Radiation Basics and the General Circulation SO 254 Spring 2017 Earth-Sun Geometry The earths axis is tilted at an angle of (why we have seasons) Boreal (N.H.) Winter Solstice (~21 Dec) Sun directly overhead Tropic of Capricorn Sun never rises above Arctic Circle Sun never sets below Antarctic Circle Tropic of Vernal Equinox (~20 March) Sun directly overhead equator The term Austral is used in the S.H. Borealreference Summer to Solstice (~21 Jun)
(the is Tropic concurrent SunAustral directlysummer overhead of Cancer with the Boreal winter) Sun never sets above Arctic Circle Sun never rises below Antarctic Circle Cancer (~ N) 23.5 Arctic Circle (~ N) Tropic of Capricorn
(~ S) Antarctic Circle (~ S) Autumnal Equinox (~22 September) Sun directly overhead equator Solar radiation (in brief) Radiation from the sun may be characterized by its equivalent blackbody temperature which, in accordance with Plancks Law of blackbody radiation, determines its spectrum. The empirically derived Planck function is: = 1 5 ( 2 / 1) emittance or flux per unit wavelength ()
wavelength () where 1 absolute temperature () constants The spectrum for a particular temperature is illustrated by its Planck curve which plots emittance as a function of wavelength How mathematically would you determine the wavelength where is a maximum? Differentiate the Planck function with respect to , set the derivative equal to zero, and solve for =0 Solar radiation (in brief) How mathematically would you determine the wavelength where is a maximum? Differentiate the Planck function with respect to , set the derivative equal to zero, and solve for This process yields Weins Law: max = where
Solar radiation is concentrated in the visible spectrum (). What would be an estimate for the solar surface temperature? 5780 K (this is pretty close) =0 Solar radiation (in brief) How mathematically would you find the total emittance (or flux density) from the sun at all wavelengths? Integrate over all values of This produces the Stefan-Boltzmann law: = 4 Where is the Stefan-Boltzmann constant
Solar radiation received at the Earth (primarily visible) is referred to as shortwave radiation or INcoming SOLar radiATION (insolation) Solar emittance attenuates through spreading loss as it travels to earth approximately via a formulation of the inverse square law Solar radiation (in brief) The flux density at any two distances from a point source is given by: 1 12 = 2 22 Calculate the solar flux density reaching the orbital radius of Earth () given a solar surface temperature of and solar radius of Flux density decreases proportionally with the inverse of the square of the distance from the source 1 2
From the Stefan-Boltzmann law, (at the solar surface) is Thus (at Earths orbit) is Terrestrial radiation (in brief) Some incoming solar radiation is reflected back into space vice being absorbed by the Earth system The ratio of reflected to incoming solar radiation is called albedo and is given by: = Albedo varies by surface: Clouds, ice, and snow are particularly good reflectors Global average albedo is about 30% Terrestrial radiation (in brief) Given the average Earth albedo of 30%, we can calculate the equivalent blackbody temperature of the Earth if we assume radiative equilibrium (i.e., no net gain or loss in energy due to radiative transfer)
Solar flux density: Solar radiation is intercepted over the area and terrestrial radiation is emitted over the area ( radius of the Earth) Using Stefan-Boltzmanns law: Here, Solving for gives: As is in the infrared, terrestrial radiation is referred to as longwave radiation h Surface Radiation Budget The net radiative flux at a point on Earths surface has contributions from: Shortwave radiation (insolation) Atmospheric longwave radiation Reflected shortwave radiation Terrestrial longwave radiation = + + + Typical diurnal cycle (fluxes are positive upward)
night night Surface Radiation Budget Lower sun angle What accounts for the difference here? The surplus of incoming solar radiation over outgoing longwave radiation at low latitudes and the deficit at high latitudes results in differential heating This process drives the global-scale general circulation of winds Terminology subpolar extratropical
zonal subtropical meriodonal mer io don al polar tropical zonal Lines of latitude are parallels 6 East/West winds are zonal winds 30N Lines of longitude are meridians North/South winds are meriodonal
winds 0 low latitudes 60 W 90 W mid-latitudes high latitudes Terminology Local maxima in the pressure field are high pressure centers or highs (H) Local minima in the pressure field are low pressure centers or lows (L) H pressure gradient force L The applied pressure gradient force causes wind to blow from high to low pressure though other forces deflect air motion to varying degrees On larger scales, the rotation of the Earth imparts a significant deflection on winds
Northern Hemisphere H + Terminology Anticyclonic circulation (clockwise) Around low pressure, winds circulate cyclonically (in the same sense as Earths rotation looking down on the pole) Cyclonic circulation (counterclockwise) Areas of low pressure are also referred to as cyclones NP L Southern Hemisphere H
