SUSTAINABLE AG SYSTEM
SUSTAINABLE AG SYSTEM AOM 4932
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This 5 page Class Notes was uploaded by Anabel Lemke PhD on Friday September 18, 2015. The Class Notes belongs to AOM 4932 at University of Florida taught by Staff in Fall. Since its upload, it has received 20 views. For similar materials see /class/206894/aom-4932-university-of-florida in Agricultural & Resource Econ at University of Florida.
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Date Created: 09/18/15
AOM 4932 Earth39s Energy Balance The hydrologic cycle is fueled by energy from the sun Planetary geometry creates areas of energy surpluses and de cits which drive all active meteorological processes 3 These processes originate to redistribute energy throughout the system Earth and the atmosphere are the media through which the energy transport occurs 3 oceans and atmosphere are more active in this redistribution than land mass 3 Water transport and phase changes ie liquid oceans gt vapor humidity gt liquid precipitation play a major role energy transport Energv ow for the earth as a whole shortwave solar radiation enteri atmosphere 99998 longwave radiation emanated 70 re ected from particulates in air clouds and the earth 5 surface 30 Klongwave radiation from clouds vapor etc back radiation from earth 20 absorbed by atmosphere water vapor dust clouds 19 earth heat entering absorbed by earth 51 am pherzf 039002 latent heat and sensible gee em heat ux 30 N m energy energy Joules calories cal Radiation rate measured in 27 or 27 Notel 2 l langley 1y 90 cm sec cm s unit area me Radiation measured with a pyranometer or radiometer Important aspects of the earth s energy balance 1 Shortwave energy from the sun moves through the atmosphere to the earth more easily than longwave energy can move from earth through the atmosphere This keeps the planet warm similar to greenhouse glass 2 Planetary geometry creates areas of energy surpluses and de cits Incoming solar radiation is uneven because the earth is a sphere which rotates on a tilted axis Outgoing radiation is more uniform because the temperature of earth s atmosphere does not vary all that much from the equator to the poles N 30 C Energy gradients drive global energy transport processes such as wind and ocean currents 3 Net radiation balance is positive for latitudes below 35 receive more radiation than is emitted and negative for latitudes above 35 3 Therefore there is a net poleward transport of energy to maintain a balance 23 of this transport occurs in atmosphere and 13 in the oceans 4 Radiation both short and longwave is the energy source leading to evaporation Large quantities of energy are carried by water vapor This is the energy absorbed by molecules during phase change from liquid to vapor Brief Review of Radiation Ph sics All matter at a temperature above absolute zero radiates energy in the form of electromagnetic waves that travel at the speed of light f c The rate at which this energy is emitted is given by the Stefan Boltzmann law a EO T4 absolute temperature of the surface of the body K rate of energy emission per StefanBoltzmann constant 567 X 10398 WattsmZK l unit area per time 138 X 103912 calcmZK lsec emissivity dimensionless 828 X 103911 calcmz K min The value of E ranges from 0 to 1 depending on the material and teXture of the surface E l 3 Blackbody Re ects no radiation Absorbs and reemits radiation in proportion to surface area E lt l 3 Grey body Radiates a fixed proportion less of blackbody radiation at all wavelengths for a given temperature Blackbody radiation intensity is distributed over various wavelengths Spectrum of radiation of a black body Radiation Wiens Displacement Law 4 le peak always at AT 3000umK a BA T5 area under curve is G wavelength gt 7LT4 temperature Blackbody radiation spectrum follows this curve at all temperatures Note Sun radiates energy approximately as a black body at 6000 K 3 high temperature 3 short wavelengths Not all this energy reaches the earths surface Some is absorbed by atmospheric gases ie 02 and 03 absorb UV radiation which can be harmful to biota 3 Depletion of 03 will increase UV incidence at earth s surface 3 concern about ozone hole Earth radiates energy approximately as a black body at 290 K 3 lower temperature 3 longer wavelengths Again some of this radiation is absorbed by atmospheric gases ie H20 and C02 absorb infrared IR radiation 3 greenhouse effect Without H20 and C02 the earth s surface would have a temperature of N 18 C 3 Concern that fossil fuel combustion increases the CO2 levels which increases the temperature of the earth 3 global warming Based on the sun s temperature and the Stefan Boltzmann law the total energy emitted by the sun is w EaT4 1828x10 11 If cal 1i 6000K41x105 cm m1nK cm min mm Because of the earth s distance from the sun only a small fraction of this total energy is received at the outer edge of the earth s atmosphere Intensity of solar radiation at a plane on the upper atmosphere J to incoming solar radiation is called the solar constant mu 5 214m 1350 2 mln m sec solar constant Since the earth is a sphere which rotates on a tilted aXis while revolving around the sun the intensity of solar radiation at a plane J to earth s atmosphere varies in space due to spherical earth and time due to tilted aXis which leads to variation of climate around the earth and with time of year Solar radiation mo spread over larger surface area gt less 1 q radiationarea time gt lower temperatures Fgt solar altitude angle of incoming radiation with lt plane tangent to earthatmosphere surface declination of the sun latitude at which sun is directly overhead ranges from 2317OS to 2317 N lt7 Rsinsolation effective radiation intensity incident at outer edge of atmosphere RS nosinoc If earth s aXis were perpendicular to plane of revolution or would be a function of latitude only or 90 1 This would mean there would be no seasons and all parts of the earth would be illuminated 12 hours per day at all times It would be colder at the poles than at the equator but temperatures would be uniform throughout the year 23170 N151 x 106 km N145 X 106 km northern summer southern winter northern winter S southern summer However because of the angle of revolution 0c varies with latitude declination time of year and longitude Equation for total daily insolation RS 2010 Tamer sin6sin cos6cos where angular velocity of the earth39s I local latitude rotation 02618 radianshr 5 declination of the sun latitude at which sun is directly overhead 5 2345 cos 2 172 F also tabulated in places like the CRC 180 365 Handbook Julian Day dechnatron 1 366 in radians Tsuuset Number of hours after solar noon that sunset occurs Note sunrise and sunset occur at equal times before and after solar noon cos71 tan 5tan a T sunset This equation gives radiation at outer edge of atmosphere This solar radiation is further reduced as it moves through the atmosphere by scattering by molecules and particulates and absorption and scattering by clouds The net radiation received at the earth s surface is further reduced by absorption by vegetation and re ection by earth materials albedo A Re ectance of solar radiation by earth materials 3 Earth s average albedo for shortwave radiation AS 032 It ranges from 008 for black moist soil to 04 08 for snow 3 Longwave albedo is essentially zero for all earths surfaces except water For water A1 003 Net radiation received at earth surface short wave albedo long wave albedo dominant Rn RS1 AS R10 Al Rb earth as a black body EeoTe l longwave radiation emitted from madam 501 radiation at earth s longwave radiation received from atmosphere emitting surface a er planetary geometry as a black body and atmospherics re ection an absorption are accounted for l E 024 longwav e radiation received from atmosphere