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Global Scale Winds

By Anthony Morales,2014-05-07 17:33
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Global Scale Winds

    Global Scale Winds Global Scale Winds

    Chapter 7 Chapter 7

    Ancient Knowledge of Winds Ancient Knowledge of Winds Mariners have known since before Columbus that winds blow ? ? Mariners have known since before Columbus that winds blow easterly off of Africa out to the Atlantic Ocean easterly off of Africa out to the Atlantic Ocean

    These winds were called the trade winds ? ? These winds were called the trade winds

    Columbus used the trade winds to travel out to the New World ? ? Columbus used the trade winds to travel out to the New World ? In addition, mariners knew to avoid areas around 30? N/S ? In addition, mariners knew to avoid areas around 30? N/S Latitude where winds were light or calm Latitude where winds were light or calm

    ? This area was known as the horse latitudes ? This area was known as the horse latitudes

    Conceptual Models Conceptual Models

    ? A conceptual model is a simple representation of reality, while ? A conceptual model is a simple representation of reality, while still having all the major features needed still having all the major features needed

    ? Our conceptual model of the global circulation of the ? Our conceptual model of the global circulation of the atmosphere needs to include or explain: atmosphere needs to include or explain:

     The steady winds observed by mariners The steady winds observed by mariners

     The lack of winds in certain regions The lack of winds in certain regions

     Regions of low and high precipitation Regions of low and high precipitation

     The global pattern of cloudiness The global pattern of cloudiness

     The existence of jet streams The existence of jet streams

    Simplest Model Simplest Model

    What if the Earth had just one simple convection cell in each ? ? What if the Earth had just one simple convection cell in each hemisphere? hemisphere?

    Rising air would occur near the Equator where solar heating is ? ? Rising air would occur near the Equator where solar heating is strongest strongest

    ? Sinking air would occur at the Poles where the air is coldest ? Sinking air would occur at the Poles where the air is coldest Air would flow from the Poles to the Equator at the surface to ? ? Air would flow from the Poles to the Equator at the surface to replace the rising air at the Equator replace the rising air at the Equator

    ? Air would flow from the Equator to the Poles aloft to replace the ? Air would flow from the Equator to the Poles aloft to replace the air moving away from the Poles air moving away from the Poles

    Simplest Model Simplest Model

? Unfortunately, that fails to explain many surface features ? Unfortunately, that fails to explain many surface features

    Simplest Model Fails Simplest Model Fails ? For example, if the air flows from North to South in the Northern ? For example, if the air flows from North to South in the Northern Hemisphere, why do winds switch between the Trade Winds and the Hemisphere, why do winds switch between the Trade Winds and the Westerlies at mid-latitudes? Westerlies at mid-latitudes?

    The rotation of the Earth prevents this very simple model from existing ? ? The rotation of the Earth prevents this very simple model from existing The Coriolis Force causes air to deflect to the right in the Northern ? ? The Coriolis Force causes air to deflect to the right in the Northern Hemisphere Hemisphere

    Air is unable to make the entire trip from the poles down to the Equator ? ? Air is unable to make the entire trip from the poles down to the Equator before turning right and blowing parallel to the Equator before turning right and blowing parallel to the Equator

    The Conceptual Model of Global Circulation The Conceptual Model of Global Circulation ? It turns out, we need three cells in each hemisphere to ? It turns out, we need three cells in each hemisphere to account for all the weather features mentioned earlier account for all the weather features mentioned earlier ? One convection cell exists in the tropics, called the ? One convection cell exists in the tropics, called the Hadley Cell Hadley Cell

    ? Another exists in the polar regions ? Another exists in the polar regions

    ? In the mid-latitudes, a coherent convection cell does ? In the mid-latitudes, a coherent convection cell does not exist not exist

    Jet Streams Jet Streams

    ? A jet stream is a swift river of air found in the upper ? A jet stream is a swift river of air found in the upper troposphere troposphere

    ? Two are usually found in each hemisphere: ? Two are usually found in each hemisphere:

     Polar jet stream Polar jet stream

     Subtropical jet stream Subtropical jet stream

    ? Each jet stream is formed by different processes ? Each jet stream is formed by different processes

    Subtropical Jet Stream Subtropical Jet Stream

    ? The radius from the axis of the Earths rotation out to ? The radius from the axis of the Earths rotation out to the atmosphere is largest at the Equator the atmosphere is largest at the Equator

    ? As one goes towards the Poles, that radius decreases ? As one goes towards the Poles, that radius decreases ? At the Pole, the radius shrinks to zero ? At the Pole, the radius shrinks to zero

    ? As air moves poleward, the air has its radius decrease ? As air moves poleward, the air has its radius decrease from the axis from the axis

    Conservation of Angular Momentum Conservation of Angular Momentum

    Angular momentum = the quantity of mass times rotational ? ? Angular momentum = the quantity of mass times rotational velocity times radius velocity times radius

    Angular momentum is conserved (kept a constant) ? ? Angular momentum is conserved (kept a constant) If radius decreases, the rotational velocity must increase to ? ? If radius decreases, the rotational velocity must increase to keep the angular momentum a constant keep the angular momentum a constant

    ? This explains why ice skaters spin faster when they pull their ? This explains why ice skaters spin faster when they pull their arms in toward them arms in toward them

    Hadley Cell Hadley Cell

    ? The trade winds in the Northern and Southern hemisphere all ? The trade winds in the Northern and Southern hemisphere all converge at or near the Equator converge at or near the Equator

