earth’s climate system (part 2) revisiting the radiation budget heat capacity heat transfer...
Post on 19-Dec-2015
217 views
TRANSCRIPT
Earth’s Climate System (part 2)
• revisiting the radiation budget • heat capacity• heat transfer• circulation of atmosphere (winds)• Coriolis Effect• circulation of oceans (currents)
Earth’s climate system
• climate driven by “solar energy”
• climate operates to distribute solar energyacross surface
From last time:
Revisiting the radiation budget
energy in = energy used for warming
+ energy radiated back to space
Unequal distribution across Earth
Energy input & output averaged over year
Earth’s spin axis is inclined, so we get seasons
23.5o
Energy input by latitude & month
Radiation budget
energy in = energy used for warming
+ energy radiated back to space
Energy transferred to Earth:Raises temperature, drives winds, ocean currents
Energy input & output averaged over year:
Excess heat in equatorial areas, heat deficit in polar areas
Average surface temperatures:
Higher in equatorial than polar areas
Response to seasonal forcing: temperature changes
Northernhemisphere
Response to seasonal forcing:
average surface temperature changes over year
Response to seasonal forcing: albedo changes
(temperature-albedo feedback)
Why does land temperature undergo bigger temperature changes, and change more rapidly, than ocean temperature?
Because of differences in “heat capacity”.
Ocean Land
Northern hemisphere
Heat capacity
-- quantity that measures the ability of a
substance to absorb heat
heat capacity = density x specific heat
cal / cm3 g / cm3 cal / g
Heat capacity
• water has higher heat capacity than rock
• water has a greater ability to store heat
(it is a good “heat sink”)
• it takes more energy to raise temperature of water
than rock
Heat capacity
Heat capacity = Density x Specific Heat (cal/cm3) (g/cm3) (cal/g)
For water: 1 g/cm3 1 cal/g
Ratios of heat capacities:
water : ice : air : land = 60 : 5 : 2 :1
so water has a capacity to absorb heat that is 60 times that of the land’s capacity fo absorb heat
On average, surface heats up moreat equator than at poles
• drives winds in atmosphere
• drives ocean currents
• strongly affects climate (& weather)
Heat transfer
• heat flows from hot to cold
• heat transfer by various means-- conduction-- convection-- radiation
• should get flow of heat from equator to poles
• heat imbalances drive winds, precipitationpatterns & ocean currents
Why?
We get flow of air & heat from ground upwards.
“Warm air rises, cold sinks”.
Circulation of atmosphere (winds)
Because:
• most heating at surface
• warm air has lower pressure & density than cold air
• lower density air moves up, higher density air moves down
“Warm air rises, cold sinks”.
Wind
Uneven heating of atmosphere causes it tomove vertically & horizontally acrossthe ground.
Air that moves across surface is called“wind”.
We get systematic wind patterns on planets.
Venus:(1) rotation rate very slow (243 Earth days)(2) get simple wind circulation pattern (northern and southern Hadley Cells)
Hadley Cells
Earth:(1) rotation rate fast(2) get complex wind circulation pattern owing to Hadley Cells + Coriolis Effect
Coriolis Effect
• apparent deflection of moving objects (e.g. airmasses, ocean currents) on planet caused by planetary rotation
• deflection to right in northern hemisphere,to left in southern hemisphere
In red:
apparentpath of objectsmoving towards or away from equator
westerlies
easterlies
Why?
Flow of heat in atmosphere also determinesprecipitation patterns.
“It rains most at the equator, and least in the tropics (+- 30o latitude) and poles”.
Because:
• Warm air can hold more water vapor than cold air
• When warm air rises, it cools
• Equator has lots of warm, wet, rising air
• Subtropics & poles have dry, sinking air
“It rains most at the equator, and least in the subtropics (+- 30o latitude) and poles”.
desert belt
rain belt
desert belt
Circulation of oceans (currents)