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The Atmospheric Circulation System

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1 The Atmospheric Circulation System
Cumulonimbus “Thunderhead” Storm Cloud with Anvil Flatiiron Top Lots of vertical convection and mixing Usually Vapour, liquid droplets and ice crystals coexist in this mixed dynamic system Geos 110 Lectures: Earth System Science Chapter 4: Kump et al 3rd ed. Dr. Tark Hamilton, Camosun College

2 Overall the Earth’s Climate is in Balance
In Balance Kind-of: But you have to average over night and day It helps to average for many seasons or years And we need to overlook trivialities like burning all of Earth’s fossil Carbon from the past ~350 Ma in < 3 centuries! However: Region to region there are hot and cold spots, wet and dry places, rain forests and deserts, mountains and plains, seas and glaciers, tropics and polar climes & a whole lot of weather!

3 The Ideal Gas Law: Relationships of Pressure, Temperature, Volume & Moles
P V = n R T; P=Pressure, T=Temperature, n=moles of gas particles (with mass), R=ideal gas constant Special Case 1 – Boyle’s Law: T=constant) Pinitial Vinitial = Pfinal Vfinal PV has units of work e.g. F/d2 x d3 = F x d At constant E, a P increases V decreases Special Case 2 – Charles’ Law: P=constant) Vinitial / Tinitial = Vfinal / Tfinal

4 There are Big Latitudinal Differences
Fig 4.1 The Tropics have Energy Surplus The Poles run a Deficit Temperate zones have transitory seasonal swings

5 There are Big Latitudinal Differences IR emission doesn’t match
There are Big Latitudinal Differences IR emission doesn’t match? How does heat move? Fig 4.2 The Tropics have a Net Radiation Surplus (Sin>Eout) The Poles run a Net Radiation Deficit (Sin<Eout) Temperate zones have transitory seasonal swings

6 There has to be a Global Circulation System
Fig 4.3 Ferrel Cell Ferrel Cell IR conversion to Latent Heat (LiquidVapor) Convection driven by density and pressure differences between different air masses

7 Convergent versus Divergent Winds at Earth’s Surface
Rising light warm air of the Tropic Lows is replaced laterally by denser air flowing in from higher latitudes & converges towards the ~Equator This position changes seasonally by ~5° of Latitude Descending cold dense air from the Horse Latitude Temperate Highs hits the Earths surface and gently diverges This position is fixed by the stable Tropopause The troposphere is well mixed by convection and is heated from below. The overlying stratosphere is warmer, heated from above and puts a lid on this mixing 7

8 Weather & Climate Vary Across the Globe
Fig 4.4 Wind & Ocean Currents Redistribute Solar Heating Solid Earth processes buffer CO2 levels by weathering rocks over few hundred Ka to Ma Eddies on all spatial & temporal scales prevent the heat redistribution from being complete or even. Very long term ~100’s of Ma heat redistribution in the core and mantle affect the shapes and positions of continents and ocean basins via solid state convection and plate dynamics.

9 Eastern Pacific & Central America w/ ITCZ Intertropical Convergence Zone
Fig 4.5 NOAA Satellite Image Cloud Band marks ITCZ at top of Troposphere The Troposphere, heated from below convects

10 Convective Towers Cumulonimbus drive Hadley Cells of ITCZ
Fig 4.5 Cloud Band marks ITCZ at top of Troposphere Solar Evaporation & Latent Heat from Condensation make the heat pump that drives the Convection

11 Horizontal & Vertical Air Movements result from Temperature & Pressure differences driving Buoyancy
Buoyancy is due to density contrasts, Δmass/volume Fast molecules, more collisions more F/A = Pressure Temperature increase  Pressure increase Pressure increase  Volume increase, buoyancy Air columns heated from below expand and rise Other denser air moves in laterally to replace it Cooling upper Troposphere cools air shrinks & sinks

12 Mid-latitude Convective Mixing
Fig 4.6 This calls for steep lateral temperature gradients and abrupt storm fronts with all kinds of wind and weather blowing through, especially at the polar front zone ~ 60°N & S. Cold fronts descend from higher latitudes Replacing/passing beneath tropical warm fronts This rapid mixing of air masses is an ever changing recipe for weather

13 N-S Meridional Mixing of Troposphere
Fig 4.7 Tropics to Horse Latitudes - Hadley Cells Mid-Latitudes – Ferrel Cells High Latitude - Polar Front 13

14 Hadley Cells Individual atmospheric cells
Between the Equator and 30-35° N Over the ocean in Atlantic and Pacific Driven by heat from below absorbed by ocean The rocky planet rotates faster than the atmosphere Hadley Cells are broken up by continents

