PHYC40050-L04-GenCirc

PHYC40050-L04-GenCirc - Climate Climate Change Nuclear...

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Unformatted text preview: Climate, Climate Change Nuclear Power and the Alternatives Climate, Climate Change Nuclear Power and the Alternatives PHYC 40050 Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences University College Dublin PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics Lecture 4 Atmospheric Forces and Wind FORCES THAT MOVE THE AIR Gravitational force: Is directed downward , normal to the Is earth Equal to the mass of the air times the Equal gravitational acceleration: mg The General Circulation Pressure gradient force PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics PRESSURE GRADIENT FORCE Pressure gradient force Is a function of the pressure difference and air Is density: F = (1/ρ) ∂p/∂n where F = Pressure gradient force per unit mass Pressure ρ = Density of air p = Pressure Pressure n = Distance normal to isobars Distance Newton’s law states: Newton’s force=mass times acceleration F is force per unit mass, equivalent to acceleration. is PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics 1 APPARENT FORCES IN THE ATMOSPHERE Coriolis Force Coriolis Centrifugal Force/Centripetal Centrifugal acceleration CORIOLIS EFFECT PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics CORIOLIS EFFECT Weather map shows that winds go Weather around a low pressure area, not across the isobars. This deviation is a result of the Earth's This rotation and has been named the coriolis effect (coriolis force) (coriolis This force always acts at right angles to This the direction of motion. PHYC 40050 Environmental Physics CENTRIFUGAL FORCE PHYC 40050 Environmental Physics THE MECHANISM OF GEOSTROPHIC FLOW GEOSTROPHIC FLOW A balance is reached between the balance coriolis force and the pressure gradient force. If there is no friction, this occurs If when the parcel of air is parallel to the isobars. At this point there is no net force on At the air parcel - no acceleration - it now moves with constant velocity. PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics 2 GEOSTROPHIC FLOW … Under these idealized conditions the Under airflow is said to be in geostrophic balance. Wind generated is called the Wind geostrophic wind. Normally only applies to winds aloft. Normally Surface winds are subject to friction. Surface Fig. 6-15, p. 172 PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics GRADIENT WINDS Winds around centers of high or low Winds pressure follow curved paths in order to stay parallel with the isobars. These winds are called gradient winds. These winds. Low pressure centers are called Low cyclones - rotation is counter-clockwise counter– the same as the earth. Centers of high pressure are called Centers anti-cyclones. anti- cyclones. In southern hemisphere the directions of flow are reversed. PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics Fig. 6-24, p. 181 3 SURFACE WINDS Friction affects winds only close to the Friction earth's surface. Here we must balance three forces Here coriolis, pressure gradient and friction. coriolis, Net effect is to induce a net inflow around a Net cyclone, an effect known as convergence. convergence. Around an anticyclone we get a net outflow, Around outflow, divergence. divergence. The General Circulation PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics HADLEY CELL GLOBAL CIRCULATION George Hadley first suggested in 1735 George the general concept of atmospheric circulation – a single cell, to explain the existence of the easterly winds at the surface Cold air at pole - high pressure at Cold surface. Warm air at equator - low pressure at surface. Pressure gradient force at surface will move air from pole to equator at surface. Return path at high altitudes. Coriolis force produces easterlies Coriolis PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics Composite (clouds, surface temperature (colors)) image. Note the line of clouds along the ITCZ Fig. 7.10 Three cell model PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics 4 GLOBAL CIRCULATION But in reality we have three cells, with But boundaries at about 30º and 60º latitude. 30º 60º This results in sinking air at 30ºN and 30ºS. This 30º 30º But sinking suppresses cloud development and precipitation. Hence most of the worlds deserts occur along these latitudes. These are the horse latitudes. These Circulation at the surface moves from 30º Circulation 30º to the equator – easterly winds. PHYC 40050 Environmental Physics GLOBAL CIRCULATION Between 30º and 60º the circulation Between at the surface is from 30º to 60º, giving rise to westerly winds. Between 60º and the Poles, the flow Between is from the Pole to 60º, leading to easterly winds at the surface. Upwelling at the equator and at 60º – Upwelling precipitation. The Doldrums. The PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics 5 JET STREAMS Conservation of Angular Momentum: As the parcel of air moves from the equator to 30º at As 30º high altitudes, its velocity increases to conserve angular momentum. At the same time the coriolis force acts to produce a At strong westerly wind – the sub-tropical jet stream subA similar jet forms at 60º – the polar jet, or polar similar 60º front jet PHYC 40050 Environmental Physics Cloud band from Pacific to Florida follows Sub-tropical jet PHYC 40050 Environmental Physics SUBTROPICAL AND POLAR JET STREAMS IN RELATION TO THE THREE CELLS WESTERLIES In the upper troposphere there is high In pressure over the equator, and low pressure over the poles. This produces a net flow from the equator to This the poles. This flow plus the coriolis force produces This westerlies. westerlies. Winds are geostrophic Winds Pressure gradient increases with altitude. Pressure Thus so does the wind speed Jet streams are part of the westerlies Jet PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics DISH-PAN EXPERIMENT PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics Fig. 7-14, p. 199 6 500 MB WINDS (A) Zonal flow pattern – air flows nearly parallel to latitudes. (B) Meridional flow pattern. (C) Combination of the two flows. PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics Fig. 7-16, p. 200 WAVES IN THE WESTERLIES Dish pan experiment. Dish C. G. Rossby. C. Rossby. Waves along the jet streams are known Waves as Rossby waves. Three to six of them around the globe. Three The air flow along the edge of the The waves can be rapid, however the waves move slowly – about 15º per day. 15º Higher jet stream speeds in the winter. Higher Jets shifts south in the winter, north in Jets the summer. PHYC 40050 Environmental Physics WESTERLIES AND THE HEAT BUDGET Major function of atmospheric dynamics is Major to move heat from the equator to the poles. But how can winds move heat when the But predominate wind direction is zonal (E to W, or W to E)? The meanderings of the jet streams The continually mix cold and warm air, thus transporting heat. PHYC 40050 Environmental Physics POLEWARD TRANSPORT OF HEAT BY THE OCEANS AND ATMOSPHERE Mean position of the ITCZ in January and July PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics 7 THE SW MONSOON PRECIPITATION PATTERNS AND TOPOGRAPHY PHYC 40050 Environmental Physics PHYC 40050 Environmental Physics End of Lecture 4 PHYC 40050 Environmental Physics 8 ...
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