Bài giảng khí hậu học chương 7

G304 – Physical Meteorology and Climatology Chapter Atmospheric circulation By Vu Thanh Hang, Department of Meteorology, HUS 7.1 Single-cell Model - A simple circulation pattern called the single-cell model to describe the general movement of the atmosphere - In the single-cell model, air expands upward, diverges toward the poles, descends, and flows back toward the equator near the surface - Winds blowing east-to-west or westto-east are referred to as zonal winds; those moving north-to-south or southto-north are called meridional winds 7.1 Single-cell Model (cont.) • Hadley’s idealized scheme assumed a planet covered by a single ocean and warmed by a fixed Sun that remained overhead at the equator • Hadley’s main contributions were to show that differences in heating give rise to persistent large-scale motions (called thermally direct circulations) and that zonal winds can result from deflection of meridional winds Æ Not so realistic 7.2 Three-cell model - The three-cell model divides the circulation of each hemisphere into three distinct cells: the heat-driven Hadley cell that circulates air between the Tropics and subtropics, a Ferrel cell in the middle latitudes, and a polar cell - Each cell consists of one belt of rising air with low surface air pressure, a zone of sinking air with surface high pressure, a surface wind zone with air flowing generally from the high-pressure belt to the lowpressure belt, and an air flow in the upper atmosphere from the belt of rising air to the belt of sinking air 7.2 Three-cell model (cont.) • The Hadley cell: - Along the equator, strong solar heating causes air to expand upward and diverge toward the poles, creating a zone of low pressure at the equator called the equatorial low or the Intertropical Convergence Zone (ITCZ) - The ITCZ is the rainiest latitude zone in the world and is observable as the band of convective clouds and showers extending from northern South America into the Pacific on the satellite image - The ITCZ is sometimes called the doldrums 7.2 Three-cell model (cont.) ITCZ on satellite images 7.2 Three-cell model (cont.) • The Hadley cell (cont.): - At about 20° to 30° latitude, air in the Hadley cell sinks toward the surface to form the subtropical highs, large bands of high surface pressure Æ Cloud formation is greatly suppressed and desert conditions are common in the subtropics - In the NH, as the pressure gradient force directs surface air from the subtropical highs to the ITCZ, the weak Coriolis force deflects the air slightly to the right to form the northeast trade winds - In the SH, the northward-moving air from the subtropical high is deflected to the left to create the southeast trade winds 7.2 Three-cell model (cont.) • The Ferrel and polar cells: - Immediately flanking the Hadley cell in each hemisphere is the Ferrel cell, which circulates air between the subtropical highs and the subpolar lows - On the equatorial side of the Ferrel cell, air flowing poleward away from the NH subtropical high undergoes a substantial deflection to the right, creating a wind belt called the westerlies - In the SH, the pressure gradient force propels the air southward, but the Coriolis force deflects it to the left, producing a zone of westerlies in that hemisphere as well 7.2 Three-cell model (cont.) • The Ferrel and polar cells (cont.): - In the polar cells of the three-cell model, surface air moves from the polar highs to the subpolar lows - Very cold conditions at the poles create high surface pressure and low-level motion toward the equator In both hemispheres, the Coriolis force turns the air to form a zone of polar easterlies in the lower atmosphere 7.2 Three-cell model (cont.) 7.4 The upper troposphere (cont.) • Troughs and ridges: - The 500 mb surface reveals that heights decrease from south to north but also rise and fall through the ridges and troughs - Height contous are displaced toward the equator in troughs and toward the pole in ridges - Air flows poleward around ridges and equatoward around troughs 7.4 The upper troposphere (cont.) • Rossby waves: - Ridges and troughs give rise to wavelike flow in the upper atmosphere of the mid-latitudes - The largest of these are called long waves or Rossby waves - There are from three to seven Rossby waves circling the globe, each with a particular wavelength and amplitude - Rossby waves often remain in fixed positions, but also migrate west to east Æ transporting warm air from subtropical regions to high latitudes, or cold polar air to low latitudes 7.4 The upper troposphere (cont.) • Rossby waves (cont.): - Changes from summer to winter Æ fewer in number, have longer wavelength, strongest winds in winter 7.4 The upper troposphere (cont.) • Rossby waves (cont.): 7.5 The Oceans • Ocean currents: - Ocean currents are horizontal movements of surface water that have an impact on the exchange of energy and moisture between the oceans and the lower atmosphere 7.5 The Oceans (cont.) • Ocean currents (cont.): - Ocean currents are driven by winds in the lower atmosphere that exert a drag on the water at an angle 45° to the right (NH) and continue to shift clockwise as their speed decreases - At a depth of about 100 m, the direction of the current is in opposite with the direction of the wind Ekman spiral 7.5 The Oceans (cont.) • Upwelling: - Strong offshore winds along a coastal region sometimes drag the warmer surface waters seaward, which draws up cooler waters from below to take their place - This process, called upwelling, greatly influences sea surface temperatures over the eastern portions of the major oceans 7.6 Major wind systems • Features such as the Intertropical Convergence Zone, the westerlies, and large Rossby waves exist on a global scale • Smaller features, such as cyclones, anticyclones, troughs, and ridges, exist at the synoptic scale, covering hundreds or thousands of square kilometers • Mesoscale events are on the order of tens of square kilometers and last for periods as brief as half an hour • The smallest exchanges of mass and energy operate at the microscale 7.6 Major wind systems (cont.) • Monsoons: - refers to the climatic pattern in which heavy precipitation alternates with hot, dry conditions on an annual basis