The Structure and Motion of the Atmosphere OCEA 101
Why should you care? - the atmosphere is the primary driving force for the ocean circulation. - the atmosphere controls geographical variations in ocean salinity. - atmospheric winds carry trace metals in dust to the ocean
The prevailing winds deliver iron-rich dust to the ocean in the north Atlantic.
Overview The earth heat budget Distribution of radiation The greenhouse effect The general circulation of the atmosphere
Heating and Cooling of the Earth s Surface Radiative equilibrium: total amount of radiation absorbed = total amount of radiation emitted Radiative equilibrium yields a constant temperature: the radiative equilibrium temperature, T e For Earth, T e ~255K (-18C) The global ave surface T s ~288K (15C)
Blackbody Radiation Law of Blackbody Radiation: ALLobjects (above absolute zero) radiate energy Energy emitted (color) is dependent on the temperature (Kelvin) of the object E(λ,T) = 2πhc 2 / λ 5 (e hc/λkt -1) Energy (W/m 2 /µm) Planck s constant Boltzmann s constant
Planck curves
Sun emits primarily in the visible Earth emits primarily in the infrared
How is radiation energy distributed geographically? Earth receives solar radiation from the Sun. Earth emits infrared radiation (IR) back to space. Some of the IR is trapped in the atmosphere by virtue of greenhouse gases, hence T s > T e.
Incident Solar Radiation
Latitudinal Distribution of Incident and Outgoing Radiation
The Earth Heat Budget
Gains and Losses of the Heat Budget In order to maintain its long-term average equilibrium surface temperature of near 15C, the earth must on average re-radiate as much heat back to space as it receives from the sun. If this balance is upset (e.g. by the emission of greenhouse gases) then the earth will seek a new equilibrium surface temperature.
The heat budget is a follows: - 100 units of incoming solar radiation - 28 units reflected from atmosphere - 3 units reflected from surface - 3 units absorbed by ozone - 66 units absorbed by earth system 45 absorbed by earth surface 21 absorbed by atmosphere Determined from satellite observations
To balance the earth heat budget and maintain a radiative equilibrium, what must happen?
To balance the heat budget and yield a radiative equilibrium the 66 units absorbed by the earth and atmosphere MUST return to space. 66 units of radiation lost to space 8 units lost from earth surface 58 units lost from atmosphere Determined from satellite observations
What do the numbers tell us? Earth surface absorbs +45 units Earth surface loses -8 units Net gain by surface +37 units Atmosphere absorbs +21 units Atmosphere loses -58 units Net loss by Atmos -37 units
For equilibrium, the 37 units gained by the Earth surface must be given up to the atmosphere (i.e. the troposphere is heated from below). Earth surface: 37 units gained 23 units transferred to atmos by evaporation and liberated by condensation (latent heat loss) via to convection. 14 units transferred to atmos by re-radiation associated with the greenhouse effect.
An very important fact The earth heat budget tells us something very important and perhaps counter intuitive: The atmosphere is heated primarily from below by the earth surface.
Atmosphere Energy Balance
Specific Heat Capacity Specific heat capacity = amount of heat required to raise unit mass of substance by 1K. Variations in specific heat from place to place strongly influence the earth surface temperature.
July Surface Temperature Jan Surface Temperature
Vertical Structure of the Atmosphere The troposphere is heated from below by the Earth surface. The stratosphere is heated a mid-levels by absorption of UV by ozone.
Composition of the Atmosphere Greenhouse Gases (GHGs)
The Greenhouse Effect In the absence of GHGs, the Earth surface temperature would be T e ~-18C Instead, the average surface temperature is ~15C The difference is due to the greenhouse effect.
Water Vapour Rotation Band
How much radiation do gases in the atmosphere absorb? The answer is provided by the atmospheric absorption spectrum. Sun A Atmosphere Radiation absorbed= Radiation measured at A - Radiation measured at B Space-based radiometer B Earth Ground-based radiometer
Infra-Red Atmospheric Absorption Spectrum (Clouds are important here)
The Greenhouse Effect The atmosphere is transparent to incoming visible radiation (SWR). SWR is absorbed by the surface of the earth. The earth surface warms and emits infrared radiation (LWR). The atmosphere is opaque to LWR because it is absorbed by greenhouse gases (GHGs) (and clouds). The GHGs (and clouds) re-emit LWR both upwards back to space and downwards towards the surface. The earth surface absorbs the LWR and warms further. Some of this LWR is re-emitted by earth surface and absorbed by GHGs (and clouds) in the atmosphere.
A Simple Model of the Greenhouse Effect S 0 (1 α) 4 4 σt s 4 σt e 4 σt e Atmosphere Surface The greenhouse effect can be illustrated with a simple model of radiative equilibrium. The incident solar radiation is given by S 0 (1-α)/4 where S 0 is the solar constant and α is the average Earth albedo. According to the Stefan-Boltzmann Law, the outgoing long wave radiation emitted by the Earth surface is σt 4 s and at the top of the atmosphere is σt e4 where σ = 5.67 10 Wm K 8-2 -4
Radiative equilibrium at top of atmosphere implies: S 0 (1 α ) = σt 4 Radiative equilibrium at earth surface implies: Combining (1) and (2) yields: 4 e S 0 (1 α) + σt = σt 4 T s = 1/4 (2) 4 4 e S T e (1) (2) ~1.20
Trends in Atmospheric Carbon Dioxide
Atmospheric Aerosols Atmospheric aerosols are small particles in the atmosphere. They can be natural: pollen, salt crystals, ash, dust They can be anthropogenic: soot, acid droplets Aerosols play two v. important roles in atmos: 1. They are hygroscopic and act as cloud condensation nuclei (CCN) 2. They reflect incident solar radiation.
DMS and Clouds Dimethylsulphide (DMS) is a gas produced by phytoplankton In the atmosphere, DMS combines with water vapour to create sulphate aerosol. Sulphate aerosols are excellent CCN. Variations in cloud cover directly influence the amount of photosynthetically available radiation.
The Atmosphere in Motion Rule of thumb: - warm surface associated with low surface pressure - cold surface associated with high surface pressure Air accelerates from regions of high pressure to regions of low pressure Newton s Laws: accelerations are the result of an applied force. The pressure gradient force
Example A sea breeze
The Coriolis Force
Objects moving relative to the rotating Earth are deflected to the RIGHT of their path in the NH Objects moving relative to the rotating Earth are deflected to the LEFT of their path in the SH The Coriolis Force VANISHES at the equator. Coriolis force is proportional to: 1. Sine of latitude 2. Speed of the object
Centrifugal Force The Centrifugal Force causes deflection of objects moving east-west. The Centrifugal Force VANISHES at the equator.
Movie!
The 3 Cell Model of the Atmosphere (on an aqua planet) Polar Cell Ferrell Cell Hadley Cell Rules of thumb: Rising air associated with precipitation Descending air associated with evaporation Precipitation releases latent heat of vapourisation which drives more vigourous circulations.
The General Circulation of the Atmosphere The land-sea distribution significantly influences the atmospheric circulation because of differences in specific heat capacity. In addition, the pressure gradient force and Coriolis force balance, leading to geostrophic balance. In geostrophic balance, winds blow parallel to isobars: - (counter) clockwise around a (L) H in NH - (counter) clockwise around a (H) L in SH
Wind Pattern Development
Geostrophic Winds in NH Summer
Geostrophic Winds in NH Winter
The prevailing winds deliver iron-rich dust to the ocean.