Atmospheric Forces Wind Relationships

Atmospheric Forces Wind Relationships Pressure Forces Apparent Forces Friction Confluence and Difluence Gradient winds Super and Subgradient Winds Geostrophic Winds

Forces of Nature Pressure Gradient (PG) - The rate of pressure change with distance. Always measured perpendicular to the isobars, or PG=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 of the parcel. 1. What force initiates the wind? Natural Coordinate System Pressure Gradient Force 2. If the gradient force is higher, what happens to the wind? It Increases Pressure Gradient Pressure Gradient Force Measured on the surface of the Earth.

Forces of Nature How do we find highs and lows in the upper atmosphere? Use a constant pressure surface chart Contour Gradient 500mb Surface (Avg 18 kft)

Forces of Nature Contour Gradient (CG) - The rate of height change (Dz) with distance (Dn) on a constant pressure surface. CG=Dz / Dn (Change in height over distance) Contour Gradient Force (CGF) - The force that represents PGF on a constant pressure chart. Differences in virtual temperature and quantity of mass over a given point in the atmosphere results in differences in thickness between layers. These variations in height result in the CG. 500mb A B Contour Gradient Contour Gradient Force 500mb Surface (Avg 18 kft)

Forces of Nature Coriolis Force (CoF) - An apparent force exerted on moving objects caused by the rotation of the Earth and it’s coordinate system. Zero at the Equator and maximum at the poles Velocity dependent. With constant latitude, as a parcel’s velocity increases, then CoF increases. CoF acts 90 degrees to the wind direction (toward the right of motion in the northern hemisphere) The closer you get to the poles, the stronger the CoF, if the wind speed remains the same.

What are the relationships of velocity and curvature on the Forces of Nature Centrifugal Force (CeF) - The apparent force that deflects particles and therefore winds away from the center of rotation. What are the relationships of velocity and curvature on the strength of CeF? Strength of force is dependent on the velocity of the parcel, and the rate of curvature: Centrifugal Force is always directed outward from the axis of rotation and perpendicular to rotation. CeF helps weaken the gradient in a High and tighten the gradient in a Low. Constant curvature Constant Velocity as v CeF as r Cef ? ?

Forces of Nature Frictional 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 back towards the stronger force (PGF or CGF). Wind without the effects of friction, and PGF/CGF and CoF in balance, is called a … ? Geostrophic Wind

Forces of Nature In Conclusion…. Pop Quiz! Force Effect Wind Speed… Effect Wind Direction… PGF/CGF CeF CoF Fr Yes (intiates wind) Yes (high to low) No Yes (curved flow) No Yes (90 deg to RT of flow) Yes (slows) Yes (backs)

Wind Relationships Geostrophic Wind: The wind that would result if there were a balance 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)

Wind Relationships Gradient Winds - In Curved Flow with no change in gradient - Anticyclonic Gradient Wind: Coriolis force must balance both Pressure/Contour Gradient Force and Centrifugal Force. H CeF CGF CoF Va * Va will be stronger than the expected geostrophic wind at the same latitude and PGF or CGF (supergeostrophic). ** Remember - with latitude constant, an increase in CoF will increase wind speed. Max Speed is limited due to finite CoF.

Wind Relationships Gradient Winds - In Curved Flow with no change in gradient - Cyclonic Gradient Wind: Coriolis Force and Centrifugal Force act together to balance Pressure Gradient Force. L CeF CoF CGF Vc ** Vc will be weaker than the expected geostrophic wind at the same latitude and PGF or CGF (subgeostrophic). Max speed is unlimited.

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 mixing Stability - The less stable the atmosphere, the more mixing (stronger upward vertical motion) Sfc Heating Normal Sfc Cooling Gradient Level

Wind Relationships Confluence and Difluence What is Confluence? The directional merging of wind flow. What is Difluence? The directional spreading apart of wind flow.

Wind Relationships Supergradient and Subgradient Winds Supergradient Winds - A wind adjusting to the rapid decrease in Contour Gradient Force downstream. 540 CGF CoF Vg 552 546 Winds turn “Uphill” towards higher values

Wind Relationships Supergradient and Subgradient Winds Subgradient Winds - A wind adjusting to a rapid increase in Contour gradient force downstream. CGF CoF Vg 546 546 552 CGF CoF Vg 552 CGF Vg 558 558 564 CoF Winds turn “downhill” towards lower values 564