Understanding Weather and Climate Ch 8

1
How Does Air Move Around the Globe?
2
Review of last lecture

• Know 3 Forces that affect wind speed /direction
• Especially work on Coriolis force, as this is the
  hardest to understand. Which direction is air
  deflected to by Coriolis force?
• What is the geostrophic balance? At which level
  is it valid? Difference between upper level and
  surface winds
• Troughs, ridges, cyclones and anticyclones. Do
  they correspond to high or low surface pressure?
  Is the air moving clockwise or counter-clockwise
  around them?

3
The most common atmospheric circulation structure
H
CE
L
Cooling or No Heating
Heating
Friction
H
L
CE
Imbalance of heating ? Imbalance of
temperature ? Imbalance of pressure ? Wind
4
Introduction

• Well-defined heating, temperature and pressure
  patterns exist across the globe
• These define the general circulation of the
  planet
• In describing wind motions
• Zonal winds (east-west) flow parallel to lines
  of latitude
• Flowing eastward Westerly wind
• Flowing westward Easterly wind
• Meridional winds (north-south) flow parallel to
  lines of longitude
• Flowing northward Southerly wind
• Flowing southward Northerly wind

5
Annual mean precipitation (heating)
Extratropical stormtrack Tropical
rainfall Extratropical stormtrack
6
Primary Highs and Lows

• Equatorial low
• Subtropical high
• Subpolar low
• Polar high

7
Three-cell model
Zonal mean circulation Each hemisphere is
divided into 3 distinct cells Hadley Cell
Ferrel Cell Polar Cell
8
Vertical structure and mechanisms
Hadley Cell (thermal) Heating in tropics ? forms
surface low and upper level high ? air converges
equatorward at surface, rises, and diverges
poleward aloft ? descends in the subtropics
Ferrel Cell (dynamical) Dynamical response to
Hadley and polar cells
Polar Cell (thermal) Driven by heating at 50
degree latitude and cooling at the poles
Hadley
Polar
9
Zonal mean structure of temperature

• Two characteristics
• Horizontally uniform in the tropics
• Steep gradient in the extratropics

10
Zonal mean structure of zonal wind

• Two characteristics
• Westerly winds in the extratropical troposphere
• Jet streams local maximum of winds

11
Westerly winds in the extratropical troposphere

• The existence of the upper level pressure
  gradient ? air is being pushed toward poles ?
  Coriolis effect deflects upper air (to the right)
  ? Westerlies dominate upper troposphere
• Strongest during winter ? thermal gradient is
  large
• Explains why storms move eastward, flight times

12
The Jet Streams

• Caused by steep temperature gradients between
  cold and warm air masses
• Polar front - marks area of contact, steep
  pressure gradient ? polar jet stream
• Low latitudes ? subtropical jet stream
• Stronger in winter, affect daily weather patterns
  

13
A Jet Stream seen from satellite
The subtropical jet is seen as a band of clouds
extending from Mexico on an infrared satellite
image
14
Video The jet streams
15
Semipermanent Pressure Cells

• Instead of cohesive pressure belts circling the
  Earth, semipermanent cells of high and low
  pressure exist fluctuating in strength and
  position on a seasonal basis.
• These cells are either dynamically or thermally
  created.
• Sinking motions associated with the subtropical
  highs promote desert conditions across specific
  latitudes.
• Seasonal fluxes in the pressure belts relate to
  the migrating Sun (solar declination).

16
South Pacific high
South Atlantic high
South Indian high

• For NH winter
• 1. Aleutian and Icelandic lows
• 2. Siberian and Bermuda-Azores highs
• 3. South Pacific, Atlantic, Indian highs

17
South Pacific high
South Atlantic high
South Indian high

• For NH summer
• 1. Tibetan low
• 2. Hawaiian and Bermuda-Azores highs
• 3. South Pacific, Atlantic, Indian highs

18
Low pressure clouds and precipitation
Extratropical stormtrack Tropical
rainfall Extratropical stormtrack
19
High pressure warm surface temperature, drought
and desert
Global distribution of deserts (all near high
pressure cells)
20
General circulation of the oceans

• Ocean surface currents horizontal water motions
• Transfer energy and influence overlying
  atmosphere
• Surface currents result from frictional drag
  caused by wind - Ekman Spiral

• Water moves at a 45o angle (right)
• in N.H. to prevailing wind direction
• Due to influence of Coriolis effect
• Greater angle at depth

21
Global surface currents

• Surface currents mainly driven by surface winds
• North/ South Equatorial Currents pile water
  westward, create the Equatorial
• Countercurrent
• western ocean basins warm poleward moving
  currents (example Gulf Stream)
• eastern basins cold currents, directed
  equatorward

22
Summary

• Three precipitation (heating) belts. Primary high
  and lows
• Three-cell model. Mechanism for each cell
• Two characteristics of zonal mean temperature
  structure
• Two characteristics of zonal mean wind structure.
  Why does westerly winds prevail in the
  extratropical troposphere? What cause the jet
  streams?
• Semipermanent pressure cells. Low pressure is
  associated with clouds and precipitation. High
  pressure is associated with warm surface
  temperature, drought, and desert.
• What drives the ocean surface currents? In the
  case of Ekman spiral, what is the direction of
  surface current relative to surface wind?

23
Works cited

• Images
• http//pulleysandgears.weebly.com/gears.html
• http//visual.merriam-webster.com/earth/meteorolog
  y/meteorological-measuring-instruments/measure-win
  d-direction.php