Chapter 5: Atmospheric Structure and Energy Balance

(I) Characteristics of the Atmosphere  Thickness, air pressure, density  Air pressure and density decrease with altitude  90% of its mass (5.1 x kg) is within 16 km (10 mi) of the surface (about 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

Composition of the Atmosphere Dry Air TRACE GASES Argon (.93%) and Carbon Dioxide (.03%) Ozone ( %) 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) Solid particles (dust, sea salt, pollution) also exist

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

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

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

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”

Atm. vertical structure Air pressure p at sea level is 1 atm. = 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. Temperature

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

UNBC EM radiation 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  10 8 m/s wavelength later t

UNBC longer period waves => ? wavelength c = /T => = cT = c/ longer

UNBC Energy flux Power = energy per unit time (watt W = J/s) Flux F = power per unit area (W/m 2 ) less flux high lat. => less F

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

UNBC Solar flux S falls off as Inverse-square law e.g. if r = 2r 0 => S = S 0 /4 S

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

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

UNBC Stefan-Boltzmann Law Planck function (blackbody rad. curve) wavelength Rad. flux total F = area under curve F =  T 4  = 5.67 x W/m 2 /K 4

UNBC Planetary energy balance Earth is at steady state: Energy emitted by Earth = Energy absorbed..(1) E emitted = (area of Earth)   T e 4 = 4  R e 2   T e 4 (T e = Earth’s effective rad. temp., R e = Earth’s radius) E absorbed = E intercepted - E reflected Solar E intercepted = S  R e 2 (solar flux S) Solar E reflected = AS  R e 2 (albedo A) E absorbed = (1-A) S  R e 2 (1) => 4  R e 2   T e 4 = (1-A) S  R e 2

UNBC Magnitude of greenhouse effect  T e 4 = (1-A) S/4 T e = [(1-A) S/(4  )] 1/4 (i.e. fourth root) T e = 255K = -18°C, very cold! Observ. mean surf. temp. T s = 288K = 15°C Earth’s atm. acts as greenhouse, trapping outgoing rad. T s - T e =  T g, the greenhouse effect  T g = 33°C

UNBC Greenhouse effect of a 1-layer atm. Earth TsTs Atm. TeTe S/4 AS/4 (1-A)S/4 Ts4Ts4 Te4Te4 Te4Te4 Energy balance at Earth’s surface:  T s 4 = (1-A)S/4 +  T e 4..(1) Energy balance for atm.:  T s 4 = 2  T e 4.. (2)

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

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

UNBC CumulusCumulonimbus StratusCirrus

UNBC 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 IR rad. from clouds at  T 4 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 Earth’s global energy budget

UNBC 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 CO 2 exp. GFDL, Princeton

UNBC 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. CO 2 => +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 Water vapour feedback If T s incr., more evap. => more water vapour => more greenhouse gas => T s incr. If T s decr., water vap. condenses out => less greenhouse gas => T s decr. Feedback factor f = 2. From RCM:  T 0 = 1.2°C (without feedback) =>  T eq = f  T 0 = 2.4°C. TsTs Greenhouse effect Atm. H 2 O (+)

UNBC Snow & ice albedo feedback If T s incr. => less snow & ice => decr. planetary albedo => T s 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. TsTs planetary albedo snow & ice cover (+)

UNBC IR flux/Temp. feedback So far only +ve feedbacks => unstable. Neg. feedback: If T s incr. => more IR rad. from Earth’s sfc. => decr. T s TsTs Outgoing IR flux (-) But this feedback loop can be overwhelmed if T s is high & lots of water vap. around => water vap. blocks outgoing IR => runaway greenhouse (e.g. Venus)

UNBC Uncertainties in cloud feedback Incr. T s => 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. T s => more cirrus clouds => warming => positive feedback. =>  T eq = 2 -5°C for CO 2 doubling