大氣環(huán)流第一章

1、The General Circulation成都信息工程學(xué)院大氣科學(xué)系陳科藝Reference:/group/dave/at605.htmlThe General CirculationIntroductionAn overview of the observationsMomentum and EnergyThe general circulation as turbulenceTropical atmosphere-ocean interactionsNumerical model1 Introduction1 Introduc

2、tion1.1 What is the GAC?1 Introduction1.1 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 s

3、tate of the atmosphere.GAC is also understood to mean the combination of some large-scale statistical characteristics of the atmosphere.1 Introduction1.1 What is the GAC?The circulation is global in extent. The circulation transports dry air, energy, three phases of water, momentum, and too many add

4、itional quantities to list here. In the Earths atmosphere, the circulations of energy and water are closely linked.The circulation is constantly fighting against friction, but it is sustained by thermal forcing.In general, atmospheric circulation is the large-scale movement of air, and the means (to

5、gether with the ocean circulation) by which heat is distributed on the surface of the Earth.1 Introduction1.1 What is the GAC?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-la

6、titude depressions, or tropical convective cells - occur randomly“.Zonal and Meridional Flow Zonal flow normal flow of winds flattened jet stream Meridional flow disturbance in the flow of winds curvy jet streamMeridionalZonalZonal circulation:氣流沿緯圈方向運動稱為緯圈（向）緯圈（向）環(huán)流環(huán)流meriodional circulation:沿經(jīng)圈方向移動

7、稱為經(jīng)圈（向）經(jīng)圈（向）環(huán)流環(huán)流。Zonal winds blow east to west or west to eastMeridional winds blow south to north or north to southVideo 1Video 2fundamental driving force The Earths atmosphere is put into motion because of the differential heating of the Earth surface. Tropics receive more heat than the poles Equa

8、torNSHADLEY CELL IS A THERMALLY-DIRECT CIRCULATIONSurface circulation published by Halley(1686)Edmond Halley(November 8, 1656 January 14, 1742) was an English astronomer, geophysicist, mathematician, meteorologist, and physicist.In 1686 Halley published the second part of his expedition, being a pap

9、er 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.Single cell model George Hadley(1735)The earth is warmed at the equator.Air expands upward an

10、d diverges toward the poleAt the pole air cools and flows back toward the equatorSingle cell modelBecause the earth is rotating the wind is shifted to the right (left in SH)Not a very realistic modelGeorge Hadley (February 12, 1685 June 28, 1768) was an English lawyer and amateur meteorologist who p

11、roposed the atmospheric mechanism by which the Trade Winds are sustained.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. coolingheating Westerly must be southward for the continuity of mass.

12、But, observed wind in mid-latitude is Southwesterly!（Is Hadleys model wrong?)Hadleys work was not noticed for 50 years until mentioned by Dalton (1793).coolingheatingSolar heating is the forcingConservation of absolute velocity drifting westward when moving southwardFriction will slow down rotation

13、of the Earth. There must be westerly in the high latitude to maintain the Earths rotation.The Hadley Cell3-cell model Each hemisphere is divided into 3 distinct circulation cells3-cell model Polar cell Ferrel cell Hadley cellEach cell has associated pressure and wind patterns3-cell modelITCZ = Inter

14、tropical convergence zoneCreates the equatorial low, rainiest regions on earthLittle wind doldrums3-cell model20-300 Subtropical High very dryLocation of many of the earths desertsLittle wind horse latitudesBetween 0-200 NE trade winds3-cell modelSubpolar low plenty of rainMid latitude westerliesPol

15、ar high pretty dryPolar easterliesGlobal atmospheric circulation030306060NSLatitudeHighHighLowLowSurface Pressure SystemsHADLEY CELLS ARE THERMALLY DIRECT CIRCULATIONSFERREL CELLS ARE THERMALLY-INDIRECT CIRCULATIONSAs are (weak) polar cellsLow030306060NSLowLatitudeHighHighLowLowAtmospheric Vertical

16、CirculationSurface Pressure Centers030306060NSWesterlyEasterlySurface Winds1 Introduction1.2 What makes it go?Basics of Atmospheric Circulation Two principles Hot air rises Air flows from high to low pressure (gradients)+ Coriolis Effect1 Introduction1.2 What makes it go? 1.The Earths radiation budg

17、et: An “upper boundary condition” on the general circulation.Radiative Budget Earth without rotation or continents Equator heats up Hot air rises LP at ground HP aloft Poles cold Cold air sinks HP at ground LP aloft1 Introduction1.2 What makes it go? Equator heats up Air rises LP at ground HP aloft

18、Poles cold Air sinks HP at ground LP aloftLPHPHPLPLPLPHPHP1 Introduction1.2 What makes it go?LPLPHPHPLPLarge circulation cells on surface of the Earth- air flows from poles toward equator at ground levelEarth without rotation or continents1 Introduction1.2 What makes it go?Atmospheric CirculationEar

19、th without rotation or continentsThe net radiation at the top of the atmosphere, which is the difference between the absorbed solar radiation and the OLR, is positive in the tropics and negative in higher latitudes. This implies that energy is transported poleward somehow, inside the system.A portio

20、n of this energy is transportedby the atmosphere, and the rest is transported by the oceans.1 Introduction1.2 What makes it go?- radiation budget Add rotation (Coriolis Effect) Earth with rotation, no continents Net result Coriolis deflects freely moving objects to: The right of their direction of m

21、otion in the NH (clockwise) The left if their direction of motion in the SH (counterclockwise)1 Introduction1.2 What makes it go?- earth rotation1 Introduction1.2 What makes it go?3. Meridional energy transports by atmos-ocean systemThe circulation of the atmosphere and oceans has a moderating effec

22、t on the global distribution of temperature, tending to warm the higher latitudes and cool the tropics. However, these same thermal contrasts represent a source of energy (called “available potential energy”) that makes the global circulations of the atmosphere and oceans possible.1 Introduction1.2

23、What makes it go?- radiation budget4. Surface boundary conditionsThe properties of the Earths surface, and their geographical variations, strongly affect the GAC. The most important properties are follows:Temperature Wetness Elevation VegetationSea ice Land ice 1 Introduction1.2 What makes it go?4.

24、Surface boundary conditionsSome observation factsSea / 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 sit

25、uation is reversedLand-Sea BreezesFigure 6.19Moonsoons, a continental scale land-sea breezeMountain-Valley breezesFigure 6.20Mountain / 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 c

26、ools an air sinks down to the valleyFoehn Winds that flow down the lee side of mountain ranges Mountains block the wind; this can be called a mechanical forcing. It will influence the geographical distribution of precipitation. Mountains can also exert a thermal forcing on the atmosphere. 4. Surface boundary conditions1 Introduction1.2 What makes it go?- radiation budget 大氣環(huán)流是指大氣環(huán)流是指大范圍大氣運動狀態(tài)大范圍大氣運動狀態(tài)。就水。就水平尺度而言，有某一大地區(qū)如歐