4: Chapter Atmosphere and Surface Energy Balances
Introduction
Earth's outpus of reflected light/emitted infrared energy fr atmosph/surface envt counter input of insolation; input + output determine net energy available to perform work;
Energy Essentials
Land/water surfaces, clouds, atmos gases, dust intercept solar energy; specific energy patterns diff for deserts, oceans, mtntops, plains, rain forests, ice-covered land; clouds reflect incoming enrgy
Energy Pathways and Principles
Transmission- passage of shortwave/longwave energy thru atmos/water; our budget of atmos'ic energy comprises s-wave radiation inputs (UV, visible, ~infrared light) & l-wave radiation outputs (thermal infrared) tht pass thru atmos by transmission Insolation- single energy input driving Earth-atmos sys; decr poleward fr 25lat in N/S Hems; constant daylength/high Sun alt produce avg annually 180-220 W/m2 thruout equatorial/tropical lats; greater insolation of 240-280 W/m2 occurs in low-lat deserts worldwide b/c ~cloudless skies there
Insolation Input
Scattering (Diffuse Radiation)
Insolation meets incr'g density of atmos gases/dust as travelssurface; gas molcs redirect radiation, changing direction of light's movement w/o altering its wavelengths- this= scattering, represents 7% of Earth's reflectivity/albedo; also due to pollutants, ice, cloud droplets, water vapor Rayleigh scattering principle- shorter wavelength= greater scattering, longer wavelength=less; shorter wavelengths visible light (blue/violet) scatter the most, dominate lower atmos; thus blue sky; angle of Sun's rays determines thickness of atmos they must pas thru to reach surface: Direct/overhead rays less scattered/absorbed b/c travel sml distance; reds/oranges at sunset b/c shrter wavelengths scatter much; sme incming insolation diffused by clouds/atmos, goesEarth as diffuse radiation- dwnwrd component of scattered light; multidirectional, casts shadowless light to ground
Refraction
As insolation enters atmos, travels thru many mediums- thus change of speed/shift in direction, a bending action called refraction; eg light thru prism to get spectrum; or mirage- image tht appears near horizon where light waves refracted by lyrs of air at diff temps on hot day Refraction adds 8 min daylight-4 min sunrise, 4 min -set; degree of refraction cont'ly varies w/atmos temp, moisture, pollutants, so cannot predict exact time of visible sunrise/set w/in thse 4 mins
Albedo and Reflection
Some arriving energy bounces directly back to space- reflection process; albedo- reflective quality/ intrinsic brightness of surface- impt control over amt of insolation available for absorption by a surface; albedo stated as % of insolation that is reflected- eg. 0% total absorption W/visible wavelengths, darker colors= lower albedos, lighter colors= higher; on water surfaces, lower angles of solar rays= greater reflection than higher angles; smooth surfaces incr albedo- opp of rough; some places have very variable albedo values during yr due to changes in cloud/ground cover Earth, its atmos reflect ~31% of all insolation/yr- avg albedo combo-21% reflected by clouds, 3% by ground/ocean surfaces, 7% by atmos; Moon sml albedo- 6-8%- earthshine 4X brighter; Jan albedos higher poleward of 40N vs. July ones b/c snow/ice; tropical forests low alb- 15%, deserts high 35% Clouds unpredictable factor in tropospheric energy budget/refining climatic models; cloud-albedo forcing- incr in albedo due to clouds; clouds also act as insulation, trapping l-wave radiation fr Earth, raising min temps; cloud-greenhouse forcing- incr in greenhouse warming due to clouds
Clouds, Aerosols, and the Atmosphere's Albedo
When Mt Pinatubo erupted 1991, tons sulfur dioxide went into stratosphere; winds spread these worldwide, increasing atmos albedo worldie, producing temp avg cooling of 0.5C; industrialization also effect on alb- pollution, subsequent chem. rxns- sulfate aerosols- insolation-reflecting haze: Causes atmos warming thru incr absorption (50%) by pollutants/surface cooling thru reduction in insolation reaching ground (10%)
Absorption