15 The Horse Latitudes The Dead Horse Shanty Oh, poor old man your horse will die And we say so, and we know so Oh, poor old man your horse will die Oh, poor old man We'll hoist him up to the main yardarm We'll hoist him up to the main yardarm Say, I old man your horse will die Say, I old man your horse will die We'll drop him down to the depths of the sea We'll drop him down to the bottom of the sea We'll sing him down with a long, long roll Where the sharks'll have his body and the devil have have his soul Spanish ships bound for the New World became becalmed w. Hi Pressure, no wind and Horses died English “Dead Horse Shanty”, working off advance

16 Idealized Tropospheric Circulation
Rising moist air cools and engenders rain or snow, especially Tropical Lows Descending cold dry air dessicates underlying landscapes making deserts & drylands, especially under Polar Highs. This idealized image would work best on an all water covered world or at least one with longitudinal oceans! If you are a convecting fluid or a fish, land kind of gets in the way! ITCZ & Polar Front Storm Belts – Hi Precipitation Horse Latitude & Polar Deserts 16

17 A Simple Pressure Model for Winds
Fig 4.8 Winds blow out of descending High Pressure limbs ~30-35° N&S between Hadley & Ferrel Cells Winds blow towards rising Low Pressure limbs on equatorial edge of Hadley Cells at ITCZ & also PCZ 17

18 Coriolis Rotational Effects on a Sphere
0 m/s Fig 4.9a N-S motions are deflected in apparent curved paths due to the rotating reference frame. E-W motions are subjected to centrifugal force due to the velocity of rotation and momentum mv 4.64 m/s Since the Earth revolves once a day…. Bantu’s and Guajiran’s move a lot faster and further Than Innu or Lapplanders! 18

19 Apparent Wind Deflection to the Right N in N. Hem
Apparent Wind Deflection to the Right N in N.Hem. (rotating reference frame) Fig 4.9b The curved paths are relative to fixed points on the ground which revolves. This is the same right wise rotation for S flowing air in the northern hemisphere This is the opposite way, leftwards deflection for S flowing air in the Southern hemisphere While the Earth revolves from AA’ & BB’ The N flowing Air moves from P1  X, This is really in a straight line viewed from Space 19

20 Coriolis (Centrifugal) Force acts on East or West moving Winds (increasing w/Latitude)
Fig 4.10 This Coriolis effect increases towards higher latitude due to decreasing rotational velocity! It is at its maximum at the poles and zero at the equator. This is like the NW Trade Winds in the Northern Hemisphere or the opposite, The SE Trade Winds in the Southern Hemisphere A Vector with 2 components in a plane defined by the spin axis and the location on the Earth’s surface 1 Component is vertical, 1 horizontal-tangent away In N Hem. E moving wind deflects Right to South 20

21 A More Realistic Model for Surface Winds: Pressure Differences, Buoyancy & Coriolis Effects
Fig 4.11 Big Seasonal Changes Tropic of Cancer 23.5°N ~1 Season Tropic of Capricorn 23.5°S Winds are names by where they blow from rather than where they blow to. This works for Sailors and Meteorologists as they really need to pay attention to where the weather is coming from. In detail the divergence zones are weak and wind speed and direction vary considerably on a day to day basis. Big Seasonal Changes The same divergence & convergence zones are shown Coriolis force effects are shown Permanent Peri-equatorial Trades & Winter Polar Easteries 21

22 High Pressure Systems tend to be Localized
Descending limbs of Hadley-Ferrel Cells in Mid latitudes tends to be fixed Trade Winds blow from the Equator-ward side of these Sub-Tropical Highs Temporary passing fronts of High or Low pressure form near the edge of the Polar Front affecting these ~1000 km wide Low Pressure systems form from T° gradients and convective winds in upper troposphere Inwards directed wind deflects to right in Northern hemisphere (Cyclonic Flow) Outwards directed flow from Highs creates Anticyclones

23 Tropical Cyclones: Hurricanes & Monsoons
Box Fig 4.1 Centripetal acceleration is experienced by objects rotating at constant tangential speed. The acceleration is inwards along the radius vector of rotation at ω2 r. This and the pressure gradient and the coriolis force all interact to determine cyclone wind speed and rotational velocity. The Circle is an Isobar = line of constant pressure High Pressure winds deflect to the right Hurricane flow is set by P gradient & Centripetal Acceleration* Cyclonic storm rotate counterclockwise in North hemisphere Cyclonic storms from at 26-27°C & > 5° Latitude from the Equator 23

24 Causes of Tropical Cyclones
Box Fig 4.1 Low Vertical Wind Shear or the storms tear apart as they build Maximal humidity in lower Troposphere, builds latent heating Steep vertical thermal gradient, promotes upwards buoyant convection Initial atmospheric disturbance from ordinary Trade wind flow: old frontal boundaries, easterly waves (off Africa or S. Pacific), usually late summer & fall when ITCZ is furthest from equator Centripetal acceleration is experienced by objects rotating at constant tangential speed. The acceleration is inwards along the radius vector of rotation at ω2 r. This and the pressure gradient and the coriolis force all interact to determine cyclone wind speed and rotational velocity. 24

