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Unformatted text preview: Energy Processes and Energy Budgets Radiant Energy Processes: 3
Scattering: dispersal of radiant energy equally in all directions because of interaction of a light beam with dust particles or atmospheric gases. Explains why areas not facing the Sun are illuminated, as well as blue sky and red sunsets. Blue sky: wavelengths close to the peak of the Sun spectrum are prone to scattering by atmospheric gases and dust. Thus bluish light is seen to come fro m all directions, even when facing North, away from the Sun. 1 • Red sunsets: shorter wavelengths are selectively scattered back to space by dust particles suspended in the lower atmosphere when the sun angle is low. Thus the spectrum of sunlight is shifted towards longer wavelengths and thus appears redder or more orange in colour: Radiant Energy Processes: 4 Absorption: incident radiant energy which strikes a body and is not reflected. Absorbed energy is stored as heat energy within the body and raises the temperature of the surface absorbing the radiant energy. Remember that the amount of energy absorbed by a surface is strongly controlled by the albedo of the surface. E.g., high albedo means strong reflection, therefore low energy absorption and low heating. 2 Earth and Energy Systems: 4 • Global Radiation Balance • Net radiation • Nonradiative energy forms Sensible Heat Latent Heat • Total energy budget of a location Global Radiation Balance: Overview Fig 2.8 Wrong! If Earth is to maintain a longterm constant average temperature, the absorption of shortwave so lar energy must balanced by emissio n of lo ngwave energy back to space fro m Earth. If an imbalance develops, then the temperature of the Earth must either progressively decrease (e.g. an Ice Age), or increase (e.g. modern Global Warming). 3 Symbols Used for Radiant Energy Terms Incoming shortwave energy emitted by Sun = K↓ Outgoing longwave energy emitted upwards from Earth’s surface or atmosphere = L↑ Incoming longwave energy emitted back down to the surface by Earth’s atmosphere = L↓ Outgoing shortwave energy reflected from Earth = K↑ In these terms, Albedo = α = K↑/K↓ Note: L↑ is NOT a reflected energy term! K↑ is NOT an emitted energy term! Concept of Net Radiation Net Radiat ion = total inco ming total outgoing radiant energy radiant energy Q* = K↓ + L↓ K↑ L↑ Q* = (K↓ K↑) + (L↓ L↑) Q* = net shortwave + net longwave energy energy Q* can be posit ive, negat ive, or zero depending on the relative magnitudes of the various radiant energy terms. 4 Sche matic view of radiant energy terms K¯, K-, L¯, L- K- L- K- K ¯ L ¯ L- Figure 4.1 Influence of clouds on net radiation terms: shortwave energy K- L- K¯ • Clouds are strong reflectors of inco ming shortwave radiat ion, K¯. • A large part of the Earth’s albedo is due to reflection fro m clouds. • Under cloudy condit ions, K¯ at the surface is reduced, and therefore less heat ing of the surface can occur. K ¯ at surface reduced Fig 47 5 Influence of clouds on net radiation terms: longwave energy • Clouds strongly absorb outgoing longwave energy L-. K- L- to space K ¯ • Some of this absorbed energy is then radiated back down to the surface as L¯. • Cooling of the surface is therefore reduced. L- L ¯ • The remaining lo ng wave energy is radiated to space as L- from the cloud tops. Fig 47 Net Radiation Under ClearSky Conditions: 1 Daytime conditions • K¯ high because of no reflect ive loss of K- fro m clouds tops. The surface beco mes strongly heated, and the dayt ime maximum temperature Tmax is high. • Because the surface is heated, emissio n of L- energy is large. Because the atmosphere is much cooler than the surface, however, downward return of longwave energy is L¯ is less than L-. So (L¯ L-) is negat ive by day. • Net radiat ion Q* is posit ive by day because the net gain in shortwave energy exceeds the net lo ngwave energy loss. L ¯ L- K ¯ 6 Net Radiation Under ClearSky Conditions: 2 Nighttime conditions • K¯ is now turned off because the sun has set, so that (K¯ K-) is zero. • Because there are no clouds to absorb the outgoing longwave radiation fro m the surface, the surface cools rapidly and therefore chills the lower atmosphere in contact with it. L¯ is st ill less than L-, so the longwave budget for the surface (L¯ L-) is negat ive. Overnight minimum temperature, T in is low. m • Net radiat ion Q* is negat ive by night because it is controlled solely by the lo