+ SP L Anticyclonic circulation (counterclockwise) Cyclonic circulation (clockwise) Around high pressure, winds circulate anticyclonically (opposite the sense of Earths rotation looking down on the pole) Areas of high pressure are also referred to as anticyclones General Circulation aqua Earth (sun overhead equator) Differential heating causes rising motion within a few degrees of the equator H H Ha d
H ley L L L L H a d le H y H H This promotes surface low pressure and equatorward flow at low levels which is deflected westward by Earths rotation
Rising air encounters the tropopause where it is inhibited from further rising by strong static stability in the stratosphere Rising air diverges poleward, is deflected eastward by Earths rotation, and sinks in the subtropics promoting surface high pressure and closing the loop These mirror-image cells are called Hadley cells General Circulation aqua Earth (sun overhead equator) Pol a Surface flow spreading poleward out of the descending branch of the Hadley cell rises again at higher latitudes where it subsequently diverges r rre Fe l Ha d H
ley L H a d le y H This process forms a mid-latitude Ferrel cell which has a vertical circulation counter to the Hadley cell and a highlatitude polar cell F e r r e l ar Pol General Circulation aqua Earth (sun overhead equator)
e d tra I T wi At the surface, the low-level winds of the Hadley cell called trade winds converge heat and moisture where they meet along the intertropical convergence zone (ITCZ) s d n C Z Surface winds along the ITCZ are generally light (doldrums) As this air rises, it cools and condenses moisture forming clouds and precipitation The sinking air in the descending branch
of the Hadley cell, on the other hand, is characteristically dry and forms subtropical highs where surface winds are also generally light (horse latitudes) Surface Circulation aqua Earth (sun overhead equator) H Polar easterlies L H L H H H L Subpolar low Westerlies L
L H H L L H Subtropical high NE trade winds ITCZ SE trade winds Subtropical high Westerlies Subpolar low Polar easterlies At the surface in the midlatitudes, winds vary in direction with the passage of extratropical cyclones which generally move eastward in a prevailing westerly flow (the westerlies) Under the rising branch of the
Ferrel cell are subpolar lows Near each pole is a climatological polar high Between the polar high and the subpolar lows is a region of winds called the polar easterlies Upper-level Circulation aqua Earth (sun overhead equator) L lar o p jet subtropical H jet H H jet
subtropical po lar L jet In the upper-troposphere, easterly winds and high pressure prevail above the ITCZ whereas westerly winds prevail elsewhere A region of strong westerly winds called the subtropical jet overlies the descending branch of the Hadley cell An additional polar jet is present at higher latitudes and supports Rossby waves which arise from instabilities in the flow A polar low is present at each pole in the upper-troposphere General Circulation aqua Earth (Boreal summer) Max insolation is displaced into the summer hemisphere and the Hadley cells become asymmetric as the ITCZ migrates northward (~ latitude) The winter hemispheres Hadley cell
becomes the major cell with stronger circulation due to the greater zonal temperature contrast The vigorous circulation in the major cell acts to balance extreme temperature contrasts by transporting significant heat away from the tropics The setup is reversed in the Austral summer General Circulation real Earth polar easterlies L westerlies H trade winds In the North Atlantic, these are known as the Azores (or Bermuda) high and the Icelandic low The high is most discernable in summer and the low is strongest in winter Over the oceans, surface winds are very similar to aqua Earth The subtropical high pressure belt, however, is not continuous but forms
distinct subtropical anticylones centered over the mid-oceans These carry (or advect) cooler, dryer air equatorward on the eastern side of the ocean basins and advect warmer, more humid air into the mid-latitudes on the western side The subpolar low pressure belt likewise forms distinct mid-ocean cyclones General Circulation real Earth H H H In the Indian Ocean basin the presence of landmasses has a pronounced influence on observed wind circulations In the boreal summer, intense heating over Asia (relative to the tropical ocean) causes ascent and disrupts the northern Hadley cell circulation eliminating the subtropical anticyclone In the boreal winter, the tropical ocean is
warm (relative to cooling over Asia) causing the pattern to reverse H This seasonal reversal of surface winds is called the monsoon circulation General Circulation real Earth Surface winds Surface winds Which of these profiles represents the December- JanuaryFebruary (DJF) average of global surface winds? December-January-February (DJF) averages Aleutian low Icelandic low Indian
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