25 Extratropical Cyclones
Box Fig 4.1 From outside the tropics > 23.5° N or S latitude Flow of Warm air from Equator hits cold air from High Latitudes These air masses do not mix well so Warmer less dense humid air rises above a cold front Lots of Mid-latitude rain or snow Lots of daily weather variations due to transient fronts While polar easterlies are stable all winter, in summer this flow breaks down and the storms can migrate to the Arctic. 25

26 Flow of Troposphere Fig 4.7 Surface Flow is dominated by latitudinal belts  Upper Level Flow is Dominantly Polewards! 26

27 Upper Level Tropospheric Flow
Fig 4.12a The troposphere is warmer and thicker in the tropics & Colder and thinner at the Poles 27

28 Tropospheric Pressure Surfaces
Fig 4.12b Tropics more expanded & < vertical pressure gradient Poles are more compressed w/ > vertical pressure gradients

29 Mid-latitude Upper Level Jet Streams
 Flow  Fig 4.12c At any elevation there is Hi P towards the Equator Flow naturally moves from High to Low Pressure These control the paths of Low Pressure Storms 29

30 Geostrophic Wind Fig 4.13 There are also curved Geostrophic currents from storm set up across broad shallow continental shelves. Pressure Gradient decreases upwards (less mass) Coriolis Force decreases downwards, net Geostrophic Right/Left flow Centrifugal & Centripetal Forces contribute around Lows/Highs (Similar curved flow occurs across mid latitude continental shelves) 30

31 Friction acts near surface at High Pressure
Fig 4.20 Fig 4.13 There are also curved Geostrophic currents from storm set up across broad shallow continental shelves. Slows and deflects wind < 90° from coriolis Causes winds to spiral in cyclonic storms 31

32 Height of the 300 mb Geopotential Surface in January (Winter N. Hem.)
Fig 4.14 As per the previous 3 figures, this show the Polar Low & Equatorial High 32

33 Seasonal Variation in Insolation
Fig 4.15 Perhelion Aphelion Obliquity (tilt) affects vertical incidence & heating More than Eccentricity (elliptical orbit) At Spring-Fall equinoxes Sun is Overhead 33

34 The Analemma & Equation of Time
The maximum noon shadow And Elevation of the Sun trace out the Figure of 8 or Analemma over the year. More heat at top and less at bottom

35 Seasonal Migration of Atmospheric Circulation Patterns
The circulation is weaker on the Summer hemisphere because of less thermal gradient Fig 4.16 The ITCZ shifts to the summer hemisphere side of the Equator and the weaker circulation cells shift Polewards Discreet Subtropical Highs mark descending Hadley Cells 35

36 Diurnal Wind Changes on Arid Coasts Strong Onshore breeze by day Weak Offshore breeze by night
Ships sail in by day and out by night 36

37 Coastlands heat by day creating Low Pressure
Fig 4.17a Water has 3-4X the heat capacity of dry land. 1cal/g°C Counterintuitively, this makes the land heat 3-4 times faster than the sea! Land, especially dark or forested land heats quickly but transfers this heat up by evaporation and convection. This sets up small thermal lows, ideal for birds along the coast. Oceans with lower albedo heat better at the same latitude. They are also more transparent than rock or dirt so they absorb heat below the surface too, but thy also transfer this heat both ways by convection in the top of the sea and convection of overlying air. Especially large flat layers like shallow bays have high Renyolds’ numbers favouring turbulent mixing downwards. This and the larger thermal mass of the ocean makes for very small temperature changes. Great for sailboarding! Coastlands heat by day creating Low Pressure This “sets-up” Onshore Adiabat winds As denser High Pressure Cool Air flows in to replace 37

38 Land cools faster than sea, less water & thermal mass
Fig 4.17b As the weak daytime lows on shore collapse by rapid cooling, the air falls bringing high pressure down especially along arid coastlines. The Ocean cools relatively little so it now gets to play the low as weak breezes flow out to sea. Land cools faster than sea, less water & thermal mass Cool high pressure air falls on land & flows to the Bay 38

39 Continentality: Land Heats & Cools Faster than the Ocean: Winter in North
Fig 4.18a Greenland & Siberia hit -48°C While Australia, Madagascar & Brazil pass +24°C The Thermal Equator shifts to about 10°South This deflection of the isotherms at sea makes the Northern Continents colder and the Southern continents warmer. This is because in the North the cold of the seas is felt preferentially further south (arctic fronts), weather from the Gulf of Alaska. By contrast the tropic equatorial air bends further south across the southern lands warming them and giving them summer. The interior of Antarctica warms to a balmy -30°C! January Isotherms deflect Southwards in Northern Hemisphere & in the Southern Hemisphere too! 39