ngwave budget (L¯ L-). • The range in temperatures over 24 hours (T ax – T in) will be m m large under clearsky condit io ns. Net Radiation Under Cloudy Sky Conditions: 1 Daytime conditions • K¯ at the surface is reduced because of reflective loss of K- from cloud tops. Thus less K¯ is absorbed tha n under clearsky conditions. The surface is less heated, so temperature T ax is m lower. • A cooler surface means less L- ener gy, and much of this ener gy is absorbed by the cloud layer, which becomes war med. Clouds then reradiate L¯ back to the surface. So the longwa ve budget (L¯ L-) is less negative than under clearsky conditions. • Net radiation Q* is positive by day but is a lower value tha n under clearsky conditions. 7 Net Radiation Under Cloudy Sky Conditions:2 Nighttime conditions • K¯ is now turned off because the sun has set, so that (K¯ K-) is zero. • The cloud layer cont inues to absorb outgoing lo ngwave radiation fro m the surface, and counterradiates much of it back as L¯. Since clouds are generally co lder than the surface, (L¯ L-) is negat ive, but is less negative than under clearsky condit ions. Thus the overnight minimum temperature, T in is higher. m • Net radiat ion Q* is negat ive by night because it is controlled solely by the lo ngwave budget (L¯ L-). • The range in temperatures over 24 hours (T ax – T in) will be m m much smaller than under clearsky condit io ns. Counterradiation & the Greenhouse Effect A = LW directly to space from surface B = LW absorbed by the atmosphere C = LW to space from atmosphere D = LW from atmosphere back to surface as counterradiation Like a greenhouse, Earth’s at mosphere allo ws the entry of solar short wave energy, and like a greenhouse, Earth’s at mosphere is fairly opaque to outgoing longwave energy, much o f which is absorbed and returned to Earth’s surface. This is known as the Greenhouse Effect and is the principal mechanism in Global Warming. 8 Atmospheric Components Important in the Greenhouse Effect The following atmospheric components are consider ed to be very important in the Greenhouse Effect, and ther efor e Global Warming, because they are all strong absorbers of outgoing longwave energy: • Clouds (water and ice). Cloud type is important (see next slide) • Carbon dioxide, CO . Derived principally from volcanic eruptions and 2 combustion. Although less than 1% of the total atmospher e by volume, it is a strong absorber of L- ener gy. Increasing levels of CO due to 2 combustion of coal, oil, gas, wood etc. have probably contributed to the observed global warming in the past 50 years. • Methane, CH . Derived mainly from orga nic decay in wetla nds and 4 rice paddies and from anima l wastes (including ours). • Water vapour, H O. Ranges between 1% and 3% by volume, 2 depending on moisture content of atmospher e. The Total Energy Budget
So far we have considered only radiant energy processes. But…. What happens to all the excess energy at the surface if Q * is positive? How do we import energy to an area if Q* is . negative? To understand how we must consider nonradiative (heat) energy forms 9 Heat Energy: Sensible Heat • Sensible heat: quantity of heat held by an object that can be sensed by touching or feeling – measured by a thermo meter • Sensible heat transfer: flow of heat from one substance to another by direct contact due to temperature differences between the two substances = conduction Heat Energy: Latent Heat • Latent heat: heat energy that is either stored in, or released fro m, a substance when it changes state fro m a solid to a liquid, fro m a liquid to gas, or from a solid directly to gas • Latent heat transfer: energy flow associated with changes of state of a substance 10 Three States of Water Important points from previous slide • Because of Energy Conservat ion, there can be no net gain or loss of energy around the triangular diagram due to changes of state. • Thus if water is condensed from water vapour (latent heat release), then frozen (latent heat release), the same amount of energy must be absorbed to change ice back to vapour by sublimatio n (latent heat absorbed). Energy Conservation rules! • The latent heat of vapourization/condensat ion is 2.5 megajoules 6 per kilogram (2.5 x 10 j/kg). This is a huge amount of energy! 5 • The latent heat of fusio n/melt ing is only 3.3 x 10 j/kg, or about 7.6 times smaller. • Huge amounts of energy are invo lved in these processes globally. 