03RadiationCirculation - 10/04/2007 ESM 203: Energy balance...

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Unformatted text preview: 10/04/2007 ESM 203: Energy balance and atmospheric circulation Jeff Dozier & Thomas Dunne Fall 2007 Wavelengths of radiation and the atmosphere Ultraviolet (<0.4 m): absorbed by stratospheric ozone (less ozone more UV) Visible (0.40.7 V bl (0 4 0 7 m): scattered b air molecules, dust, soot, ) d by l l d salt, clouds Scattering by air greater for shorter wavelengths (blue). Near-infrared (0.7-3.0 m) (from Sun): scattered less, but absorbed by water vapor, especially at 1.4 and 1.9 m, and by clouds Middle infrared (3 5 m) (from Sun and Earth) and thermal (3-5 infrared (>5 m) (from Earth): absorbed by clouds, water vapor, carbon dioxide, methane, ozone, and other "greenhouse" gases Some "windows" (3.5-4.0m and 10.5-12.5m) when no clouds 1 3 The simplest climate model--energy model-- balance with a non-absorbing atmosphere nonSolar radiation at top of atmosphere (MJ m2day1) Solar radiation absorbed by whole Earth = infrared radiation emitted by whole planet h l l t i.e., net all-wave radiation = 0 300 S0 = solar radiation = planetary average albedo F = infrared radiation T = planetary surface temperature So (1 - ) = F = T 4 4 S so, T = 0 (1 - ) 4 0.90-0.95, = 5.67108 1/ 4 250 Tem perature (K) 200 150 100 S0 = 1370 W m2 normal to Sun 50 Divide by 4 to average over Earth, 342.5 Wm2 Dingman, Figure 3-6 2 0 0 0.2 0.4 Albedo 0.6 0.8 1 Albedo 0.27-0.33 (see Charlson) Thus T 255K = 18C 4 ESM 203: Energy balance and atmospheric circulation 1 10/04/2007 Interpretation If the atmosphere didn't absorb radiation, the global average temperature should be about 18C 18 C. Near the surface, average air temperature is measured to be about 16C. The discrepancy must be due to the role of the atmosphere in absorbing energy and storing it near the surface. This interaction between solar radiation and the atmosphere begins the processes of energy transfer that create climate 5 Most of the atmospheric constituents that absorb outoutgoing long-wave radiation long(relatively large asymmetric molecules) although natural, are augmented by pollutant gases. If we change these concentrations, expect more outgoing radiation to be absorbed and the atmospheric temperature to rise, especially in the lower parts of the atmosphere. Graedel, T. E. and P. J. Crutzen (1995) Atmosphere, Climate and Change 7 Variation of atmospheric temperature with elevation reflects absorption of radiation emitted from surface and absorbed by atmospheric gases < 0.1m absorbed by N2, O2, N, O < 0.2m absorbed by O2 O3 absorbs < 0.31 m and ~8 m Mean annual global energy balance for Earth's atmosphere > 0.31m warms surface, which radiates and warms atmosphere Graedel, T. E. and P. J. Crutzen (1995) Atmosphere, Climate and Change 6 Graedel, T. E. and P. J. Crutzen (1995) Atmosphere, Climate and Change 8 ESM 203: Energy balance and atmospheric circulation 2 10/04/2007 Harte's more realistic energy-balance model, energybut still 1-D (Homework 1) 1- (Homework 1) Sav=S/4 apSav Fu (1-)Fs Structure of energy balance models in general Top boundary They have compartments Energy (and mass) fluxes nergy into and out of each must balance Temperature affects some of the fluxes, so T can adjust to make them balance upper kuSav Fl 0.5L Tx,y,z lower klSav Fu H 0.5L W surface Fl 9 Layers (e atmosphere) e.g., Fluxes are: Radiative Convective (vertical) and advective (horizontal) Both sensible and latent Fs Surface (lower boundary) Conductive (not important in atmosphere) 11 Harte's 1-D climate model with atmosphere 1A simple model of this type allows us to anticipate the general nature of changes in atmospheric g p temperature if various controlling factors were to change e.g. solar radiation, albedo, or the absorbing capacity of the atmosphere caused by changes in concentrations of absorbing gases. Layered atmosphere, most infrared absorption in lower layer. y Some solar absorption in upper atmosphere. Sensible and latent heat from surface up into atmosphere. Latent heat estimated from global average of precipitation. We are concerned about such changes because we have come to recognize that surface albedo has changed due to regional-scale vegetation changes; there are feedback effects between climate and albedo because of snow and ice; several greenhouse gases have changed over recent Earth history. Also includes energy released from human activities, although negligible. This is a "steady state" model: no time element; in contrast with a transient model. 