Bài giảng khí hậu học chương 3

3.1 Atmospheric influences on Insolation• The atmosphere absorbs some radiation directly and thereby gains heat • Another portion of radiation disperses as weaker rays going out in many

Chapter 3

Energy balance and

Temperature

By Vu Thanh Hang, Department of Meteorology, HUS

G304 – Physical Meteorology and Climatology

3.1 Atmospheric influences on Insolation

• The atmosphere absorbs some radiation directly and thereby gains heat

• Another portion of radiation disperses as weaker rays going out in many different directions through a process called scattering

• Some of the scattered radiation is directed back to space

• The energy that is scattered is not absorbed by the atmosphere Æ not contribute to its heating

• The remaining insolation passes through the atmosphere without modification, reaching the surface

as direct radiation

3.1 Atmospheric influences on Insolation (cont.)

• Absorption:

- Atmospheric gases, particulates, and droplets all reduce

the intensity of solar radiation (insolation) by absorption, a process in which radiation is captured by a molecule

- Absorption represents an energy transfer to the absorber

- This transfer has two effects: the absorber gains energy and warms, while the amount of energy delivered to the surface is reduced

- The gases of the atmosphere are not equally effective at absorbing sunlight & different wavelengths of radiation are not equally subject to absorption

3.1 Atmospheric influences on Insolation (cont.)

• Absorption (cont.):

- If the atmosphere were able to absorb all the incoming

solar energy, the sky would appear completely dark

- Ultraviolet radiation is almost totally absorbed by ozone in the stratosphere

- Visible radiation passes through the atmosphere with only a minimal amount of absorption

- Near infrared radiation is absorbed mainly by two gases: water vapor & carbon dioxide

3.1 Atmospheric influences on Insolation (cont.)

• Reflection & scattering:

- The reflection of energy is a process whereby radiation making contact with some material is simply redirected away from the surface without being absorbed

- All substances reflect visible light, but with vastly differingeffectiveness

- Objects do not reflect all wavelengths equally

- The percentage of visible light reflected by an object or substance is called its albedo

- When light strikes a mirror, it is reflected back as a beam

of equal intensity, in a manner known as specular reflection

3.1 Atmospheric influences on Insolation (cont.)

• Reflection & scattering (cont.):

- When a beam is reflected from an object as a larger

number of weaker rays traveling in different directions, it

is called diffuse reflection, or scattering

- Large solid surfaces, gas molecules, particulates, and small droplets scatter radiation

- The scattered energy reaching Earth’s surface is thus

diffuse radiation, which is in contrast to unscattered

direct radiation

3.1 Atmospheric influences on Insolation (cont.)

• Reflection & scattering (cont.):

- In a scattering process, the radiation is redirected but not absorbed

- The characteristics of radiation scattering by the atmosphere depend on the size of the scattering agents relative to the wavelength of the incident electromagnetic energy

- Three general categories of scattering exist: Rayleigh scattering, Mie scattering and nonselective scattering

3.1 Atmospheric influences on Insolation (cont.)

• Reflection & scattering (cont.):

- It primarily affects shorter wavelengths

- Rayleigh scattering disperses radiation both forward and backward

Æ blue sky on a clear day, the blue tint of the atmosphere when viewed from space, the redness of sunsets and sunrises

The sky appears blue because gases and particles in the atmosphere scatter some of the incoming solar radiation in all directions Air molecules scatter shorter wavelengths most effectively Thus, we perceive blue light,

the shortest wavelength of the visible portion of the spectrum.

Fig 3-3

Sunrises and sunsets appear red because sunlight travels a longer path through the atmosphere This causes a high amount of scattering to remove shorter wavelengths from the incoming beam radiation The result is sunlight

consisting almost entirely of longer (e.g., red) wavelengths.

Fig 3-5

3.1 Atmospheric influences on Insolation (cont.)

• Reflection & scattering (cont.):

- It causes sunrises and sunsets to be redder than they would due to Rayleigh scattering alone, so episodes of heavy air pollution often result in spectacular sunsets

3.1 Atmospheric influences on Insolation (cont.)

• Reflection & scattering (cont.):

+ Nonselective scattering:

- The water droplets in clouds are considerably larger than suspended particulates reflecting all wavelengths of incoming radiation about equally Æ clouds appear white or gray

- Scattering by clouds is sometimes called nonselective scattering

- An isolated water droplet affects various wavelengths of solar radiation differently Æ a rainbow involves each wavelengths being refracted a different amount Æ the bands of individual colors

