2 Atmospheric Forces Wind Relationships Pressure Forces Apparent Forces FrictionConfluence and DifluenceGradient windsSuper and Subgradient WindsGeostrophic Winds
3 Forces of Nature Pressure Gradient (PG) - The rate of pressure change withdistance. Always measured perpendicular to the isobars, orPG=Dp / Dn (Change in pressure over distance)Pressure Gradient Force (PGF) - A force acting on a parcel of air,resulting from the difference in pressure on either side ofthe parcel.1. What force initiatesthe wind?NaturalCoordinateSystemPressure Gradient Force2. If the gradient force ishigher, what happens tothe wind?It IncreasesPressure Gradient Pressure Gradient ForceMeasured on the surface of the Earth.
4 Forces of NatureHow do we find highs and lows in the upper atmosphere?Use a constant pressure surface chartContour Gradient500mb Surface (Avg 18 kft)
5 Forces of Nature Contour Gradient (CG) - The rate of height change (Dz) withdistance (Dn) on a constant pressure surface.CG=Dz / Dn (Change in height over distance)Contour Gradient Force (CGF) - The force that represents PGFon a constant pressure chart.Differences in virtual temperatureand quantity of mass over a givenpoint in the atmosphere results indifferences in thickness betweenlayers. These variations in heightresult in the CG.500mbABContour Gradient Contour Gradient Force500mb Surface (Avg 18 kft)
6 Forces of NatureCoriolis Force (CoF) - An apparent force exerted on movingobjects caused by the rotation of the Earth and it’s coordinatesystem.Zero at the Equator and maximum at the polesVelocity dependent. With constant latitude, as a parcel’svelocity increases, then CoF increases.CoF acts 90 degrees to the wind direction (toward the right ofmotion in the northern hemisphere)The closer you get to the poles,the stronger the CoF, if the windspeed remains the same.
7 What are the relationships of velocity and curvature on the Forces of NatureCentrifugal Force (CeF) - The apparent force that deflectsparticles and therefore winds away from the center of rotation.What are the relationships ofvelocity and curvature on thestrength of CeF?Strength of force is dependent onthe velocity of the parcel, and therate of curvature:Centrifugal Force is always directedoutward from the axis of rotation andperpendicular to rotation.CeF helps weaken the gradientin a High and tighten the gradientin a Low.Constant curvature Constant Velocityas v CeF as r Cef??
8 Forces of NatureFrictional Force (Fr) - The force that opposes motion.Acts opposite to the direction of motion.No direct effect on PGF or CGF, but does affect CoF and CeF.Increasing Fr results in decreasing wind velocity.Fr causes both CoF and CeF to decrease, therefore…Winds tend to backtowards the strongerforce (PGF or CGF).Wind without the effects of friction,and PGF/CGF and CoF in balance, iscalled a … ?Geostrophic Wind
9 Forces of Nature In Conclusion…. Pop Quiz! Force Effect Wind Speed… Effect Wind Direction…PGF/CGFCeFCoFFrYes (intiates wind)Yes (high to low)NoYes (curved flow)NoYes (90 deg to RT of flow)Yes (slows)Yes (backs)
10 Wind RelationshipsGeostrophic Wind: The wind that would result if there were abalance between the Coriolis Force and Pressure Gradient Force.Assumes: Straight line flow - no curvature.Assumes: No friction. Most often possible above the Earth’s surface.Forces are in balance, so wind speed is constant.Can there ever truly be a Geostrophic wind in our atmosphere?Maybe? How about very short periods of time?Straight Line Flow (No Friction)Ageostrophic flow (friction added-straight line)
11 Wind RelationshipsGradient Winds - In Curved Flow with no change in gradient -Anticyclonic Gradient Wind:Coriolis force must balance both Pressure/ContourGradient Force and Centrifugal Force.HCeFCGFCoFVa* Va will be stronger than the expectedgeostrophic wind at the same latitude andPGF or CGF (supergeostrophic).** Remember - with latitude constant, anincrease in CoF will increase wind speed.Max Speed is limited dueto finite CoF.
12 Wind RelationshipsGradient Winds - In Curved Flow with no change in gradient -Cyclonic Gradient Wind:Coriolis Force and Centrifugal Force acttogether to balance Pressure Gradient Force.LCeFCoFCGFVc** Vc will be weaker than the expectedgeostrophic wind at the same latitude andPGF or CGF (subgeostrophic).Max speed is unlimited.
13 Wind Relationships Gradient Level Winds What is the Gradient Level? The top of the boundary layer, or the height where friction,wind shear, and/or surface stability end.Terrain - Rougher the terrain, the greater the mixing.Wind - Greater wind means more mechanical mixingStability - The less stablethe atmosphere, the moremixing (stronger upwardvertical motion)Sfc Heating Normal Sfc CoolingGradient Level
14 Wind Relationships Confluence and Difluence What is Confluence? The directional merging of wind flow.What is Difluence?The directional spreading apart of wind flow.
15 Wind Relationships Supergradient and Subgradient Winds Supergradient Winds - A wind adjusting to the rapid decreasein Contour Gradient Force downstream.540CGFCoFVg552546Winds turn “Uphill”towards higher values
16 Wind Relationships Supergradient and Subgradient Winds Subgradient Winds - A wind adjusting to a rapid increase inContour gradient force downstream.CGFCoFVg546546552CGFCoFVg552CGFVg558558564CoFWinds turn “downhill”towards lower values564
17 Jet Streams What is a Jet Stream? Relatively strong, quasi-horizontal winds concentrated withina narrow air current in the upper troposphere. “Ameandering, fast-moving river of air”.What is it’s cause?Large horizontal temperature contrasts produce strongcontour/pressure gradients and strong winds.
18 L H Jet Streams Forms at the Tropopause - CGF at it’s maximum Jet Core - region of relatively uniform wind speeds within theinterior of the Jet.Jet Axis - Axis of max wind speed at the core (remember sub/supergeostrophic) .Jet Max - Intermittent regions of strongest wind speeds. Veryimportant to the strengthening/weakening of meteorologicalphenomenon.L709011090H70
19 Jet Streams Polar Front Jet- Primary Cause: Horizontal Temperature ContrastHeight varies with the season, near 200 mb in summer,300 mb in winter. Why?The atmosphere is thicker in the summer.Migrates north and south with the seasons.Stronger in the winter than the summer. Why?Thermal contrast is strongest in the winter.Found on the 500 mb chart at approximately the 560 line(5560 meters).Found near the -17 degree line on a 500 mb chart.
20 Jet Streams Subtropical Jet- Convergence aloft and Horizontal Temperature ContrastPrimary Cause:Conservation of Angular Momentum - causes maximumsAM=m v r (mass constant, so r decreases, increasing v)Height approximately 150 to 200 mb.Location: between 25 and 30 Deg latitude, mean positionabout 28 Deg.Stronger in the winter than the summer.Confluence with the PFJ results in favorable conditionsfor severe turbulence.Diffluence of the two jets results in severe weather justdownstream of the jet max. Look for the “wedge” on asatellite picture (MCC)
21 Jet StreamsPolar Front Jet and Subtropical Jet interaction -PFJSTJ