Climate & Earth System Science. Introduction to Meteorology & Climate. Chapter 06. Newton s Laws. First Law (Law of Inertia):

Transcription

1 Climate & Earth System Science Introduction to Meteorology & Climate MAPH Peter Lynch Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Chapter 06 Atmospheric Forces and Wind Newton s Laws First Law (Law of Inertia): A body at rest tends to stay at rest while a body in motion tends to stay in motion, travelling at a constant speed and in a straight line, until acted upon by an outside force 1

2 Newton s Laws Second Law (Law of Momentum): Sum of forces A force exerted on an object causes it to accelerate in the direction of the force. Force equals Mass times Acceleration: F = m a PARALLELOGRAM OF FORCES FORCES THAT MOVE THE AIR (1) Gravitational force: Is directed downward, normal to the Earth Equal to the mass of the air times the gravitational acceleration: mg (2) Pressure gradient force (2) PRESSURE GRADIENT FORCE Is a function of the pressure difference and air density: where F = (1/ρ) p/ n F = Pressure gradient force per unit mass ρ = Density of air p = Pressure n = Distance normal to isobars 2

3 Mean sea-level pressure analysis for Sunday, 1st February, 1953 (3) APPARENT FORCES IN THE ATMOSPHERE CORIOLIS EFFECT Coriolis Force Centrifugal Force/Centripetal acceleration 3

4 Roundabout film CORIOLIS EFFECT Dublin٠٠٠٠٠٠٠ Weather map shows that winds go around a low pressure area, not across the isobars. This deviation is a result of the Earth's rotation and has been named the coriolis effect (coriolis( force) This force always acts at right angles to the direction of motion. THE 1953 STORM Occurred 31 Jan/1 Feb, Greatest storm surge on record for the North Sea 100,000 hectares flooded and more than 300 people lost their lives In eastern England Mean sea-level pressure analysis for Sunday, 1st February, dykes burst and 1,800 people drowned in Holland. The Princess Victoria sank in the North Channel. 4

5 Sea-level pressure and wind Mean sea-level wind analysis for Sunday, 1st February, 1953 A POPULAR MISCONCEPTION CENTRIFUGAL FORCE In the Southern Hemisphere, the water going down the plug-hole spins in the opposite direction. GEOSTROPHIC FLOW A balance is reached between the Coriolis force and the pressure gradient force. If there is no friction, this occurs when the movement of air is parallel to the isobars. THE MECHANISM OF GEOSTROPHIC FLOW At this point there is no net force on the air parcel - no acceleration - it now moves with constant velocity. 5

6 GEOSTROPHIC FLOW Under these idealized conditions the airflow is said to be in geostrophic balance. Wind generated is called the geostrophic wind. Normally only applies to winds aloft. Surface winds are subject to friction. THE GEOSTROPHIC WIND An exact balance between PGF and Coriolis force Low Pressure PGF 1 p = ρ n P V CF = fv P+1 n P+2 High Pressure GRADIENT WINDS Winds around centers of high or low pressure follow curved paths in order to stay parallel to the isobars. These winds are called gradient winds. Low pressure centers are called cyclones - rotation is counter- clockwise the same as the earth. Centers of high pressure are called anti-cyclones cyclones. THE GRADIENT WIND FRICTIONAL DRAG NEAR THE SURFACE Cyclonic flow Anticyclonic flow Coriolis Force CFF V PGF LOW V PGF CFF Coriolis Force HIGH Sub-geostrophic Super-geostrophic 6

7 THE SURFACE WIND Friction causes the wind back and reduce PGF FF v CF P P+ΔP SURFACE WINDS Friction affects winds only close to the earth's surface. Here we must balance three forces - Coriolis,, pressure gradient and friction. Net effect is a net inflow around a cyclone, an effect known as convergence. Around an anticyclone there is a net outflow, divergence. 7

8 Aneroid Barometer SEA BREEZES Are the result of differential heating of the ocean and the land During the day the land heats up quickly while the ocean heats up slowly High temperature over the land, lower temperature over the ocean At the surface - high pressure over the ocean, low pressure over the land - causes wind at the surface to flow from the ocean to the land. Sea Breeze Effect LAND BREEZE Land Breeze Effect: Night-time At night the land cools rapidly to a temperaure below that of the ocean. Wind reverses - flows from the land to the ocean at the surface (land breeze) Fig

9 15:45 UTC 18:15 UTC 20:15 UTC 21:43 UTC Conclusion 9