11 Total Energy Budget for Earth’s Surface: 1 Energy conservation requires that the net radiant energy, Q*, is balanced off by other energy ter ms. A radiant energy surplus at the surface must somehow be distributed elsewhere. This is expressed in the Energy Budget Equation: Q* = Q + Q + Q H E G where Q = sensible heat transfer to/from atmosphere H Q = latent heat transfer to/fro m at mosphere E Q = sensible heat transfer to/from the ground G Note: energy budgets can also be computed for animals, trees….. NonRadiative Energy Transfer • Conduction: flow of sensible heat fro m a warmer substance to a colder one through direct contact (Q ) H • Convection: sensible heat is distributed in a fluid (air) by turbulent mixing of warm air into cooler air (Q ) H • Latent heat transfer: energy exchange, mainly during condensation/evaporation cycles (Q ) E 12 Total Energy Budget for Earth’s Surface: 2 If there is a Q* energy surplus at the surface, then the flow directions for the other energy terms are as follows: Q ↑ Q ↑ Q* > 0 H E
__________________________________________________________Surface Q ↓ G If there is a Q* energy deficit at the surface, then the flow directions for the other energy ter ms are as follows: Q ↓ Q ↓ Q* < 0 H E
____________________________________________________________Surface Q ↑ G Total Energy Budget for Earth’s Surface: 3 Energy flows for a dry surface: Q ↑ H Q ↓ G Energy flows for a moist or vegetated surface: Q ↑ H Q ↓ G Q ↑ E Q* > 0 Q = 0 E Q* > 0 __________________________________________________________Surface ____________________________________________________________Surface 13 Global View of Energy Budgets • The global energy budget of the Earth’s atmosphere and surface is a full accounting of all the important energy flows – not an easy task! • There must be a balancing of radiant energy, as well as sensible and latent heat energy. Overview of Earth’s Energy Budget: Shortwave portion Total incoming = 100 Reflect ion loss = 31 Absorbed by atmosphere = 24 Absorbed by surface = 45 Total absorption = 69 Check: 69 + 31 = 100 Note that atmosphere is not heated much by shortwave energy (21 units) but that surface is (45 units). 14 Overview of Earth’s Energy Budget: Longwave portion From last slide: 69 units must be lost back to space as L- if surface energy balance is to be maintained. If not, then global climate either warms or cools over time: From surface, 45 units lost as: Q = 19 E Q = 4 H L- to space = 8 (small) L- to atmosphere = 14 (large) (Note the importance of latent heat transfer, Q ) E Total L- back to space = 69 Q E Q H Latitudinal variation of net radiation Polewards of about 36° lat itude, in both hemispheres, there is a deficit of net radiant energy at the surface. Therefore, energy in other forms (Q , Q ) H E must be imported to those areas from the tropics. 36° 36° Heat transport occurs via Earth’s atmosphere and ocean currents. 15 Air Temperature Changes • • • • • • Surface Temperature Air Temperature Daily Cycle of Air Temperature Temperature Structure of the Atmosphere Annual Cycle of Air Temperature World Patterns of Air Temperature Surface Temperature • Surface temperature is determined by the budget of the various energy flows that move across it • By day: incoming solar radiation normally exceeds outgoing terrestrial (IR) radiation = positive net radiation – surface warms • At night: outgoing IR radiation normally exceeds incoming IR radiation = negative net radiation – surface cools 16 Heat Flow in Air, Water and in Soil • Dominant heattransfer process in air and water is convect ion, which invo lves rapid, turbulent mixing and overturning within a fluid. •Dominant heattransfer process in so il or rock is conduction, which invo lves a very slow mo lecule tomo lecule transfer of heat. •Conduction process in air is about 10,000 times slower than convection in air. •Conduction in water is about 5,000 times slower than convection in water. Implications of conduction versus convection During the course of a typicalday, convect ion distributes heat through 1 – 5 km of Earth’s atmosphere, and through depths of 1020 m within water bodies. Over the same time period, heat flow by conduct ion within the soil occurs only wit hin the uppermost < 0.5 m This means that heating is much more concentrated within solid objects such as rock and soil, whereas in air and water the same heat energy is distributed within very large vo lumes. Thus, we would expect much greater temperature changes to occur at the surface of a so lid than within a fluid, given the same energy input over the same time interval. 