3.1 Atmospheric influences on Insolation (cont.)

• Transmission:

- When solar radiation travels the vacuum of outer space, there is no modification of its intensity, direction, or wavelength

- When it enters the atmosphere, only some of the radiation can pass unobstructed to the surface

- There is a reduction in the amount of radiation reaching the surface due to scattering process in cloudy days

3.2 The fate of solar radiation

• Seasonal variations in the availability of insolation, with almost 7% more solar energy available on perihelion than

on aphelion

• Planetary albedo is 30% (25 from the atmosphere and 5 from the surface of solar radiation are scattered back to space)

• The surface, the atmosphere, and the planetary system must give up as much energy as they obtain

• To achieve this energy balance, huge quantities of energy must be transferred from the Earth system and within the system between surface and atmosphere

Incoming solar radiation available is subject to a number of processes

as it passes through the atmosphere The clouds and gases of the atmosphere reflect 19 and 6 units, respectively, of insolation back to space The atmosphere absorbs another 25 units Only half of the insolation available at the top of the atmosphere actually reaches the surface, of which another 5 units are reflected back to space The net solar radiation absorbed by the surface is 45 units.

Fig 3-7

3.3 Energy transfer processes between the

surface and the atmosphere

• Surface-atmosphere radiation exchange:

- Earth’s surface and atmosphere radiate energy almost in the longwave portion of the spectrum

- Longwave radiation emitted by Earth’s surface is largely absorbed by the atmosphere Æ increases the atmosphere temperature

- The energy radiated by the atmosphere is transferred in all directions, including downward Æ surface receives a portion

of this radiation Æ causes surface heating Æ increases in longwave radiation emission from the surface… Æ an infinite cycle of exchange with energy transferring back and forth

3.3 Energy transfer processes between the

surface and the atmosphere (cont.)

• Surface-atmosphere radiation exchange (cont.):

- Water vapor, CO2, other greenhouse gases are good at absorbing most wavelengths of longwave radiation, a band from 8 to 12μm can pass through the atmosphere unimpeded Æ atmospheric window

- The difference between absorbed and emitted longwave radiation is referred to as the net longwave radiation

- When either is absorbed, the absorber is warmed Æcombine longwave and shortwave into net allwave radiation (net radiation), defined as the difference between absorbed and emitted radiation, or equivalently, the net energy gained

or lost by radiation

Figure created by Leland McInnes from

published EPICA data

Net radiation is the end result of the absorption of insolation and the absorption and radiation of longwave radiation The surface has a net radiation surplus of 29 units, while the atmosphere has a deficit of 29 units Fig 3-10

3.3 Energy transfer processes between the

surface and the atmosphere (cont.)

- The temperature increase resulting from a surplus of

energy receipt also depends on the mass of a substance

- Sensible heat travels by conduction through the laminar boundary layer and is then dispersed upward by convection

3.3 Energy transfer processes between the

surface and the atmosphere (cont.)

Both the surface and atmosphere lose exactly as much energy as they gain The surface has a surplus of 29 units of net radiation, which is offset by the transfer of sensible and latent heat to the atmosphere The atmosphere offsets its 29 units of radiation deficit by the

receipt of sensible and latent heat from the surface.

Fig 3-14

3.4 The Greenhouse effect

• The interactions that warm the atmosphere are often referred to as the greenhouse effect

• The greenhouse gases of the atmosphere do not impede the transfer of latent and sensible heat

• If the atmosphere had none of the “greenhouse gases”that absorb outgoing longwave radiation Æ Earth would be colder, temperature would oscillate wildly from day to night

• The greenhouse effect keeps Earth warmer by absorbing most of the longwave radiation emitted by the surface Æwarming the lower atmosphere Æ emits radiation downward

3.5 Global temperature distributions

• One of the most immediate and obvious outcomes of radiation gain or loss is a change in the air temperature

• The map depicts differences between mean temperatures in January and

July through the use of isotherms

3.5 Global temperature distributions (cont.)

• Temperatures tend to decrease poleward in both hemispheres

• The latitudinal temperature gradient is greatest in the winter hemisphere

• The isotherms shift poleward over land in the hemisphere experiencing summer and shift equatorward over land during the winter

• The NH has a steeper temperature gradient in its winter than the Southern Hemisphere does in its winter

3.6 Influences on temperature

• Latitude:

- Outside the tropics, the annual mean temperature decreases with latitude

- Affects seasonal patterns

- The tilt of Earth’s axis influences the amount of solar radiation available at any latitude on any particular day

- Within the tropics, energy receipts exhibit little change during the year

- Outside the tropics, the noontime solar angles exhibit a range of 47o with the lowest solar angles coinciding with the periods of shorter days Æ availability of incoming radiation (temperature) is more variable as distance from the equator increases

3.6 Influences on temperature (cont.)

• Altitude:

- Any point in the atmosphere has some particular altitude

(height above mean sea level)

- Altitude is not synonymous with elavation, the distance above sea level for a land surface

- Altitude and elevation both deal with position relative to sea level, but altitude is related to the atmosphere and elevation refers to land

- Temperatures in the troposphere typically decrease with altitude above sea level

3.6 Influences on temperature (cont.)

• Atmospheric circulation patterns:

- An organized pattern of mean atmospheric pressure and air flow across the globe strongly influences the movement

of warm and cold air Æ directly effects on temperature

- The large-scale circulation patterns also influence the development of cloud cover Æ an indirect effect on temperature

3.6 Influences on temperature (cont.)

• Contrast between land and water:

- Water bodies are far more conservative than land surfaces with regard to their temperature, taking longer to warm and cool when subjected to comparable energy gains and losses

- The specific heat of water is about 5 times as great as that of land

- The warming of a water surface can be reduced because

of the vast supply of water available for evaporation

- Water can be easily mixed both vertically and horizontally

3.6 Influences on temperature (cont.)

• Warm and cold ocean currents:

- The warm currents typically move poleward in the western portion of the ocean basins near the east coasts

of continents in the mid-latitudes

- Along the eastern margins of oceans, cold ocean currents dominate in the mid-latitudes

- Where the water temperatures are high, heat is transferred to the atmosphere and promotes higher air temperatures

Fig 3-16 The circulation of ocean currents Those moving warm water are depicted by red arrows, those moving cold water by blue arrows

3.6 Influences on temperature (cont.)

- Vegetation patterns often respond to the change in microclimate

- Densely wooded areas also have different temperature regimes than areas devoid of vegetation cover

3.7 Daily and Annual temperature patterns

• At solar noon the Sun achieves its greatest angle above the horizon and the surface receives its greatest input of solar radiation Æ max air temperature occur at least a couple hours afterward

• Surface temperature increases as long as energy gained by the surface is greater than energy lost

• The amplitute of the daily

temperature pattern is depended on

a lot of factors: cloud, strong wind,

temperature difference between

surface and air, …

• The relative amounts of incoming

and outgoing radiation affect energy

budgets and annual temperature

• Max temp is in July or Aug in NH

3.8 Measurement of temperature

• When a group of molecules (microscopic) move predominantly in the same direction, the motion is called wind (macroscopic)

• When they move in random directions, the motion is associated with temperature Æ higher temperatures are associated with greater average molecular speed

Celsius Temperature = (oF - 32) / 1.8Fahrenheit Temperature = (1.8 x oC) + 32

Kelvin Temperature = oC + 273

3.8 Measurement of temperature (cont.)

• The measurement of temperature is a simple and routine procedure in which the expansion and contraction of the fluid

- must contain mercury

- in the tube just beyond the bulb is a very narrow constriction that allows the mercury to expand outward when the temperature increases but prevents it from contracting back when temperature decreases

3.8 Measurement of temperature (cont.)

• A minimum thermometer is also similar to a regular thermometer

• If the index is at the end of the alcohol and temperature is decreasing, surface tension pulls the index toward the bulb

• Thermograph gives a continuous record of temperature

• Temperature measuring instruments should always be kept in an instrument shelter

3.8 Measurement of temperature (cont.)

• The daily mean is defined as the average of the maximum and minimum temperature for a day.

• The daily temperature range is obtained by subtracting the minimum temperature from the maximum

• The monthly mean temperature is found by summing the daily means and dividing by the number of days in the month

• The annual mean temperature is obtained by summing the monthly means for a year and dividing by 12

• The annual range is obtained as the difference between the highest and lowest monthly mean temperatures

If low temperatures are accompanied by windy conditions,

a person’s body loses heat much more rapidly than it would

under calm conditions due to an increase in sensible heat loss.

It is common for weather reports to state both the actual temperature and how cold that temperature actually feels,

the wind chill temperature index.

3.8 Measurement of temperature (cont.)