground Temperature • We can calculate the worldwide average ground temperature if we know the skin temperature altitude and the lapse rate • If the lapse rate is 6K/km, and the skin altitude is 5 km, the calculation is as follows: • Tground = Tskin + 6K/km • 5 km , or Tground = Tskin + 30K

34. Changing Skin Altitude • If GHG concentration goes up, so does skin altitude • This shifts the point at which the moist adiabat intercepts the ground to a higher temperature • Thus, greenhouse warming

35. Changing Skin Altitude • We can rewrite the equation for changing ground temperature with changing skin altitude, as follows: • ΔT= Δzskin • 6K/km , where • ΔT is the change in temperature • Δzskin is the change in skin altitude

36. Incompressible Atmosphere • If the atmosphere were incompressible, convection would keep the temperature equal at all altitudes, thus making the lapse rate zero • ΔT= Δzskin • 0K/km = 0 • There would be no greenhouse effect

37. Ground Temperature Sensitivity • The lapse rate determines the sensitivity of the ground temperature to increasing GHG concentration • Thus, this is a critical parameter for model calculations