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Wind • Wind is the motion of the fluid we call the atmosphere. To understand why this (and all other fluids) move we must consider the FORCES that act on the fluid. • There are several, but not all of them are necessarily acting at a given time. • The major forces acting on the atmosphere are: – gravity – pressure gradient force – Friction – Coriolis Force (not a real force, but more on that later) Force as a vector • Two things to remember about a force: • forces have a magnitude (how “strong” is it?) and • forces have a direction (what direction does it work in?) • We can represent this pictorially with an arrow, with the length the magnitude Weaker force Stronger force Pressure Gradient force • DEFINE PRESSURE GRADIENT: a change in pressure over a distance. • Any time a pressure gradient exists in a fluid, there is a pressure gradient force acting toward the lower pressure: The pressure gradient force is the main cause of atmospheric motion !!! Just what is a force anyway??? • DEFINE FORCE: some agent that causes an object at rest to move, or alters the movement of an object already in motion. • Examples: • gravity (causes motion towards center of earth) • magnetic (attracts iron objects toward magnet) • impulsive force (e.g. blow w/ a baseball bat, explosion,... impulsive forces are often very strong, but applied over a short amount of time) Vectors • You are already familiar with other vector quantities like velocity: • velocity magnitude is called “speed” (how fast?) • A more correct definition of FORCE might be to say that a FORCE changes the DIRECTION and/or MAGNITUDE of a body’s VELOCITY • NOTE: 1) If a body is moving at a constant velocity (no change in speed or direction, i.e.,straight line), NO FORCES ARE ACTING ON IT! • 2) FORCES ADD UP! Two forces of the same magnitude but opposite direction cancel each other out Pressure gradient force (cont.) • Remember, nature usually acts in a way that tries to eliminate a gradient! • An everyday example happens when you fill a bathtub. Not all the fluid stays at one end...right? Nature distributes the fluid in such a way that it eliminates the pressure gradient. (it makes a flat top surface... right?) • But... what about the vertical pressure gradient in the atmosphere? • After all, pressure decreases with height, right? • Why doesn’t nature eliminate this gradient? 1 Hydrostatic balance • in the vertical, another force (almost) perfectly balances the pressure gradient, resulting in: hydrostatic balance Coriolis Force 500 mb Fp G 1000 mb • Most of the time, we are interested in horizontal, rather than vertical winds. Also we are interested in the horizontal pressure gradient force (PH) Here gravity does not enter. (why...?) • The other important “force” for horizontal motion is the coriolis force, which is not really a true force, but kind of acts like one. • The coriolis force is simply a result of the fact that our coordinate system (the latitude/longitude pattern on Earth) is moving (rotating). WHAT IS THIS FORCE ACTING IN THE VERTICAL?!! Coriolis Force (cont.) • DEFINE CORIOLIS FORCE (Fc): • An apparent horizontal force, resulting from the rotation of the planet, that always acts perpendicular to the motion (velocity vector). • The magnitude of the coriolis force is proportional to the speed (magnitude of the velocity vector). Faster-moving objects (parcels) “feel” a stronger coriolis force Coriolis Facts Fc acts to the right in Northern Hemisphere to the left in Southern Hemisphere • Fc never changes the magnitude of the velocity vector, just the direction it points. • Fc is strongest at the poles and vanishes at the equator • Fc is a weak force but acts constantly on all moving objects on the Earth • SO... it takes Fc a long time (several hours) to change the flow. AND... • Fc is most important in processes with big time scales (hours to days) and big space scales (100 km and up). Figure 9.20 now we have the conceptual tools to understand the winds! • DEFINE GEOSTROPHIC BALANCE: a balance between the PH (hor. pressure gradient force) and the coriolis force, Fc. The resulting so-called geostrophic wind 1) does not change speed and 2) goes in a straight line (has no curvature). • GEOSTROPHIC FLOW has the wind blowing parallel to isobars (lines of constant pressure) • 1) most straight-line large scale flow in the atmosphere is nearly geostrophic. • 