due to the seasonal reversal in surface winds caused by an oscillation between high- and lowpressure cells - During winter (top), dry southward from the Himalayas air flows - When summer arrives (bottom) moist air is drawn northward from the equatorial oceans - Surface heating, convergence, and a strong orographic effect cause heavy rains over the southern part of the continent 7.6 Major wind systems (cont.) • Foehn wind: - Foehn is the generic name for synoptic scale winds that flow down mountain slopes, warm by compression, and introduce hot, dry, and clear conditions to the adjacent lowlands Foehn effect 7.6 Major wind systems (cont.) • Sea and land breeze: - During the daytime, land surfaces warm more rapidly than the adjacent water (a) Æ the air column overlying the land to expand and rise upward (b) - At a height of about 1km, the rising air spreads outward (c) - The air over the water moves toward the low-pressure area over the land, which sets up the daytime sea breeze - At night the land surface cools more rapidly than the water The air over the land becomes dense and generates a land breeze 7.6 Major wind systems (cont.) • Valley and moutain breeze: - A valley breeze (a) forms when daytime heating causes the mountain surface to become warmer than nearby air at the same altitude - The air expands upward and the air flows from the valley to replace it - Nocturnal cooling makes the air dense over the mountain and initiates a mountain breeze (b) 7.7 Air-Sea interactions • ElNiño, LaNiña and the Walker circulation: - ElNiño is a recurrent event in the tropical eastern Pacific in which sea-surface temperatures are significantly above normal - LaNiña is the inverse event (cold sea-surface temperatures) - The Walker circulation is an east-west circulation pattern of the Tropics, characterized by several cells of rising and sinking air connected by horizontal motions along parallel lines of latitude - The Southern Oscillation is the reversal of surface pressure patterns over the tropical Pacific associated with ElNiño events 7.7 Air-Sea interactions (cont.) • ElNino, LaNina and the Walker circulation (cont.): 7.7 Air-Sea interactions (cont.) • ElNino, LaNina and the Walker circulation (cont.): [...]... parallel lines of latitude - The Southern Oscillation is the reversal of surface pressure patterns over the tropical Pacific associated with ElNiño events 7. 7 Air-Sea interactions (cont.) • ElNino, LaNina and the Walker circulation (cont.): 7. 7 Air-Sea interactions (cont.) • ElNino, LaNina and the Walker circulation (cont.): ... transporting warm air from subtropical regions to high latitudes, or cold polar air to low latitudes 7. 4 The upper troposphere (cont.) • Rossby waves (cont.): - Changes from summer to winter Æ fewer in number, have longer wavelength, strongest winds in winter 7. 4 The upper troposphere (cont.) • Rossby waves (cont.): 7. 5 The Oceans • Ocean currents: - Ocean currents are horizontal movements of surface water... generates a land breeze 7. 6 Major wind systems (cont.) • Valley and moutain breeze: - A valley breeze (a) forms when daytime heating causes the mountain surface to become warmer than nearby air at the same altitude - The air expands upward and the air flows from the valley to replace it - Nocturnal cooling makes the air dense over the mountain and initiates a mountain breeze (b) 7. 7 Air-Sea interactions... changes in position and intensity over the course of the year • Some of these cells result from temperature differences and others from dynamical processes 7. 3 Semipermanent pressure cells (cont.) January 7. 3 Semipermanent pressure cells (cont.) July 7. 3 Semipermanent pressure cells (cont.) The Sahel is a region of Africa bordering the southern Sahara Desert During the summer (left), the ITCZ usually... temperature gradient (9-12km above sea level) 7. 4 The upper troposphere (cont.) • The polar front and jet streams (cont.): - Wind speeds average about 180km/hr in winter and about half that in summer, peak winds can exceed twice these values - Near the equator is the subtropical jet stream, associated with the Hadley cell, can bring with it warm, humid conditions 7. 4 The upper troposphere (cont.) • Troughs... much of the year, the ITCZ is located south of the Sahel, and the region receives little or no precipitation (right) 7. 4 The upper troposphere • Upper tropospheric heights decrease poleward from lower latitudes due to the increased density of colder air Decreasing heights with latitude 7. 4 The upper troposphere (cont.) • Westerly winds in the upper atmosphere: - Height differences correspond to pressure... and the lower atmosphere 7. 5 The Oceans (cont.) • Ocean currents (cont.): - Ocean currents are driven by winds in the lower atmosphere that exert a drag on the water at an angle 45° to the right (NH) and continue to shift clockwise as their speed decreases - At a depth of about 100 m, the direction of the current is in opposite with the direction of the wind Ekman spiral 7. 5 The Oceans (cont.) • Upwelling:... strong orographic effect cause heavy rains over the southern part of the continent 7. 6 Major wind systems (cont.) • Foehn wind: - Foehn is the generic name for synoptic scale winds that flow down mountain slopes, warm by compression, and introduce hot, dry, and clear conditions to the adjacent lowlands Foehn effect 7. 6 Major wind systems (cont.) • Sea and land breeze: - During the daytime, land surfaces... level westerlies are strongest in winter Æ affect aviation 7. 4 The upper troposphere (cont.) • Westerly winds in the upper atmosphere (cont.): - Wind speeds generally increase with height between the surface and the tropopauseÆ because of decreasing of friction and PGF is stronger at high altitudes - The surfaces representing the 900, 800, and 70 0 mb levels all slant downward to the north, but not by... same amount - Higher surfaces slope more steeply, which means that the pressure gradient force is greater 7. 4 The upper troposphere (cont.) • Westerly winds in the upper atmosphere (cont.): The difference in heights between successive surfaces continues to increase upward, leading to stronger winds 7. 4 The upper troposphere (cont.) • The polar front and jet streams: - The polar front is a strongly sloping