VIEWS: 3 PAGES: 40 POSTED ON: 3/25/2011
Outline for Lecture 13 Factors Affecting Wind Winds Aloft and Geostrophic Flow Curved Flow and the Gradient Wind Surface winds How Winds Generate Vertical Air Motion 3/13/03 Formation of A Sea Breeze (a) No pressure gradient, no wind. (b) Unequal heating creates pressure differences aloft which causes air to flow. (c) The transfer of air aloft Returning air (from the land to the sea) creates a surface high over the sea which results in a flow of air from sea to land Sea Breeze (a.k.a. a sea breeze). Pressure Gradient Force steep pressure gradient shallow pressure gradient (fast winds) (slow winds) pressure p n n distance The Coriolis Force Named after the French Scientist Gaspard Gustave Coriolis free moving objects are deflected to the right of their path in the Northern Hemisphere (to the left of their path in the Southern Hemisphere) because of the Earth’s rotation. It depends on an object’s speed—higher speed means stronger Coriolis Force. The Coriolis Force Coriolis deflection of winds blowing eastward at different latitudes Coriolis force also Strong increases with increasing Latitude… Middling strong over poles, no effect over equator. Weak None Friction • Friction acts at the surface. winds at the surface aren’t as strong as those at higher altitudes Altitude • Air is a little viscous, so the layer next to the surface is also affected, but not as much. Winds Aloft and Geostrophic Flow Force: Geostrophic Flow. Balance pressure gradient force Coriolis Geostrophic winds: •go in a straight line •go parallel to the isobars •have speeds proportional to the pressure gradient force. Geostrophic Flow • WHAT!?!? • pressure difference starts wind • wind gets going a little, starts being deflected by Coriolis force • wind goes faster in response to pressure difference, gets deflected more by Coriolis Force • Eventually, the two balance Newton’s Laws of Motion (condensed version) I. An object in motion tends to stay in motion (unless acted upon by an external force) II. F = ma (that’s pretty much it.) Put down the pencils Let’s break down the forces Just a Pressure Gradient Start with just a View from top pressure gradient Then wind blows straight from High P to Low H L P And accelerates as it goes Pressure 1017 1015 1013 1011 1009 1007 1005 return Just a Pressure Gradient Wind blows straight from High to Low, eventually H L evening out the pressure and stopping the wind. Pressure 1014 1012 1010 1008 Just a Pressure Gradient The wind would blow at the same speed regardless of altitude. H L Pressure 1013 1011 1009 1007 1005 return Now add Friction Then wind still View from top blows straight from High P to Low P, but it doesn’t get H moving as fast L as soon, especially near the ground Pressure 1017 1015 1013 1011 1009 1007 1005 back up Friction and Pressure Gradient The pressure difference also evens out eventually, H L though it might take a bit longer. Pressure 1014 1012 1010 1008 Friction and Pressure Gradient Friction slows the wind at the ground—its effects decrease as you go up in H L the atmosphere. Pressure 1013 1011 1009 1007 1005 Friction and Pressure Gradient Friction slows the wind at the ground—its effects decrease as you go up in H L the atmosphere. Pressure 1013 1011 1009 1007 1005 back up Pressure Gradient and Coriolis Forget friction. The wind starts out straight, but as soon as it speed, the H starts building up L Coriolis force turns it a bit to the right. The wind can’t accelerate any more over here because it’s going parallel to the isobars This is the Geostrophic Flow Pressure Gradient and Coriolis The Coriolis force limits the wind speed by redirecting it AND it prevents wind H L from blowing straight from H to L Pressure Gradient and Coriolis Since the wind never really reaches the low, the pressure difference is H L maintained, and the low never fills! Reality: Pressure Gradient, Coriolis Force, and Friction Coriolis Force turns the wind some, friction 30º slows the wind some, and the result is roughly H L a 30º angle between isobars and wind. Reality: Pressure Gradient, Coriolis Force, and Friction The wind doesn’t blow straight from High to Low, but it does 30º out the pressure H eventually get in there and even L difference, so H and L don’t last forever without a source of energy Reality: Pressure Gradient, Coriolis Force, and Friction Friction slows the wind at the ground—its effects decrease as you go up in H L the atmosphere. Pressure 1013 1011 1009 1007 1005 Reality: Pressure Gradient, Coriolis Force, and Friction Coriolis Force is turning the wind toward us in the right part of the picture. H L Pressure 1013 1011 1009 1007 1005 Reality: Pressure Gradient, Coriolis Force, and Friction Since the Coriolis Coriolis Force depends on wind speed, its effect decreases toward the H L ground where the wind speed Friction is slower. Pressure 1013 1011 1009 1007 1005 Pencils are now allowed Winds Aloft and Geostrophic Flow Geostrophic winds are up high and go straight: only Coriolis and Pressure Gradient Forces are important. Friction is important down low: below about 1500 meters. How do the Coriolis and Pressure Gradient forces change? Coriolis Force Pressure Gradient Wind speed Coriolis Force Wind flows from high to low pressure. Latitude Coriolis Force “Isobaric packing” P.G.F. Winds Aloft and Geostrophic Flow Wind direction is directly linked to the prevailing pressure pattern. Dutch meteorologist Buys Ballott, 1857 Buys Ballott’s Law states: In the Northern Hemisphere if you stand with your back to the wind, lower pressure will be found to your left and higher pressure will be found to the right. Best when there are no frictional forces or topography involved . Curved Flow and the Gradient Wind Cyclonic Flow Anticyclonic Flow Actual flow around pressure systems are never this regular (because of small changes in the pressure field). Surface winds—high pressure (anticyclone)COLD DRY H WARM MOIST Surface winds—low pressure (cyclone) COLD DRY L WARM MOIST Upper Level Weather Chart 500mb chart Variations in height are analogous to variations in pressure. High height fields correspond to high pressure fields. Troughs and Ridges RIDGE RIDGE TROUGH Troughs and Ridges • An elongated region of low pressure (trough) or high pressure (ridge) • Tend to be quite common at higher altitudes • At the surface, a trough is usually a fairly weak feature How Winds Generate Vertical Air Motion Around a surface low pressure center, a net inward transport of air causes a shrinking of the area occupied by the mass. This is known as horizontal convergence. Airflow Associated with Cyclones and Anticyclones “Upper level support” is important in cyclone development Wind speeds and isobars STRONG WINDS Slack winds Slack winds The tighter they’re packed, the stronger the wind Factors that promote vertical airflow • Friction: air flow from ocean to land (upward motion) air flow from land to ocean (downward motion) •Mountain ranges Wind Measurement Wind roses provide a method of representing prevailing winds by indicating the percentage of time the wind blows from various directions Go Terps! ACC Tournament this Weekend
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