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Air Pressure and Winds-II Air Pressure and Winds-II GEOL 1350: Introduction To Meteorology 1 Why don’t wind always blow directly from higher towards lower pressure ? 2 Forces that influence the winds 1. Pressure gradient force 2. Coriolis force 3. Centrifugal force 4. Frictional force 3 Coriolis Force – apparent force due to the rotation of the earth Rest, ball goes in a straight line rotation, ball seems to veer to its right 4 Actual & Observed Paths Airplane travel paths have an apparent curvature, just as Coriolis forces affect winds. Again, the deflection between actual and observed paths is greater near the poles. 5 What determine the magnitude of Coriolis force ? CoriolisCoriolis forceforce == ff ••VV V is wind speed f is the Coriolis parameter f = 2 × earth’s rotational rate × sin of latitude Earth’s ratation rate (7.3 • 10-5 radian/s) 6 Coriolis Force (CF) . Apparent force due to the rotation of the earth . Magnitude depends on latitude and the speed of the air parcel The higher the latitude, the larger the Coriolis force Zero at the equator, and maximum at the poles The faster air moves, the larger the Coriolis force . Causes the parcel to deflect to the right of its intended path in the northern hemisphere to the left of its intended path in the southern hemisphere. Only influence wind direction, no effect on wind speed ! 7 Centrifugal Force • When viewed from a fixed reference frame, a ball swung on a string accelerates (changes direction) towards center of rotation (centripetal acceleration). • When viewed from a rotating reference frame, this inward acceleration (caused by the string pulling on the ball) is opposed by an apparent force (centrifugal force). 8 Centrifugal Force • Magnitude CENTF = mV2/R – m is the mass – R the radius of curvature of the curved path – V is the speed of the air parcel • Direction – Pointing away from the center of the curve – The faster the speed and the tighter the curve of the path traveled (i.e., the smaller R), the larger the centrifugal force. 9 Frictional Force • Frictional drag of the ground slows wind down. FF = -kV – Magnitude • Depends upon the speed of the air parcel (V) • Depends upon the roughness of the earth’s surface (k) – Direction • Always acts in the direction opposite to the movement of the air parcel (minus sign emphasizes this) – Important in the friction layer (planetary boundary layer) • ~lowest 1000 m of the atmosphere 10 Summary of forces effecting winds . Pressure gradient force PGF = - the rate of pressure changes over horizontal distance. Always pointing towards lower pressure. The force that initiate winds . Coriolis Force CF= f V f = 2 × earth’s roation rate × sin of the latitude zero at equator and maximum at pole deflect parcel to the right of its motion in NH . Centrifugal force CENTF= V2/R important only during high winds that curve sharply (those that have a small turning radius such as hurricane and tornado) . Frictional force FF = - k V, strong in low levels 11 If there is no wind, are there any CF, CENTF, and FF ? PGF acts to cause the wind, whereas the Coriolis force, centrifugal force, and frictional force react to the wind to change its speed and/or direction It is the balance of these forces that usually determines the strength and direction of the wind. Putting force together – Balance of the forces and wind 12 Winds Geostrophic Wind Gradient Wind 13 Geostrophic Wind Geostrophic balance - the most fundamental horizontal force balance when the pressure gradient force is counter balanced by the Coriolis force PGF + CF = 0 The resulting wind is called Geostrophic wind. Geostrophic wind is good approximation to winds above earth’s surface 14 Geostrophic Wind At 1, PGF acts on the parcel, accelerating it northward. Coriolis force deflects the air towards the right of the motion (towards east here). The faster air move, the higher the Coriolis force, bending the wind more and more to its right. Eventually, at 5, the two forces cancel each other and the wind flow15 in a straight line parallel to isobars. Geostrophic Wind When the pressure gradient force is balanced by the Coriolis force, the wind is called geostrophic wind. Geostrophic wind blows in a straight line parallel to isobars (constant pressure lines) Lower PGF 994 mb Pressure V 996 mb g 998 mb CF Higher Pressure 16 How to compute geostrophic wind ? Geostrophic wind represents a balance between pressure gradient force and Coriolis force CF = PGF CF = ff ••VV wherewhere ff == 22ΩΩ sinsinφφ PGFPGF == (1/(1/ρρ)) •• ((ΔΔPP // d)d) f •V = (1/ρ) • (ΔP / d) Vg = (1/f) • (1/ρ) • (ΔP / d) 17 Buys Ballot’s Law: Stand with your back towards the wind (upper-level wind), low pressure is on your left and high pressure on your right. 18 Buy’s Law explains a fundamental feature of wind around surface low and high pressure Because the wind must blow with low pressure on its left, it must blow clockwise around high pressure center and counter clockwise around low pressure center L H Coriolis force Pressure gradient force 19 Wind Speed & Pressure Contours Just as a river flow speeds and slows when its banks narrow and expand, geostrophic winds speed as contour intervals narrow, and slow as contour intervals widen. 20 Upper Air Wind Prediction The geostrophic balance/geostrophic wind/Buys Ballot’ Law enable us to look at the upper level pressure maps and predict upper level wind direction and speed. 21 Geostrophic flow is often a good approximation high in the atmosphere (> 850 meters). Winds on a weather map are strong where the isobars or isoheights are close together and winds are weak where they are far apart 22 Gradient Balance – The Gradient Wind • The Gradient Wind is flow around a curved path where there are three forces involved in the balance (gradient balance): – 1. Pressure Gradient Force – 2. Coriolis Force – 3. Centrifugal Force Gradient balance: PGF+CF+CENTF = 0 • Important near high or low pressure centers 23 Gradient Balance – The Gradient Wind 24 Why winds near surface do not blow parallel to isobars ? Winds blow across isobars 25 What happens when we add friction? • Friction can only slow wind speed, not change wind direction • Therefore, in the northern hemisphere, if the wind speed is decreased by friction, the Coriolis force (CF=fV) will be decreased and will not quite balance the pressure gradient force – Force imbalance (PGF > CF) pushes wind in toward low pressure center and outward away from high pressure center – Angle at which wind crosses isobars depends on surface roughness • Average ~ 30 degrees 26 PGF+CF+FF=0 Lower Pressure PGF 994 mb V 996 mb FF CF 998 mb Higher Pressure The wind no longer blows parallel to the isobars, but is deflected toward lower pressure; this happens close to the ground where terrain and vegetation provide friction 27 Friction & Surface Winds Surface objects create frictional resistance to wind flow and slows the wind, diminishing the Coriolis force and enhancing the effect of pressure gradient forces. The result is surface winds that cross isobars, blowing out from highs, and in toward lows. 28 Relationship between horizontal and vertical air Motion Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules. These updrafts may cause cloud formation. Likewise, diverging air molecules from a high pressure area are replenished by downward winds. 29 SUMMARY 1. Coriolis force causes the wind to bend to the right of its path in the Northern Hemisphere and to the left of its path in the Southern Hemisphere. 2. Coriolis force increases with increasing latitude and with increasing wind speed. It only effect the wind direction, not the wind speed. 3. Where the wind blows in a straight line path, and a balance exists between the pressure gradient force and the Coriolis force, the wind is termed geostrophic. 4. Where the wind blows parallel to curved isobars, the wind is called a gradient wind. 5. Sinking air occurs above a surface high-pressure area; rising air above a surface low-pressure area. 30.
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