ATMOS 5400 The Climate System 01. Introduction 2. Energy fluxes a. effect of clouds b. turbulent heat fluxes c. greenhouse gases

2a. Effect of clouds

Effect of clouds Cirrus: Allow shortwave through (low- α ) Have low outgoing long wave radiation (OLR) because they have low temperature (high in atmosphere) Net warming effect NASA Cirrus Cumulus

Effect of clouds Cumulus: Reflect shortwave (high- α ) Have high OLR (low-altitude = high-T) Net cooling effect NASA Cirrus Cumulus

Effect of clouds R TOA is the net radiation at the top of the atmosphere (energy retained in system) is the change in retained energy when a certain cloud is added If ΔR TOA is positive, cloud caused system to retain more energy (to gain energy and warm) If ΔR TOA is negative, cloud caused system to retain less energy (to lose energy and cool)

Effect of clouds Simple model – Assume cloud absorbs all long wave radiation from below – Assume no absorption of long wave radiation above height of cloud top (z ct ), meaning complete transmission “Albedo effect” “OLR effect”

Effect of clouds Albedo effect: increasing α reflects more solar radiation  tends to make Δ R TOA negative OLR effect: increasing cloud top height decreases OLR  tends to make Δ R TOA positive “Albedo effect” “OLR effect”

Effect of clouds Why does Δ R TOA vary as it does along the arrows here?

2b. Turbulent heat fluxes The energy transfer associated with heat is quantified as heat flux: flow of energy per unit time per unit area [ J s -1 m -2 = W m -2 ] Measuring heat flux

Heat flux Climate system has four basic types of energy transfer mechanisms 1.Radiation: waves travelling at speed of light 2.Convection: parcels with different energy amounts change places 3.Conduction: collisions between atoms or molecules 4.Latent heat (motion of water vapor)

Heat flux 4. Moving water vapor can release energy into system, so it is a form of energy transfer

How we organize thinking about these energy transfer mechanisms Radiative heat flux Radiation Turbulent heat flux Sensible heat flux: heat transfer by convection or conduction Latent heat flux: heat transfer by motion of water vapor which could undergo a phase change

Sensible heat flux 1. Surface heated by sun 2. Surface warms overlying air by conduction 3. Mixing by turbulence moves warm air up convection Positive sensible heat flux is upward energy transport

Sensible heat flux 1.Surface cools by emitting radiation 2. Air loses heat to surface by conduction 3. Mixing by turbulence moves warm air down convection Negative sensible heat flux is downward energy transport

Latent heat flux 1.Water evaporates from surface 3. Mixing by turbulence moves moisture (latent heat) up convection Positive latent heat flux is upward energy transport 2. Air moistened by evaporation

Latent heat flux 1.Water condenses onto surface 3. Mixing by turbulence moves moisture (latent heat) down convection Negative latent heat flux is downward energy transport 2. Air dried by condensation

Global energy balance: energy flow Recall our simple 1-layer climate model: Atmosphere is assumed to – be a perfect absorber and emitter of terrestrial – be transparent to solar Ignored sun angle Independent of height (1-layer) Ignores non-radiative (turbulent) heat fluxes No clouds = σ T A 4 = σ T e 4 atmosphere T a =T e σTS4σTS4 σTA4σTA4

Global energy balance: energy flow

Energy balance: flows Short wave (“solar”) radiation 31% is reflected back to space 49% absorbed by surface 20% absorbed by atmosphere and clouds Long wave (“terrestrial”) radiation About 90% of heat emitted by surface is absorbed by the atmosphere Atmosphere in turn emits radiation up and down (greenhouse effect) Turbulent heat fluxes Some of the surface’s heat is returned to atmosphere as – sensible heat flux – latent heat flux Top of atmosphere (TOA): 100 – = 0 Atmosphere and clouds – 95 – 60 = 0 Surface – 114 – =

Global energy balance: energy flow Stephens et al. (2012) Nature Geoscience

2c. Greenhouse gases from: Peixoto & Oort

What’s special about greenhouse gases? Greenhouse gases absorb the energy of long wave radiation because it causes them to “vibrate” (stretch and bend) and rotate Vibration (< 20 μm) Vibration of atoms in a molecule about a stable state is quantized (happens in response to specific frequencies) Three independent modes: stretching, antisymmetric stretching, and bending

Low Energy Interaction Rotation Also quantized, and happens at smaller energy than vibration (λ<10,000  m, IR). Purely rotational absorption requires a “dipole moment” – Center of mass is not collocated with center of charge; charges on bonds in molecule do not cancel each. – Atoms in molecule have different electronegativity (tendency to attract electrons): O > C > H – In H 2 O, permanent nonzero dipole moment is directed from O to center of H’s. In CO 2, no permanent dipole moment. See this by adding vectors directed from neg. to pos. charge. - water carbon dioxide

Low Energy Interaction Rotational-vibrational absorption Combination of rotation and vibration Water is a bent triatomic molecule with a permanent dipole moment, so can undergo pure rotational and also rotational- vibrational transitions Carbon dioxide is a symmetric linear molecule (no pure rotation transitions) but may take on a temporary dipole moment under bending, allowing rotational-vibrational transitions - watercarbon dioxide

IR Absorption Most of the atmosphere is made up of diatomic gases (N 2, O 2 ), which have uniform charge, zero dipole moment, and thus no rotational absorption of long wave radiation. Few polyatomic trace gases determine the entire IR transmissivity of the atmosphere: Terrestrial or water vapor “window”: 8-12  m. H 2 O 6.3  m (vib.-rot. band) and continuum beyond 12  m (most important GHG due to purely rotational) CO 2 15  m (strong vibrational-rotational band) near peak of terrestrial radiation. O  m in the center of the “window”.

The greenhouse effect Contributors to natural greenhouse effect – Water vapor accounts for 50% – Clouds account for 25% – CO 2 accounts for 20% – Remaining 5% is other absorbers like methane Remains true under climate change Schmidt G. et al. (2010) J. Geophys. Res.