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The Atmospheric Circulation System Geos 110 Lectures: Earth System Science Chapter 4: Kump et al 3 rd ed. Dr. Tark Hamilton, Camosun College.

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Presentation on theme: "The Atmospheric Circulation System Geos 110 Lectures: Earth System Science Chapter 4: Kump et al 3 rd ed. Dr. Tark Hamilton, Camosun College."— Presentation transcript:

1 The Atmospheric Circulation System Geos 110 Lectures: Earth System Science Chapter 4: Kump et al 3 rd 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 Ideal Gas Law: 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) P initial V initial = P final V final PV has units of work e.g. F/d 2 x d 3 = F x d At constant E, a P increases V decreases Special Case 2 – Charles’ Law: P=constant) V initial / T initial = V final / T final

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

5 There are Big Latitudinal Differences IR emission doesn’t match? How does heat move? The Tropics have a Net Radiation Surplus (S in >E out ) The Poles run a Net Radiation Deficit (S in

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

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

8 Weather & Climate Vary Across the Globe Wind & Ocean Currents Redistribute Solar Heating Solid Earth processes buffer CO 2 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. Fig 4.4

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

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

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 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 Fig 4.6

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

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 Spanish ships bound for the New World became becalmed w. Hi Pressure, no wind and Horses died English “Dead Horse Shanty”, working off advance 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 manWe'll hoist him up to the main yardarmSay, 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

16 Idealized Tropospheric Circulation ITCZ & Polar Front Storm Belts – Hi Precipitation Horse Latitude & Polar Deserts

17 A Simple Pressure Model for Winds 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 Fig 4.8

18 Coriolis Rotational Effects on a Sphere Since the Earth revolves once a day…. Bantu’s and Guajiran’s move a lot faster and further Than Innu or Lapplanders! Fig 4.9a 4.64 m/s 0 m/s

19 Apparent Wind Deflection to the Right N in N.Hem. (rotating reference frame) 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 Fig 4.9b The curved paths are relative to fixed points on the ground which revolves.

20 Coriolis (Centrifugal) Force acts on East or West moving Winds (increasing w/Latitude) 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 Fig 4.10

21 A More Realistic Model for Surface Winds: Pressure Differences, Buoyancy & Coriolis Effects The same divergence & convergence zones are shown Coriolis force effects are shown Permanent Peri-equatorial Trades & Winter Polar Easteries Fig 4.11 Tropic of Cancer 23.5°N Tropic of Capricorn 23.5°S ~1 Season Big Seasonal Changes

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 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 Box Fig 4.1

24 Causes of Tropical Cyclones 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 Box Fig 4.1

25 Extratropical Cyclones 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 Box Fig 4.1

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

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

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

29 Mid-latitude Upper Level Jet Streams 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 Fig 4.12c  Flow 

30 Geostrophic Wind 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) Fig 4.13

31 Friction acts near surface at High Pressure Slows and deflects wind < 90° from coriolis Causes winds to spiral in cyclonic storms Fig 4.13 Fig 4.20

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

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

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 ITCZ shifts to the summer hemisphere side of the Equator and the weaker circulation cells shift Polewards Discreet Subtropical Highs mark descending Hadley Cells Fig 4.16

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

37 Coastlands heat by day creating Low Pressure This “sets-up” Onshore Adiabat winds As denser High Pressure Cool Air flows in to replace 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!

38 Land cools faster than sea, less water & thermal mass Cool high pressure air falls on land & flows to the Bay Fig 4.17b

39 Continentality: Land Heats & Cools Faster than the Ocean: Winter in North January Isotherms deflect Southwards in Northern Hemisphere & in the Southern Hemisphere too! Fig 4.18a Greenland & Siberia hit -48°C While Australia, Madagascar & Brazil pass +24°C The Thermal Equator shifts to about 10°South

40 Continentality: Land Heats & Cools Faster than Ocean: Summer up North July Isotherms deflect Northwards in Northern Hemisphere & in the Southern Hemisphere too! Fig 4.18b 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

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

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

43 Average Sea Level Pressure July 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! Fig 4.19b

44 Wind 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

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

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

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 Fig 4.21b

48 The Hydrologic Cycle – Blue Planet Latent Heat: Ocean, Clouds, Ice Caps H 2 O 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 Fig 4.22

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

50 Fig 4.24

51 Vapour Pressure – Precipitation, Evaporation P total = P N2 + P O2 + P Ar + P H2O + P CO2 + P trace gases P H2O = Vapour Pressure, varies with T°C & Water Condensation H 2 O (L)   H 2 O (V) Evaporation Sat’n Fig 4.24

52 Global Hydrologic Cycle Approximately balanced +/- ice/snow storage on land Evaporation puts latent heat into atmosphere Precipitation leaves latent heat in atmosphere Fig 4.24b Why is Atmospheric water load only 13x10 12 m 3 if the evaporation & precipitation are 5 to 20 x this big?

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 Vapour

54 Fig 4.26

55 Global precipitation on Land: January Fig 4.26a

56 Global precipitation on Land: July Fig 4.26b

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

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