    InterTropical Convergence Zone (ITCZ): Region where trade ? ? InterTropical Convergence Zone (ITCZ): Region where trade winds converge at the surface winds converge at the surface

    The winds beneath the ITCZ are often light, resulting in the ? ? The winds beneath the ITCZ are often light, resulting in the doldrums doldrums

    Sinking portion of each Hadley Cell creates high pressure called ? ? Sinking portion of each Hadley Cell creates high pressure called subtropical highs subtropical highs

    ? The subtropical high creates the light winds of the horse ? The subtropical high creates the light winds of the horse latitudes latitudes

    Polar Cell Polar Cell

    ? Air sinks at the Poles, creating the Polar highs ? Air sinks at the Poles, creating the Polar highs

    ? Air blows from the Poles down towards the Equator ? Air blows from the Poles down towards the Equator Coriolis force deflects the air to the right, resulting in the polar ? ? Coriolis force deflects the air to the right, resulting in the polar easterlies easterlies

    ? The boundary between the polar easterlies and the westerly ? The boundary between the polar easterlies and the westerly winds of the midlatitudes is called the polar front winds of the midlatitudes is called the polar front The polar front separates cold polar air from more temperate ? ? The polar front separates cold polar air from more temperate air to the South air to the South

    Polar Jet Stream Polar Jet Stream

    ? As we discussed in the last chapter, strong winds ? As we discussed in the last chapter, strong winds should exist above regions where the temperature should exist above regions where the temperature gradient is large gradient is large

    ? The polar jet stream forms because of this temperature ? The polar jet stream forms because of this temperature gradient gradient

    ? Therefore, the polar jet stream is found above the polar ? Therefore, the polar jet stream is found above the polar front front

    Midlatitude Circulation Midlatitude Circulation

    Air moves away from the subtropical highs at the surface ? ? Air moves away from the subtropical highs at the surface

    ? Winds blow up towards the midlatitudes from the South ? Winds blow up towards the midlatitudes from the South

    ? Coriolis Force deflects the winds to the right, resulting in the Prevailing ? Coriolis Force deflects the winds to the right, resulting in the Prevailing Westerlies (or midlatitude westerlies) Westerlies (or midlatitude westerlies)

    ? The midlatitudes dont have a complete cell (winds dont blow down to the ? The midlatitudes dont have a complete cell (winds dont blow down to the subtropical highs aloft like your book shows) subtropical highs aloft like your book shows)

    ? If there was that return flow southward aloft, the Coriolis force would ? If there was that return flow southward aloft, the Coriolis force would deflect those winds to the right and we would observe easterly winds aloft deflect those winds to the right and we would observe easterly winds aloft ? That is not the case - they are westerly! ? That is not the case - they are westerly!

    Upper-Air Midlatitude Westerlies Upper-Air Midlatitude Westerlies ? Winds above 500 mb tend to flow in wavelike patterns ? Winds above 500 mb tend to flow in wavelike patterns ? Troughs elongated regions of low pressure (dips) ? Troughs elongated regions of low pressure (dips) ? Ridges elongated regions of high pressure (bumps) ? Ridges elongated regions of high pressure (bumps) ? Typically 3-5 waves will encircle the Northern ? Typically 3-5 waves will encircle the Northern Hemisphere called Rossby Waves Hemisphere called Rossby Waves

    Upper-Air Midlatitude Westerlies Upper-Air Midlatitude Westerlies ? Zonal flow a small amplitude pattern where winds ? Zonal flow a small amplitude pattern where winds blow mostly west-to-east blow mostly west-to-east

    ? Meridional flow a large amplitude (deep troughs and ? Meridional flow a large amplitude (deep troughs and peaked ridges) pattern where cold air moves peaked ridges) pattern where cold air moves equatorward and warm air flows poleward equatorward and warm air flows poleward

    Flow Patterns Flow Patterns

    ? Zonal Index the average speed of the west-to-east ? Zonal Index the average speed of the west-to-east 500 mb wind between 35? and 55? latitude 500 mb wind between 35? and 55? latitude

    ? High zonal index zonal flow ? High zonal index zonal flow

    ? Low/Negative zonal index meridional flow ? Low/Negative zonal index meridional flow

    ? Split flow pattern zonal flow exists near the poles with ? Split flow pattern zonal flow exists near the poles with a meridional flow pattern further to the south a meridional flow pattern further to the south

    Flow Patterns Flow Patterns

    Blocking Patterns When large pools of cold or warm air get ? ? Blocking Patterns When large pools of cold or warm air get separated from the stronger flow separated from the stronger flow

    ? Blocking patterns can persist for extended periods of time ? Blocking patterns can persist for extended periods of time resulting in extreme weather events resulting in extreme weather events

    ? Index cycle irregular oscillation between patterns (meridional ? Index cycle irregular oscillation between patterns (meridional to zonal) to zonal)

    ? Short waves superimposed smaller waves that travel through ? Short waves superimposed smaller waves that travel through Rossby waves Rossby waves

    Poleward Transport of Energy Poleward Transport of Energy ? There is an energy surplus at the tropics and an energy ? There is an energy surplus at the tropics and an energy deficit at the poles deficit at the poles

    ? The circulations in the atmosphere act to redistribute ? The circulations in the atmosphere act to redistribute the heat the heat

    ? Therefore, our conceptual model also explains this ? Therefore, our conceptual model also explains this transfer of heat transfer of heat

    Seasonal Variations Seasonal Variations

    The global circulation pattern shifts with the Sun and the seasons ? ? The global circulation pattern shifts with the Sun and the seasons ? The ITCZ shifts location dur