Absorption- assimilation of radiation by molcs of matter, its conversion fr one form of energy to another; direct/diffuse insolation not part of the 31% reflected is absorbed; is converted to infrared radiation or chem. energy by photosynthetic plants; thus incr'g temp of absorbing surface: This warmer surface radiates more total energy at shorter wavelengths; hotter surface= shorter wavelengths emitted; absorption also occurs in atmos gases/dust/clouds/stratospheric ozone
Conduction, Convection, and Advection
Conduction- molc-to-molc transfer of heat energy as it diffuses thru a substance; diff materials (gas, liquid, solid) conduct sensible heat directionally fr areas of higher temp to those of lower temptransfers energy thru matter at diff rates, depending on its conductivitybetter= land, moist air Gases, liquids transfer energy by convection- phys mixing involves strong vertical motion; whn horiz motion dominates,=advection; in atmos/water bodies, warmer (less dense) masses rise, cooler sink- sets up patterns of convection; examples of each phys transfer mechanism: o Conduction- surface energy budgets, temp diffs btwn land/water bodies, heating of surfaces/ overlying air, soil temps o Convection- atmos'ic/oceanic circulation, air mass movements & weather sys's, internal motions w/in Earth tht produce magnetic field, movements in crust o Advection- horiz movement of winds fr land to sea & back, fog that forms, moves to another area, air-mass movements fr source regs
Energy Balance in the Troposphere
Earth-atmos energy sys budget naturally balances itself in steady-state equilibrium; infrared energy radiated back to space- this+ reflected energy= initial solar input; atmos greenhouse gases delay losses to space, warm lower atmos
The Greenhouse Effect and Atmospheric Warming
Sun hot-body radiator- emits shorter wavelengths fr its surface; Earth cool-body radiator- some infrared radiation emitted absorbed by lower atmos gases, emitted back to Earth; this delays energy loss to space, is impt factor in warming troposphere= greenhouse effect: Energy absorbed more than released; w/greenhouse, opening roof vent lets air inside mix w/outside, removing by heat moving air physically btwn places- convection; atmos a little diff- infrared radiation not trapped, but its passage to space delayed as absorbed by gases, clouds, dust... & emitted to Earth's surface; incr'g CO2 concentration forcing more absorption in lower atmos, creating warming trend, changes in Earth-atmos energy sys
Clouds and Earth's "Greenhouse"
%, type, height, thickness (water content/density) of clouds affect heating of lower atmos; lower cloud cover reflects ~90% of incoming insolation; jet contrails (condensation trails) produce high cirrus clouds aka false cirrus clouds; high-altitude/icy; reduce outgoing infrared radiation &... Contribute to global heating; diurnal temperature range (DTR)- diff btwn daytime max/nighttime max temps- incr'd in absence of contrails; max temps esp responsive; lgst effects in regs where greatest # of flights occur in fall; less heat energy escapes fr tropical lands due to tall, thick clouds: Over equatorial reg+ higher s-wave reflection; sub-tropical desert regs- more l-wave radiation emitted due to little cloud cover, greater radiative energy losses fr surfaces tht've absorbed much energy
Earth-Atmosphere Radiation Balance
Neither Earth/atmos hve balanced radiation budget; avg annual energy distribution positive (surplus) for Earth, negative (deficit) for atmos; poss to construct overall energy balance, but reg'ly/seasonlly, Earth absorbs more energy in tropics, less in polar regs=>imbalance tht drives global circ'tn patterns
Earth's avg albedo 31%; out of 100% solar energy arriving, Earth eventlly emits thermal infrared part of budget into space: 21% atmos heating- atmos heat input; 3%- stratospheric ozone absorption, radiation; 45%- incoming insolation (surface heating); 69%- ozone emission; see pg. 99 Natural energy balance occurs thru energy transfers fr surface tht're both nonradiative (phys motion), radiative; former- convection, conduction, latent heat of evaporation (energy tht's absrbed/ dissipated by H2O as evaporates/condenses); latter- infrared radiation btwn surface, atmos, space Radiation balance for all s-, l-wave energy by latitude: o Btwn tropics, angle of incoming insolation high & daylength consistent w/little seasonal variation, so more energy gained than lost= energy surpluses o Polar regs- Sun low in sky, surfaces light/reflective, no insolation ~6 mths; energy deficits o 36lat, balance btwn energy gains/losses for Earth-atmos sys Imbalance drives vast global circulation of energy, mass; meridional (N-S) transfer agents: winds, ocean currents, dynamic weather sys's etc; dramatic ones- hurricanes, typhoons
