energy balance and temperature - uio

© 2015 Pearson Education, Inc.

Energy Balance

and Temperature

Chapter 3 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo


Quiz om stråling

• Hva er forskjellen på en gul genser og en gul

flamme?


Quiz om stråling


flamme?

• Hvorfor er himmelen blå?


Quiz om stråling


flamme?

• Hvorfor er himmelen blå?

• Hvorfor kan solnedgangen være rød?


The Global Radiation Budget


Atmospheric Influences on Insolation

• Absorption

– Particular gases, liquids, and solids in the atmosphere

reduce the intensity by absorption.

– Less energy is transferred to the surface.

– Atmospheric gases are overall poor absorbers of energy.


• Reflection and Scattering

– Energy is redirected by objects through reflection without

being absorbed.

– Albedo is the percentage of energy reflected by an object.

– Specular reflection is reflection of energy as an intense

beam.




– Energy reflected as disperse energy into less intense beams

is diffuse reflection, or scattering.

– Gases in the atmosphere scatter radiation.

– Energy that reaches the surface is scattered and different in

intensity from direct radiation.




– Scattering of light by agents smaller than 1/10 the wavelength

of incoming radiation is known as Rayleigh Scattering.

– Partial to shorter wavelength energy.

– Rayleigh Scattering results in our blues skies. It’s why our

skies are blue to the human eye.





– Mie scattering, scattering sunlight, is predominantly forward

scattering, diverting relatively little energy backward to space.

– Mie scattering causes sunrises and sunsets to be more red,

when pollution is present.


• Transmission

– The fraction of energy transmitted through the atmosphere to

the surface.

– Transmission is dependent upon the atmosphere’s ability to

absorb, scatter, and reflect.

– Transmission of energy varies from place to place.



The Global Radiation Budget


The Fate of Solar Radiation

• Atmospheric reflection

averages 25 units, 19 of which

are reflected to space by

clouds and 6 units are

back-scattered to space from

atmospheric gases.


• Five units are reflected back to space.

• These five units combined with the 25 scattered to space

from the atmosphere (clouds, etc.) equate to a total

albedo of 30 percent for Earth.

• The remaining 45 units of energy at the Earth’s surface is

absorbed and this heats the surface from the ground up.

• Earth processes and transfers this energy back to space.



Energy Transfer Processes

• Surface–Atmosphere Radiation Exchange

– Earth’s surface and atmosphere radiate longwave energy.

– Longwave radiation emitted from the surface is largely

absorbed by the atmosphere. This increases the temperature

of the atmosphere, which causes it to radiate energy in all

directions, including toward the surface.

– This causes additional surface heating, and the cycle repeats.

– To describe longwave energy, we begin with 116 units of

radiation.

– 104 units are absorbed by the atmosphere.



• Surface–Atmosphere Radiation Exchange

– Water vapor and CO2 are the primary absorbers of longwave

radiation (greenhouse gases).

– The range of wavelengths, 8-15 μm, matches those radiated

with greatest intensity by the Earth’s surface.

– This range of wavelengths not absorbed is called the

atmospheric window.

Atmospheric window


• Conduction

– As the surface warms, a temperature gradient develops in the

upper few centimeters of the ground.

– Temperatures are greater at the surface than below.

– Surface warming also causes a temperature gradient within a

very thin (a few millimeters) sliver of adjacent air called the

laminar boundary layer.



• Convection

– The temperature gradients in the laminar boundary layer

induce energy transfer upward through convection.

– This occurs any time the surface temperature exceeds the

air temperature, typically occurring in the middle of the day.

– At night, the surface cools more rapidly that air and energy

is transferred downward.

– Convection can be generated by two processes in fluids. • Free Convection

– Mixing related to buoyancy, warmer, less dense fluids rise

• Forced Convection – Initiated by eddies and other disruptions to smooth, uniform flow



Free Convection

Forced Convection



• Sensible Heat

– When energy is added to a substance, an increase in

temperature can occur.

– Eight units of energy are transferred from the surface to the

atmosphere as sensible heat.



• Latent Heat

– Energy required to induce a change of state in a substance.

– In the atmosphere, we relate this to water.

– Energy must be supplied in order to melt an ice cube,

freeze water, evaporate water, or boil it to water vapor.



The Global Energy Budget


• Net Radiation and Global Temperature

– Earth’s radiation balance is a function of an incoming and

outgoing radiation equilibrium.

– Balances occur on an annual global scale and diurnally over

local spatial scales.


(1-α) I = σ T4

α albedo

I solar constant / 4

T = [(1-α)I/σ]-4

T = -18 °C


The Greenhouse Effect

• Terrestrial radiation is trapped by

certain atmospheric gases, allowing

solar radiation to enter but trap

outgoing heat energy.

• Without atmospheric gases (H2O,

CO2, and CH4) trapping outgoing

terrestrial radiation, average Earth

temperatures would be about

-18°C. Very cold.

• Increases in greenhouse gas

concentrations through human

activities may lead to future climatic

changes and potential warming.


Influences on Temperature


• Altitude and Elevation

– Temperatures in the troposphere decrease with altitude.

– Earth heats from the ground up.

– Temperatures at high altitudes remain fairly constant.

– Air at high elevations (but near a surface) is influenced

more by rapid diurnal temperature fluxes than air at

lower elevations.



• Atmospheric Circulation Patterns

– Latitudinal temperature and pressure differences cause

large-scale horizontal energy transport through advection.

– Also influences latitudinal moisture and cloud cover, which

then impact temperatures.



• Contrasts Between Land and Water

– Surface composition affects atmospheric heating.

– Water bodies heat slower than land.

– Continentality is the effect of inland location that favors greater

temperature extremes.

– Maritime locations experience more moderate seasonal

temperature extremes due to the presence of water bodies,

which change temperature very slowly. The water acts like a

temperature regulator.

– Water heats less due to higher specific heat, transparency,

evaporative cooling, and horizontal and vertical mixing factors.



• Local Conditions

– Small topographical features impact temperatures.

– Slopes facing toward the equator heat more quickly than

slopes facing the poles.

South-facing slopes are typically more

vegetated than north-facing slopes.



Daily and Annual Temperature Patterns

Vegetation reduces surface radiation

during the day and traps it at night.

• Daytime Heating and Nighttime Cooling

– Forest regions reduce surface insolation during the day and

trap radiation at night leading to cooler daytime temperatures

and warmer nighttime temperatures.


Our Warming Planet

• In 2007, the Intergovernmental Panel on Climate

Change (IPCC) issued a report that concludes a rapidly

warming planet. There are many other reports and

accounts since this report that conclude the Earth is

warming.

• Global temperatures have increased during the 20th

century.

• Extreme hot and cold events are also believed to be an

indicator of climate change.

• Scientists use complex mathematical programs called

general circulation models (GCMs) to examine how

the atmosphere might respond to increasing greenhouse

gas concentrations in the future.


Our Warming Planet


• The IPCC paid close attention to volcanic eruptions,

solar radiation variability, and other factors that

could influence global climates but still identified

human greenhouse gas emissions as the primary

cause of warming.

• The effect of humans on climate is one of the

foremost matters that societies will have to deal with

for some considerable time to come.

Our Warming Planet

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