Atmospheric Circulation Chapter 7 Atmospheric circulation Global Covers

Atmospheric Circulation Chapter 7

Atmospheric circulation • • Global – Covers major portions of the earth Synoptic – covers 100 s to 1000 s of km 2 Mesoscale – covers 10 of km 2 Microscale – small exchanges of mater and energy

Global Scale Circulation

Single cell model • George Hadley The earth is warmed at the equator. Air expands upward and diverges toward the pole At the pole air cools and flows back toward the equator

Single cell model Because the earth is rotating the wind is shifted to the right (left in SH)

Single cell model Zonal winds – blow east to west or west to east Meridional winds – blow south to north or north to south

Single cell model Not a very realistic model

3 -cell model • Each hemisphere is divided into 3 distinct circulation cells

3 -cell model • Polar cell • Ferrel cell • Hadley cell Each cell has associated pressure and wind patterns

3 -cell model ITCZ = Intertropical convergence zone Creates the equatorial low, rainiest regions on earth Little wind – doldrums

3 -cell model 20 -300 Subtropical High – very dry Location of many of the earth’s deserts Little wind – horse latitudes Between 0 -200 NE trade winds

3 -cell model Subpolar low – plenty of rain Mid latitude westerlies Polar high – pretty dry Polar easterlies

Global Atmospheric Circulation Model

3 -cell model Is OK at explaining some pressure and wind phenomena. Ex ITCZ

The “real” world • Not covered by distinct belts that completely encircle the earth • Circulation is dominated by semi-permanent cells of low or high pressure

Winter • Weather in the North America is dominated by the Aleutian Low and Icelandic Low

Summer • Weather in North America is dominated by the Hawaiian High and the Bermuda. Azores High

Seasonal Pressure and Precipitation Patterns

The jet stream The pressure gradient force increases with height

The jet stream The Polar front is a boundary between very cold polar air and warmer mid-latitude air Creates an extreme Pressure Gradient force

The jet stream The result is a powerful flow of air, the jet stream, located near the tropopause at the polar front

Synoptic scale circulation

Ridges and troughs • Ridges – thicker regions of the atmosphere with higher pressure, often associated with surface divergence • Troughs – thinner regions of the atmosphere with lower pressure, often associated with surface convergence

Ridges and troughs Ridge Trough

Ridges and troughs • Can best be seen on a 500 mb map • Troughs point toward the equator • Ridges point toward the pole 500 mb map

Rossby Waves The ridges and troughs form long waves, called Rossby waves, which circle each hemisphere. Rossby waves can remain stationary or move west to east Movement of the waves can transport vast amounts of air

The Jet Stream and Rossby Waves

Rossby waves Groovy web animation

Ocean Currents • Large scale movements of surface water that can exchange energy and moisture with the atmosphere

Ocean Currents

Ocean Circulation

Foehn winds • Winds that flow down mountain slopes, warm by compression and bring warm dry air to the lowlands – Chinook – formed by air descending the eastern slopes of the Rocky Mountains – Santa Ana – formed by high pressure over the Rockies and Great Basin

Chinook

Santa Ana

Mesoscale circulation

Sea / land breezes • Sea breeze – Land warms faster in the day then water – Causes air to expand, rise, diverge and thus create low pressure – Air moves from the sea to balance the pressure • Land breeze – At night the land cools faster than the water and the situation is reversed

Sea / land breezes

Mountain / valley breezes • Valley breeze – Daytime heating of the mountain causes air to warm, rise and be replaced by – air from the valley • Mountain breeze – At night the mountain cools an air sinks down to the valley

Have a great day!