2) geostrophic balance assumes no frictional forces. • This is OK well above the surface, but becomes less true as you get closer to the ground 2 Curved flow At position 1), the parcel starts to accelerate down the pressure gradient (purple vectors are wind velocity) • The main problem with geostrophic wind is that it assumes straight line flow, and the is NOT how the atmosphere usually works, especially with big storm systems (low-pressure systems). • NO PROBLEM! • Let’s just adjust our geostrophic wind results to make curved flow. • Remember from the example of geostrophic adjustment that to make the flow curve, we need to apply a net force perpendicular to the velocity At position 5), the parcel is in geostrophic flow (balance) Here the purple arrows represent the instantaneous velocity of a ball on a string. The heavy black arrows represent the force always towards the center of the circle that the string must exert on the ball to keep it going in a circle. This force, always perpendicular to the motion, is called the centripetal force, in this case caused by the tension of the string. A similar force is required to make air parcels curve Remember that CF (Coriolis Force) always acts to towards the right of the flow. So to have the flow curve to the RIGHT, we need CF (coriolis force) to be a bit stronger than PGF (pressure gradient force) This slight imbalance gives us the anticyclonic (clockwise) flow we observe around highs Notice that to make CF stronger, we need to increase the velocity from geostrophic balance, which makes anticyclonic flow supergeostrophic in speed Centripetal force • We have all the forces necessary to create a centripetal force in the atmosphere, we just need to not have the Pressure Gradient Force (PH) and the Coriolis Force (Fc) not quite cancel each other out, as they did in the case of geostrophic flow. • This slight imbalance between these 2 forces provides a net force that can act as the centripetal force to make the flow curve. So to have the flow curve to the LEFT, we need CF (coriolis force) to be a bit weaker than the PGF (pressure gradient force) This slight imbalance (PGF>CF ) gives us the cyclonic flow (anticlockwise) we observe around lows (cyclones) Notice that to make CF weaker, we need to decrease the velocity from geostrophic balance, which makes cyclonic flow subgeostrophic in speed 3 Gradient Wind Balance Gradient Winds (cont) This cyclonic flow around low pressure centers and anticyclonic around highs is called gradient wind balance Points to remember about gradient flow: • 1) In the southern Hemisphere, these directions are reversed. • 2) The coriolis force gets weaker near the equator, making long-lasting lows and highs very rare in the tropics. • 3) This result means that low-pressure systems have a hard time “filling up”, since air cannot “get into them”, but rather circles the low • 4) remember that Fc takes a relatively long time to have an effect, so phenomena like tornadoes, thunderstorms, etc. are NOT affected by it. • 5) In valleys btwn mountains, where flow is channeled, Fc is not important and the wind will blow towards the low pressure as much as possible. This means that valley winds might well be perpendicular to the flow above ridge top!!! • 6) Cyclones/anticyclones are seldom exactly circular, but between gradient flow and geostrophic flow we have conceptual models that work well in most situations. Friction • We need to consider just one more important piece of the puzzle, and that is adding in friction when we are near the surface • All we need to do to add the effects of friction is to remember that it always acts to slow things down! • Since CF gets weaker as the velocity decreases, this gives a slight edge to the PGF and “unbalances” gradient balance a bit. • The net effect of friction is that the wind tends to cross isobars towards a low and away from a high Figure 9.29a CONVERGENCE, DIVERGENCE and VERTICAL MOTION • All the air converging (flowing inward) towards the center of the low has to go somewhere (like upwards) The top figures show flow aloft without the effect of friction • and diverging winds have to get their mass from somewhere (like from above...) So… friction-forced convergence causes upward motion near the surface and divergence causes downward motion (sinking or subsidence) near the surface. The bottom figures show the flow at the surface where friction is important 4 Figure 9.33 5