Chapter 5: Atmospheric Structure and Energy Balance

1. Chapter 5: Atmospheric Structure and Energy Balance

2. (I) Characteristics of the Atmosphere • Thickness, air pressure, density • Air pressure and density decrease with altitude • 90% of its mass (5.1 x 1018 kg) is within 16 km (10 mi) of the surface (about 0.0025 times the radius of the Earth) 97% of air in first 29 km or 18 mi; 99% 32 km (18 mi); 99.9% 47km (30mi) • Atmospheric motions can therefore be considered to occur “at” the Earth’s surface • The greatest and most important variations in its composition involve water in its various phases • Water vapor • Clouds of liquid water • Clouds of ice crystals • Rain, snow and hail

3. TRACE GASES Argon (.93%) and Carbon Dioxide (.03%) Ozone (.000004%) Composition of the Atmosphere Dry Air Solid particles (dust, sea salt, pollution) also exist Water vapor is constantly being added and subtracted from the atmosphere, and varies from near 0% (deserts) to 3-4% (warm, tropical oceans and jungles)

4. Vertical Structure of the Atmosphere 4 distinct layers determined by the change of temperature with height

5. Extends to 10 km in the extratropics, 16 km in the tropics • Contains 80-90% of the atmospheric mass, and 50% is • contained in the lowest 5 km (3.5 miles) • It is defined as a layer of temperature decrease • The total temperature change with altitude is about 72°C • (130°F), or 6.5°C per km (lapse rate) • It is the region where all weather occurs, and it is kept • well stirred by rising and descending air currents • The transition region of no temperature change is the • “tropopause”: it marks the beginning of the next layer

6. Vertical Structure of the Atmosphere 4 distinct layers determined by the change of temperature with height

7. Extends to about 50 km • It is defined as a layer of temperature increase and • is stable with very little vertical air motion – a good place to fly! • Why does temperature increase? • The major heating is the UV of sunlight absorbed by O3.. When the sunlight • travel down, the UV will become less and less available, so the temperature • increase with height… • The transition region to the next layer is the “stratopause”

8. Temperature Atm. vertical structure • Air pressure p at sea level is 1 atm. = 1.013 bar = 1013 mb • p decr. with altitude by factor of 10 every 16 km. • T decr. with altitude in troposphere, rises in stratosphere drops in mesosph. rises in thermosph.

9. (II) Radiation Energy Objectives: Electromagnetic (EM) radiation & spectrum Energy flux Blackbody radiation -- Wien’s Law & Stefan-Boltzmann Law Planetary energy balance UNBC

10. EM radiation wavelength later t • EM radiation includes visible light, ultraviolet, infrared, microwaves. • wavelength  • period T, frequency  = 1/T • wave speed or phase speed c = /T =  • Speed of light in vacuum: c = 3.00108 m/s UNBC

11. c = /T =>  = cT = c/  longer period waves => ? wavelength longer UNBC

12. Energy flux • Power = energy per unit time (watt W = J/s) • Flux F = power per unit area (W/m2) less flux high lat. => less F UNBC

13. EM spectrum • EM radiation classified by their wavelength or freq. UNBC

14. Inverse-square law S • Solar flux S falls off as e.g. if r = 2r0 => S = S0/4 UNBC

15. Blackbody radiation • Absolute temperature in degrees Kelvin (K) • 0 K = -273°C (coldest possible T) • All bodies emit EM radiation • e.g. humans emit mainly infrared (IR) • “Blackbody” emits (or absorbs) EM rad. with 100% efficiency. UNBC

16. Wien’s Law Planck function (blackbody rad. curve) Rad. flux max wavelength max = const./T Temp. T in K const. = 2898 m max refers to the Wavelength of energy radiated with greatest intensity. UNBC

17. Blackbody rad. curves for Sun & Earth max = const./T Temp. T in K const. = 2898 m UNBC

18. Stefan-Boltzmann Law Planck function (blackbody rad. curve) Rad. flux wavelength F = T4  = 5.67 x 10-8 W/m2/K4 total F = area under curve UNBC

19. Planetary energy balance • Earth is at steady state: Energy emitted by Earth = Energy absorbed ..(1) • E emitted = (area of Earth)   Te4 = 4 Re2   Te4 (Te= Earth’s effective rad. temp., Re= Earth’s radius) • E absorbed = E intercepted - E reflected • Solar E intercepted = S Re2 (solar flux S) • Solar E reflected = AS Re2 (albedo A) • E absorbed = (1-A) S Re2 • (1) => 4 Re2   Te4 = (1-A) S Re2 UNBC

