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Atmospheric motion (Geostrophic balance (Density of air changes very…
Atmospheric motion
Geostrophic balance
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Geostrophic wind speed is directly proportional to the pressure gradient and inversely dependent on latitude
For a fixed pressure gradient, the geostrophic wind speed decreases towards the poles
Density of air changes very little at a fixed altitude, and is usually assumed constant, but decreases significantly with increasing altitude
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Frictional drag
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Lower wind speed results in a smaller coriolis force, hence reduced turning to right
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Coriolis force Fc
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The coriolis force= an apparent force, introduced to account for the apparent deflection of a moving object observed from within a rotating frame of reference (Earth)
In the atm we are concerned with the horizontal component of coriolis force, with a magnitude of: 2Ω V sinlat = f V
Ω= angular velocity, V= wind speed, sinlat = f V
This is a max at the poles & zero at equator- results in reflection to the right in the NH and to the left in SH
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Centripetal acceleration
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The required centripetal acceleration is provided by imbalance between the pressure and coriolis force
For a low, the pressure force is greater than the coriolis force: gradient wind is subgeostrophic around LOW
For a high, the pressure force is less than the coriolis force
gradient wind is supergeostrophic around HIGH
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Global circulation
For a non-rotating sphere, convection could form simple symmetric cells in each hemisphere
Within each cell, Coriolis force turn winds to east or west
Coriolis force turns the air flow- stable mean temp has 6 counter rotating cells (3 in each hemisphere)
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Forces on air
Steady slow
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When the forces are balanced, the airflow is steady
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