10 Lean 2005, Physics Today 12 ESM 203: Energy balance and atmospheric circulation 3 10/04/2007 We have discussed controls on global average temperatures, but what controls climate? Solar radiation Orbital controls L i d Latitude Variability in net radiation (http://cimss.ssec.wisc.edu/wxwise/homerbe.html) http://cimss.ssec.wisc.edu/wxwise/homerbe.html) Aerosols Albedo Ab Absorption and scattering of i d i f solar radiation Condensation nuclei Clouds Albedo Atmospheric emissivity Absorption and scattering of solar radiation Land surface Albedo Evaporation Temperature Atmospheric composition (water vapor, CO2, CH4) vapor Cryosphere Albedo Water storage Absorption of solar radiation Atmospheric emissivity Oceans Albedo Evaporation Energy transfer by ocean currents & vertical mixing 13 Which ones do humans alter? Variability in planetary albedo (http://cimss.ssec.wisc.edu/wxwise/homerbe.html) http://cimss.ssec.wisc.edu/wxwise/homerbe.html) (Karl & Trenberth 2003) 14 ESM 203: Energy balance and atmospheric circulation 4 10/04/2007 Cloud effects on albedo and net radiation (http://cimss.ssec.wisc.edu/wxwise/homerbe.html) http://cimss.ssec.wisc.edu/wxwise/homerbe.html) Effects of heating the atmosphere (mainly from below) The density of the atmosphere depends on temperature, water vapor content, and pressure Heating and evaporation of water from surface lower air density relative to surrounding air and cause air to rise Denser air moves in below the rising low-density air (lowpressure air since pressure is the force due to the overlying co u oc u t ov y g column of air) Spatially unequal heating causes air to rise in some places and to descend in other, cooler places Relate to your Santa Barbara experience Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 19 But the picture is not static --- Radiation imbalance varies with latitude: it drives circulation of the atmosphere and latitude: ocean, ocean, which reduce these latitudinal differences excess Force balances, hydrostatic: if only the pressure-gradient force and gravity were acting -- a good pressureapproximation locally, such as over Santa Barbara region Hydrostatic: pressure equals weight of air above a in warm air, pressure decreases more slowly with m = molar mass of air, R = gas constant height, so warm (low pressure) areas at surface are locally high pressure areas aloft Pm dP = -g = -g * a dz RT inversely related to T Absorbed solar radiation Emitted infrared radiation deficit (Lower P) pressure gradient (Higher P) (Higher P) pressure gradient (Lower P) 0 30 latitude 60 90 Water (cool) 18 Land (warm) 20 ESM 203: Energy balance and atmospheric circulation 5 10/04/2007 Effects of spatially variable heating on a uniform, non-rotating Earth nonHeating at equator and cooling near poles would cause a single convection cell in the atmosphere if Earth were covered with a uniform surface and if Earth did not rotate Coriolis force Displacement of the air parcel is to the right in N hemisphere appears to be subject to a [Coriolis] force that increases with increasing latitude and air speed Using same reasoning, imagine the effect on air moving south (toward faster rotating surface) in the hemisphere i th N h i h In the S hemisphere, the sense of the force is reversed (i.e. to left) whether moving north or south Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 23 Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 21 Movement across Earth's rotating surface A substance that moves across Earth's rotating surface moves from a place where the planet is rotating at one h th l t i t ti t speed to a position where it is rotating at a different speed (measured in m/s, not in radians/s or deg/s) Ignoring friction, if an air parcel moves directly north in N hemisphere, it begins with a faster W E velocity than does the place it is heading for. Also, as it moves "inward" relative to Earth's axis of rotation the air's rotation speeds up because its angular momentum (mr, where is angular velocity) must remain constant., like a spinning skater. Coriolis force If air is moving due E or W, it still moves to the right in the N. hemisphere. The reason is a little more complicated than the N-S movement, -- it has to do with the component of the centrifugal force that acts parallel to the surface being aligned to the right of the initial motion in N. hemisphere, etc. h i h t The devoted student may wish to consult a textbook of atmospheric science or oceanography, or .... Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 24 From the point of view of an observer on the surface, the air appears to move to the right Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 22 ESM 203: Energy balance and atmospheric circulation 6 10/04/2007 Coriolis force: magnitude and direction Coriolis force on a rotating disk 87% N Maximum at Poles 60N Definitions: velocity of rotation v = r, = angular velocity (rad sec-1) r = radius of rotation angular momentum = vr = r2 r r0 50% Right in Northern Hemisphere No effect Left in Southern Hemisphere H h 50% 30N 0 Conserving angular momentum with change r=r0-r requires r02 = (+)r2 = (+v/r)r2, where v is relative velocity caused by r Solving, v = (r02/r - r) 25 30S 87% S 60S Effect of latitude, Coriolis force on a sphere N Coriolis force and cooling of air raised at equator disrupt the simple circulation of a non-rotating Earth nonAir rising at Equator moves N and increasingly to E while cooling (densifying) By time it has reached ~30N, some of it sinks and flows back along surface to S (and therefore W) and to N (E) Remainder of air aloft continues towards pole, where it sinks and flows S (W) meeting the NEflowing air at the surface Reverse in S hemisphere Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 26 28 true direction of Coriolis force Equator latitude the horizontal (only important) component, proportional to sin(latitude) ESM 203: Energy balance and atmospheric circulation 7 10/04/2007 Schematic zonal circulation is complicated by unmixed boundaries between cold and warm air, creating `fronts' Simple zonal picture of pressure distribution is complicated by... Seasonal changes in solar heating of continents and g oceans Distribution of continents Why are interiors of continents alternately locations of relatively high and low pressure? Note low pressures that drive monsoonal flow in India, Africa, and SW US. 29 31 Simplified zonal pattern of surface winds Where winds converge, air must rise, and thus pressure is lowered Where winds diverge, they must be supplied by sinking air, and the pressure must be relatively high Surface pressure, January and July From Columbia University January 1000 mb height July 1000 mb height Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 30 32 ESM 203: Energy balance and atmospheric circulation 8 10/04/2007 Force balances, geostrophic Gradients of pressure (the pressure-gradient force) drive air flow Geostrophic: balance between pressure g p p gradient and Coriolis forces where friction is negligible (aloft) Forces on air parcel in atmosphere Gravity: g (force per unit volume is density of air gravitational acceleration) Pressure gradient force: spatial variability in air pressure, from high to low, =P/distance Coriolis force: right in northern hemisphere, left in southern, magnitude depends on latitude (zero at equator) and wind speed Friction: small except near surface Centrifugal force: where winds are turning rapidly, such as in a hurricane Wind speed and direction balance all these forces. geostrophic wind blows parallel to isobars Geostrophic + friction at surface, wind slowed by friction,is not parallel to isobars but still moves to right of PGF direction F 33 35 Geostrophic winds in upper atmosphere flowing around the high- and low-pressure cells highlow- Links to some animations showing circulation Winds and clouds (NCAR) Water vapor and precipitation (NCAR) (long) Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 34 36 ESM 203: Energy balance and atmospheric circulation 9 ...
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This note was uploaded on 08/06/2008 for the course ESM 203 taught by Professor Dozier,dunne during the Fall '07 term at UCSB.

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