Energy Balance at Earth's Surface Daily Radiation Patterns
Shape, height of insolation curve (see pg. 101) diff w/season, lat; highest trend at summer solstice; air temp peaks 3-4 pm, dips ~sunrise; lag btwn air temp/insolation curves; warmest time of day occurs at moment when max of insolation absorbed, emitted to atmos fr ground As long as incoming energy exceeds outgoing energy, air temp incr's, only peaks whn incoming energy dips in afternoon as Sun's alt decr's; annual pattern of insolation, air temp shows similar lag; N Hem- Jan coldest month after Dec solstice, warmest- July/August after June solstice, longest days
Simplified Surface Energy Balance
Energy, moisture cont'ly exchanged at surface, creating worldwide boundary-layer climates of grt variety; phys conditions at/near Earth's surface studied in microclimatology- science of ths lwest part of atmos; see diagram pg. 101-102; adding, subtracting energy flow at surface completes: Calculation of net radiation (NET R)- balance of all radiation at Earth's surface; varies as parts of this simple equation vary w/daylength thru seasons, cloudiness, lat; at night, NET R value neg b/c insolation ceases at sunset, surface conts to lose infrared energy to atmos; surface rarely reaches: 0 NET R value- perfect balance- but evntlly Earth surface ntrlly balances incomng/outgoing energies NET R of all wavelengths available at Earth's surface= final outcome of entire radiation-balance process; abrupt change in radiation balance fr oceanland surfaces; highest NET R: N of equator in Arabian Sea; pattern of values ~zonal/parallel, decr'g away fr equator o LE- latent heat of evaporation- energy that's stored in water vapor as water evaporates; lg qtys of this absorbed into water vapor during it's change of state, removing this heat energy fr the surface; energy releases to envt when water vapor changes state back to liquid; main expenditure of Earth's entire NET R, esp over water surfaces o H- sensible heat- back-and-forth transfer btwn air/surface in turbulent eddies thru convection/conduction w/in materials; depends on surface, boundary-lyr temps/intensity of convective motion in atmos; 1/5 of earth NET R radiated as this, esp over land o G- ground heating & cooling- energy tht flows into/out of ground surface (land/water) by conduction; during yr, overall G value 0 b/c stored energy fr spring/summer=losses in fall/ winter; energy also absorbed in melting snow; most available energy in LE/H doing this Hi-est annual LE values in tropics, decr toward poles; high also in subtropical lats over oceans; H hiest in subtropics; most NET R expended as H in dry regs; most/vegetated surface expend more LE
Net Radiation
Sample Stations
Variation in expenditure of NET R among H (energy we can feel), LE (energy for evaporation), G produces variety of envts in nature; see pg.107 examples- El Mirage, Pitt Meadows
The Urban Environment
Urban landscape produces most temps; b/c ~50% people live in cities, urban microclimatology, othr city-related effects impt; phys characteristics of urbanized regs produce urban heat island tht has on avg both max, min temps higerh than nearby rural settings
Major cities produce own dust dome of airborne pollution, which can be blown in plumes; affect urban energy budgets; world trendmore urbanization putting more ppl on urban heat islands; temps drop over areas of trees, parks; H lessened b/c of LE + plant effects eg transpiration/shade Urban forests impt in cooling cities; buildings hottest objects; poss to reduce urban heat island effect- lighter-colored surfaces, more reflective roofs, more trees, parks, open space.