40 Continentality: Land Heats & Cools Faster than Ocean: Summer up North
Greenland & Siberia hit a “balmy” +12°C While Australia, Johannesburg & Brazil dip to a “frigid” +12°C Much smaller climate variation in the southern hemisphere, zonal air flow over southern oceans. The Thermal Equator shifts to about 10°North This deflection of the isotherms at sea makes the Northern Continents warmer and the Southern continents colder. This is because in the North the warmth of the tropic seas is felt further north inland. Mild summer southerly breezes. By contrast the cold southern seas from the Antarctic convergence zone lend cold air on land. cooling the southern lands and giving them winter. The interior of Antarctica chills like a step mother’s breath to -66°C! Polar and Continental is seemingly a real recipe for an ice age no matter what the sun and earth are doing. Fig 4.18b July Isotherms deflect Northwards in Northern Hemisphere & in the Southern Hemisphere too! 40

41 Annual Temperature Difference Between Summer and Winter
Fig 4.18c Bumpy  Flat  The Tropics and Southern Oceans Experience little ΔT While the Southern Continents get a little more Northern Continents & Oceans Get more Δ T 41

42 Average Sea Level Pressure January
Fig 4.19a Pressure in mbar, 1 atm = bar 2 Belts of Highs +/-30° from Equator & ITCZ Lows at 60-70°South 42

43 Average Sea Level Pressure July
Fig 4.19b Northern Belts of Highs +40° from Equator & ITCZ Southern Belts of Highs -25° from Equator & ITCZ Lows still at 60-70°South & Cape Stiff Blows! 43

44 Wind Field @ 00Z Aug 1, 1999 Radar Satellite Data
ITCZ ~ 10°N of equator Where NE & SW trade winds converge Left curves to south & right to north Subtropical highs as spirals Fig 4.20 Which way does the wind blow at the equator? So which way should you sail around the world? 44

45 Reversing Monsoon Flow
Summer High over Tibetan Plateau but Winter Low Weather in many continental sites changes seasonally 45

46 Summer Monsoons in India & Pakistan
Warm moist air from Highs over Indian Ocean Flow to replace the rising heated rising Lows on the Tibetan Plateau Since Miocene Uplift Began w/ collision of India & Asia and the final closing of the Tethys Ocean Fig 4.21a 46

47 Winter Droughts in India & China
Cool dense high pressure air falls onto Tibetan Plateau and out along major river valleys to the Indian Ocean Tropical heating of the ITCZ is deflected south Rains hit Java, Sumatra & Northern Territories Aus Western Austrailia is still Dry as a Bone! Fig 4.21b 47

48 The Hydrologic Cycle – Blue Planet Latent Heat: Ocean, Clouds, Ice Caps
H2O is #1 molecule 71% of Earth is Covered by Water ~3.8 km deep Polar ice caps Clouds are water condensed or xlz’d! T°Vap = 2260 kJ/Kg T°Fus = -335 kJ/Kg Latent heat of vapourization at 100°C and Latent Heat of Fusion is at 0°C. Fig 4.22

49 Water Drives Earth’s External Heat Engine
Fig 4.23 There is no other compound like it for heat capacity per weight or latent heats. Heat cycle = Water cycle 49

50 Fig 4.24 50

51 Vapour Pressure – Precipitation, Evaporation
Saturation vapour pressure = 100% relative humidity. Fig 4.24 Ptotal = PN2 + PO2 + PAr + PH2O + PCO2 + Ptrace gases PH2O = Vapour Pressure, varies with T°C & Water Condensation H2O (L)   H2O (V) Evaporation Sat’n 51

52 Global Hydrologic Cycle
Why is Atmospheric water load only 13x1012 m3 if the evaporation & precipitation are 5 to 20 x this big? There is a ~6000 year long thermal convection cycle in the oceans < 1% of water on land is in rivers and lakes, most is ground water to the amount of 5-30% by volume of the expanse of sediments and sedimentary rocks on the land surface There is a thermal convection advection cycle into and out of aquifers on land Fig 4.24b Approximately balanced +/- ice/snow storage on land Evaporation puts latent heat into atmosphere Precipitation leaves latent heat in atmosphere 52

53 Saturation Vapour Pressure for Water
Condensation happens on the Low T & Hi P side of the curve An air mass which is undersaturated can cool and start to rain An air mass which is saturated can warm and cause evaporation Fig 4.25 Liquid Liquid water which is decompressed or heated will boil. Geysers anyone? Vapour 53

54 Fig 4.26 54

55 Global precipitation on Land: January
Fig 4.26a 55

56 Global precipitation on Land: July
Fig 4.26b 56

57 Why do these occur where they do?
Deserts & Drylands Why do these occur where they do? Fig 4.27 57

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