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10/11/14 Primer on Forces •  Newton’s second law of mo/on Force = mass x accelera1on •  Push or pull that causes object at rest to move/alters velocity •  Accelera1on (a change in velocity) is a response to a force •  Newton’s first law of mo/on –  When the forces ac1ng on a parcel of air are in balance, a parcel either remains sta1onary or con1nues to move along a straight path at a constant speed Hydrostatic Balance: Pressure Gradient and Gravity Forces and Pressure To determine which direction the wind will blow we
must identify and examine all the forces that affect the
horizontal movement of air. Pressure Gradient Force Coriolis Effect Fric1onal Force Centripetal Force Gravity Pressure Gradient Force •  Vertical Pressure Gradient –  Lower pressure at top of air parcel than bottom –  Pressure gradient force is upward •  Gravitational : density times gravity •  Hydrostatic Balance ρg = −
ΔP
Δz
Pressure Gradient Force (PGF) and is directed from higher to lower pressure Pressure change at a constant al1tude (e.g., Sea Level Pressure) Pressure Gradient Force Coriolis Force •  Deflec1on due to curvature of Earth’s orbit •  Only apparent from frame of reference on Earth, looks like a straight path from outer space. •  Coriolis Effect only significantly influences the wind in large-­‐
scale weather systems dx Pressure Gradient is the change of pressure over a distance: Pressure Gradient = pressure difference / distance F = − 1 ΔP
Tighter gradient = stronger force ρ Δx
•  Deflec1on due to the Coriolis force depends upon: 1. The rota1on of the earth and la1tude 2. The object’s speed •  Acts at right angles to the wind, and only influences wind direc1on not speed. 1 10/11/14 Geostrophic Balance Pressure Gradient and Coriolis Ridge: height contours bend poleward,
associated with warm air mass
1. Air accelerates in direc1on of pressure gradient force 2. Coriolis Force acts to the right of mo1on w/speed of air parcel 3. Eventually a balance is reached between -­‐ Pressure Gradient Force -­‐ Coriolis Force This is called geostrophic balance and the geostrophic wind Trough: height contours bend equatorward,
associated with cold air mass
Es1ma1ng Geostrophic Winds Geostrophic winds assume balance between Coriolis and Pressure Gradient Force and no accelera1on ug = −
g ΔZ
f Δy
vg =
g ΔZ
f Δx
In midla1tudes f = 1x10 -­‐4 1/s Areas of low pressure (L) and high pressure (H) are shown.
Arrows indicate wind direction– the direction from which the
wind is blowing.
Note the cellular structure at surface and zonal aloft
Wind: Joining Forces Gradient Wind Geostrophic around low/high Centripetal force inward toward center, influences direc1on of mo1on On a typical surface weather map, isobars exhibit clockwise (an1cyclonic) curvature (ridges) and counterclockwise (cyclonic) curvature (troughs). 2 10/11/14 Fric1on Surface Winds Frictional Force –  Retards velocity –  Since Coriolis is a function of velocity it weakens as well –  Winds cross isobars at an angle that depends on roughness of Earth’s surface. •  Atmospheric boundary layer – zone to which fric1onal resistance (eddy viscosity) confined ~ 1000m above ground level •  Turbulence – fluid flow characterized by eddy mo1on •  We experience turbulent eddies as gusts of wind Wind changes in direction with height •  Fric1onal forces wane away from the surface and are nominal above the boundary layer. •  Convergence for surface L and rising mo1on •  Angle varies from 10-­‐45 degrees •  Smooth surface 10-­‐degrees •  Rough surface 45-­‐degrees Con1nuity of Wind •  In a surface low, horizontal winds converge toward the center. –  Air ascends due to converging surface winds and diverging winds alo\ L
Three Dimensional View Estimating Wind Direction and Pressure Aloft by Watching Clouds
Upper-level clouds moving from the southwest indicate isobars and winds aloft.
Buys-Ballot law: wind @ your back, turn 45-degrees clockwise, low on your left
3 10/11/14 Same PGF across surface Scales of Weather Systems •  Planetary-­‐scale systems: large-­‐scale wind belts encircling the planet (midlatitude westerlies, trade winds) •  Synoptic-­‐scale systems: continental or oceanic in nature (migrating cyclones, hurricanes, and air masses) •  Mesoscale-­‐scale systems: circulation systems that influence weather in part of a large city or county (thunderstorms, sea breeze) •  Microscale systems: weather system covering a very small area such as several city blocks (weak tornado) Jet Streams: Barrier between cold and warm •  Sharp gradient between cold & warm air masses –  Denser in cold side allows for pressure drops more rapidly in with height above surface –  With no ini1al pressure gradient at surface, horizontal pressure gradient alo\ increases with increasing al1tude –  Strongest gradients in what season? Cold Head, Warm Belly Westerlies Alo\ COLD AIR WARM AIR 1. Westerlies alo\ in midla1tudes is a consequence of pole to equator temperature gradient modifed by the Coriolis Force 2. Jet streams: high-­‐speed westerly winds along certain other global la1tude zones at high levels that form due to presence of strong temperature gradients. Midla1tude Waves Westerly winds follow a wavelike pattern of ridges and troughs Responsible for movement of the synoptic-­‐scale weather systems Typically 2-­‐6 large scale Rossby waves at any given time 4 10/11/14 Zonal and Meridional Flow upper-­‐level ridge Blocking Pagern Split flow omega block Blocking Pagern Short Waves •  Short Waves –  A ripple superimposed on Rossby long waves –  Propagate rapidly through the Rossby waves –  12 circle the midlatitudes at any given time –  Associated with the mid-­‐latitude weather disturbances Cyclone Development accelerate decelerate –  In flowing into a trough, Westerlies slow, inducing speed convergence aloft; in flowing into a ridge, the Westerlies accelerate, inducing speed divergence aloft –  Upper-­‐level divergence results in ________ –  Upper-­‐level convergence results in __________ 3-­‐D View of Cyclones and An1cyclones Development: convergence at the surface, divergence alo\ Cyclone Development * Accelerate * DIVERGENCE * Decelerate * Jet streak moving at ~ 25 m/s Winds move at 200 m/s CONVERGENCE In a straight jet streak, the strongest horizontal divergence is in the left-­‐
front quadrant, supplying upper-­‐air support for cyclone development. 5