Geostrophic winds (PGF + Corriolis by Ekman distribution)
Centrifugal force
Acts outwards from the centre of rotation of a point of spinning object
What is pressure gradient force
This is driven by increased
Corriolis force and trade winds
Trade winds coming down to equator... deflects to right in the NH and to the left in the SH
What are the absolute values of corriolis force with increasing latitude = deflection
0 at 0 degrees 0.25 at 10 degrees 0.5 at 20 grees 1.00 at 43 degrees and 1.458 at 90 degrees In f(10^-4s^-1) Barry, Chorley 2010
Effect of friction on windspeed and thus geostrophic winds
1) reduces wind speed 2) therefore reduces Coriolis force such that PGF>CF. means the wind is no longer geostrophic. 3) Flow therefore crosses isobars albeit at a lower surface 3) only applies at surface where friction is significant
Equation for geostrophic wind
1/P x Dp/Dn= 2wsinØV
What is the pressure at the surface and what is the pressure at 5km altitude? What does this do to the PGF
1000hPa and 500 hPa Doubles the pressure gradient force
What is the earth's angular velocity
15 degrees hr^-1 or 2pie/24 radians hr^-1
Equation for geostrophic balance (aka gradient wind)
2wsinØV + V2/r + 1/P x dp/dn = 0 2wsinØV= CF V^2/r = Cent 1/P x dp/dn = PGF
Effect of centrifugal force
Acts with or against geostrophic wind depending upon whether zone of low pressure is further from or closer to the centre of rotation
Process of generating a geostrophic wind
Air is initially at rest PGF causes it to move towards high and low pressure... acceleration is positively proportional to PGF As air accelerates CF increases proportionally with velocity of wind Wind is deflected to the right eventually PGF is equal to corriolis force Thus vectors balance and a geostrophic wind is created- this runs parallel to isobars
explanation of coriolis
At higher latitudes there is less distance travelled with the rotation of the Earth therefore angular velocity is lower than at the equator where circumference is wider and thus each point must travel further per second in a given rotation. When the cloud moves North there is the conservation of angular momentum then the cloud will be moving quicker than the surface so is deflected. The cloud moves faster than the surface when it moves from equator towards the pole. The cloud is moving faster than the surface. This causes it to appear to accelerate towards the east/right.
Ekman spiral
Decreasing influence of friction. moving away from the surface causes a directional change of the winds with height
What is the corriolis force
Deflects wind at right angles To the right in the northern hemisphere and to the left in the southern hemisphere Deflection is proportional to the wind speed
Two reasons geostrophic winds are found in upper atmosphere
Density according to 1/P x Dp/Dn Less friction
Defintion geostrophic wind:
Driven by two forces... PGF ... this is force which causes air to move from low to high pressure e.t.c CF: deflective force... conservation of angular velocity with increase latitude means wind accelerates in relation to surface... deflected to right/ left Geostrophic wind: where these are balanced .... wind thus runs parallel to two isobars.... Draw diagram go through stages below ......
Define equation for corriolis force
F =2wsinØV w= earth's angular velocity = constant Sin=constant Ø(theta)= latitude V= wind velocity
Effect of friction on anticyclonic and cyclonic zones
Friction stops wind travelling parallel to isobars (aka prevents geostrophic wind) This means that air moves out of high pressure zone (anticyclone decays) This means air moves into low pressure zone (cyclone decays)
Effect of hydrostatic equillibrium on wind velcoity
Horizontal air motion is generally 100 times faster than vertical air motion (Barry, Chorley 2010)
What drives the winds?
Horizontal pressure differentials in the atmosphere. Wind goes from high to low pressure.
Why do vertical winds not occur in the atmosphere
Hydrostatic equilibrium This is the balancing of vertical pressure gradient force and downwards pull of earth's gravitational field (Barry, Chorley 2010)
Centrifugal force when PGF is directing wind from high to low further from centre of rotation (Aka at high pressure zone)
IN this instance PGF and Cent are balancing CF and thereofre wind velocity increases - wind velocity is supergeostrophic
Effect of seasons on jet stream + consequences
In the winter, the enhanced poleward temperature gradient enhances pressure gradient between equator and poles (fixed distance) and accordingly leads to stronger jet streams... polar jet stream especially... (talk about radiation budget for poles being 0 in winter due to no daylight)! Density varies inversely with air temperature (barry, chorley 2010) Wetter weather in the UK by enhanced cyclogenesis (Met office)
Centrifugal force moving towards low closer to centre of rotation (aka in low pressure zone)
In this instance PGF is balancing corriolis and centrifugal force so CF is less than PGF and wind speed is decreased) act against it and therefore you get subgeostrophic wind
What factors increase the corriolis force
Increasing latitude at equator SinØ= 0 at poles SinØ= 1 Increasing altitude in accordance with PGF which increases wind velocity
What causes the decrease in air pressure with distance from the surface
More air above to be pulled down by gravitational pull of the earth
Geostrophic wind is idealised
Neglects friction + curvature of isobars ( and thus centrifugal force)
Where is geostrophic wind found in atmoshere
On the altitudinal axis it is found anywhere above 1km.... (density + friction) This is where density + friction low enough Parallel to two isobars .... this is where corriolis force.... Latitude: found anywhere between subtropics and poles... .... can't be found at tropics because corriolis force is too low Crucial geostrophic wind is the jet stream : polar jet stream Doesn't occur between tropics....
Three forces acting on wind besides friction (e.g above 1000m)
PGF and Corriolis force...
What is the pressure gradient force equation define the terms
PGF= 1/P X Dp/ Dn PGF= Pressure gradient force P= Density (Rho) Dp= Pressure Change Dn= distance between isobars thus Dp/Dn = pressure change over distance
Complexity of geostrophic wind
Pressure systems are rarely stationary and therefore geostrophic wind balance must constantly adjust itself
What factors increase the pressure gradient force
Reducing the density of the atmosphere (e.g increasing height) Increasing the pressure change for a given distance Reducing the distance for a given pressure change
Seasonal variability of jet stream
Strongest in winder where temp and thus pressure differential between latitudes is strongest
Where are jet streams
Subtropical (20 and 30 degrees) and polar (50-60 degrees) Barry, Chorley 2010
What is the magnitude of corriolis force directly proportional to
The horizontal velocity of the wind And the sine of the latitude
Met office notes on jet stream:
The jet stream is a core of strong winds around 5 to 7 miles above the Earth's surface, blowing from west to east. Wind speeds can exceed 200 mph but we don't see or feel this at ground level. 100km wide only a few km dee Major jet streams are polar front and subtropical jet
What causes pressure differentials in the atmosphere
Uneven heating causing differences in air density and thus air pressure (Barry, Chorley 2010) At any given, same height, because cold air is denser, the pressure in hot air will be greater than the pressure in a cold column of air and therefore air moves down this pressure gradient (Met Office)
What is the velocity of the geostrophic wind given as (mathmetically) What does this show
Vg= 1/ 2wsinØV x Dp/ Dn Velocity of geostrophic wind is inversely proportional to latitude.... (Barry, Chorley 2010)
Difference between sub and super geostrophic
Where Cent acts against PGF then you get sub-geostrophic which are slower. This is because to maintain the geostrophic balance there needs to be a reduction in wind velocity in order to to reduce CF Where Cent acts with PGF then super geostrophic. To maintain geostrophic balance velocity must increase... therefore faster
Where are geostrophic winds found
above 1km in subtropics