17 Thermal Properties of Materials Thermal Conductivity: the rate at which heat flows into a body. Fluids such as air and water have high conductivity because of turbulent mixing effects. Soil has a low conductivity because heat flow is by conduction alone. Volumetric Heat Capacity (or Specific Heat): Refer s to the amount of ener gy r equir ed to raise a unit volume (or mass) of a substance through 1°C. Very high for water, moderate for soil and rock, and very low for air. Air value is 1,000 times smaller than water value. Thus air heats up very quickly because of a low heat capacity (low density fluid) combined with a high thermal conductivity. Water heats up much more slowly than air because, although heat flows in quickly (high conductivity), a large amount of heat is required per unit volume to cause a 1°C change. Thermal Differences between Land and Water Fig 5.7 18 Air temperature: Refers to shade temperature, measured under standard conditions in a Stevenson Screen, at height of 1.2 m above the ground surface. Daily Temperature Profiles (near ground) 19 Typical Daily Cycle of Air Temperature Fig 4.14 Explanation of Diurnal Temperature Cycle • Minimum temperature near dawn – explained by fact that the surface has been losing L- all night. • Maximum temperature in midafternoon – explained by the fact that although absorbed radiat ion peaks at noon, a surplus of net radiant energy cont inues to accumulate until about 3 pm. In the late afternoon, K¯ starts to decline unt il energy losses outpace energy gains, so the surface, and thus the overlying air, start to cool down. 20 Temperature Structure of the Atmosphere • In general, air temperature decreases with altitude above the surface, since the surface is the dominant source of heat energy. • Lapse rate: refers to a change in measured air temperature (°C per 1000 m) with height Atmospheric Temperature Profile 21 Structure of Lower Atmosphere – Troposphere (016 km) : layer of strongly decreasing temperature with height (i.e., steep lapse rate); lowest atmospheric layer, containing most atmospheric moisture and clouds and weather pheno mena – Stratosphere (1650 km): layer of increasing temperature with height due to absorption of high energy radiatio n; very low mo isture content; contains strong persistent winds that blow west to east; little vertical mixing o f air between troposphere and stratosphere Temperature Inversion • On a clear night, the ground surface radiates more longwave energy to the sky than is returned as counter radiation. Thus, net radiation at the surface, Q* < 0. Thus sensible heat flow occurs fro m lower atmosphere to the chilled surface, and thus the temperature of the air next to the surface decreases. • If surface continues to stay cold, a layer of cooler air gradually accumulates beneath a layer of warmer air aloft • Temperature inversion: reversal of the normal environmental temperature lapse rate, so that temperature increases with height above surface. 22 LowLevel Temperature Inversion Effects of Surface Types on Temperature Changes • Global Scale: Basic contrast in thermal properties between land and water surfaces. Land surfaces heat up and cool down more rapidly than do water bodies. • Local/Regional Scale: Vegetated (rural) versus urban areas: rural areas transpiration and evaporation from plants and soil = cooling, because of energy dissipated as latent heat Urban areas – drier surfaces means less evaporation and drier, darker surfaces absorb radiant energy more efficiently = warmer surfaces 23 Factors Controlling Air Temperatures 1. Latitude • As latitude increases, average annual insolation decreases, so temperatures decrease 2. Maritime vs. continental contrasts • Coastal locations warmer in winter but cooler in summer. Thus coastal areas have much smaller annual temperature ranges relative to interior locations 3. Elevation • Temperatures are much cooler at higher elevations because of lower density of the atmosphere. Continental vs. Maritime Temperature Cycles Both locations at similar latitudes, hence similar annual inso lation inputs. Note much larger temperature range at Winnipeg, and the higher mean annual temperature at Vancouver. Fig 5.12 24 Mean Monthly Air Temperatures (Jan.) Mean Monthly Air Temperatures (July) 25 ...
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at The University of British Columbia.
- Spring '09