20. Magnitude of greenhouse effect •  Te4 = (1-A) S/4 • Te = [(1-A) S/(4  )]1/4 (i.e. fourth root) • Te = 255K = -18°C, very cold! • Observ. mean surf. temp. Ts = 288K = 15°C • Earth’s atm. acts as greenhouse, trapping outgoing rad. • Ts - Te = Tg, the greenhouse effect • Tg = 33°C UNBC

21. Greenhouse effect of a 1-layer atm. S/4 AS/4 Te4 Te Atm. Te4 (1-A)S/4 Ts4 Ts Earth • Energy balance at Earth’s surface: • Ts4 =(1-A)S/4 +Te4 ..(1) • Energy balance for atm.: • Ts4 = 2Te4 .. (2) UNBC

22. Subst. (2) into (1): Te4=(1-A)S/4 ..(3) (same eq. as in last lec.) Divide (2) by ; take 4th root: Ts= 21/4Te = 1.19 Te For Te = 255K, Ts = 303K. (Observ. Ts = 288K) Tg = Ts- Te = 48K, 15K higher than actual value. • Overestimation: atm. is not perfectly absorbing all IR rad. from Earth’s surface. UNBC

23. (III) Modelling Energy Balance Objectives: Effects of clouds Earth’s global energy budget Climate modelling Climate feedbacks UNBC

24. Cumulus Cumulonimbus Stratus Cirrus UNBC

25. Climatic effects of clouds • Without clouds, Earths’ albedo drops from 0.3 to 0.1. By reflecting solar rad., clouds cool Earth. • But clouds absorb IR radiation from Earth’s surface (greenhouse effect) => warms Earth. • Cirrus clouds: ice crystals let solar rad. thru, but absorbs IR rad. from Earth’s sfc. => warm Earth • Low level clouds (e.g. stratus): reflects solar rad. & absorbs IR => net cooling of Earth UNBC

26. IR rad. from clouds at T4 • High clouds has much lower T than low clouds => high clouds radiate much less to space than low clouds. => high clouds much stronger greenhouse effect. UNBC

27. Earth’s global energy budget UNBC

28. Climate Modelling • “General circulation models” (GCM): Divide atm./oc. into 3-D grids. Calc. variables (e.g. T, wind, water vapor, currents) at grid pts. => expensive. • e.g. used in double CO2 exp. GFDL, Princeton UNBC

29. Weather forecasting also uses atm. GCMs. Assimilate observ. data into model. Advance model into future => forecasts. • Simpler: 1-D (vertical direction) radiative-convective model (RCM): Doubling atm. CO2 => +1.2°C in ave.sfc.T • Need to incorporate climate feedbacks: • water vapour feedback • snow & ice albedo feedback • IR flux/Temp. feedback • cloud feedback UNBC

30. Water vapour feedback Atm. H2O Ts Greenhouse effect • If Ts incr., more evap. => more water vapour => more greenhouse gas => Ts incr. • If Ts decr., water vap. condenses out => less greenhouse gas => Ts decr. • Feedback factor f = 2. • From RCM: T0 = 1.2°C (without feedback) => Teq = f T0 = 2.4°C. (+) UNBC

31. Snow & ice albedo feedback snow & ice cover Ts planetary albedo • If Ts incr. => less snow & ice => decr. planetary albedo => Ts incr. • As snow & ice are in mid-high lat. => can only incorp. this effect in 3-D or 2-D models, not in 1-D RCM. (+) UNBC

32. IR flux/Temp. feedback Ts Outgoing IR flux • So far only +ve feedbacks => unstable. • Neg. feedback: If Ts incr. => more IR rad. from Earth’s sfc. => decr. Ts (-) • But this feedback loop can be overwhelmed if Ts is high & lots of water vap. around => water vap. blocks outgoing IR => runaway greenhouse (e.g. Venus) UNBC

33. Uncertainties in cloud feedback • Incr. Ts => more evap. => more clouds • But clouds occur when air is ascending, not when air is descending. If area of ascending/descending air stays const. => area of cloud cover const. • High clouds or low clouds? High clouds warm while low clouds cool the Earth. • GCM’s resolution too coarse to resolve clouds => need to “parameterize” (ie. approx.) clouds. • GCM => incr. Ts => more cirrus clouds => warming => positive feedback. => Teq = 2 -5°C for CO2 doubling UNBC