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60 Low
30 High
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30 High
60 Low
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Surface PressurFra bibliotek Systems
FERREL CELLS ARE THERMALLY-INDIRECT CIRCULATIONS
Atmospheric Vertical Circulation
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60
The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant. However, individual weather systems – mid-latitude depressions, or tropical convective cells - occur "randomly“.
§1 Introduction
§1 Introduction
1.1 What is the GAC?
§1 Introduction
1.1 What is the GAC?
GAC is understood to be a set of different synoptic situations which are constantly (or nearly constantly), such as action centers, stream currents, monsoon circulation etc.) GAC of the atmosphere is usually considered as an averaged-on-time state of the atmosphere. GAC is also understood to mean the combination of some large-scale statistical characteristics of the atmosphere.
HADLEY CELL IS A THERMALLY-DIRECT CIRCULATION
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Equator
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Surface circulation published by Halley(1686)
In 1686 Halley published the second part of his expedition, being a paper and chart on trade winds and monsoons. In this he identified solar heating as the cause of atmospheric motions. He also established the relationship between barometric pressure and height above sea level.
The General Circulation
成都信息工程学院 大气科学系 陈科艺
Reference:
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The General Circulation
Introduction An overview of the observations Momentum and Energy The general circulation as turbulence Tropical atmosphere-ocean interactions Numerical model
The Hadley Cell
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
Radiative Budget
§1 Introduction
3-cell model
Subpolar low – plenty of rain Mid latitude westerlies Polar high – pretty dry Polar easterlies
Global atmospheric circulation
HADLEY CELLS ARE THERMALLY DIRECT CIRCULATIONS As are (weak) polar cells
The circulation is constantly fighting against friction, but it is sustained by thermal forcing.
§1 Introduction
1.1 What is the GAC? In general, atmospheric circulation is the large-scale movement of air, and the means (together with the ocean circulation) by which heat is distributed on the surface of the Earth.
cooling
heating
Westerly must be southward for the continuity of mass. But, observed wind in mid-latitude is Southwesterly! （Is Hadley‘s model wrong?) Hadley’s work was not noticed for 50 years until mentioned by Dalton (1793).
• Two principles
– Hot air rises – Air flows from high to low pressure (gradients) + Coriolis Effect
§1 Introduction
1.2 What makes it go?
1.The Earth’s radiation budget: An “upper boundary condition” on the general circulation.
• George Hadley(1735) 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
Video 1 Video 2
fundamental driving force
The Earth's atmosphere is put into motion because of the differential heating of the Earth surface.
Tropics receive more heat than the poles
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
Zonal and Meridional Flow
• Zonal flow – normal flow of winds – flattened jet stream • Meridional flow – disturbance in the flow of winds – curvy jet stream
He related the direction of trade winds to the rotation of the Earth in his famous paper "Concerning the Cause of the General Trade Winds," published in 1735.
Meridional Zonal
Zonal circulation:气流沿纬圈方向运动称为纬圈（向） 环流
meriodional circulation:沿经圈方向移动称为经圈（向） 环流。
Zonal winds – blow east to west or west to east Meridional winds – blow south to north or north to south
Single cell model
Because the earth is rotating the wind is shifted to the right (left in SH) Not a very realistic model
George Hadley (February 12, 1685 – June 28, 1768) was an English lawyer and amateur meteorologist who proposed the atmospheric mechanism by which the Trade Winds are sustained.