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Lecture 7 • Forces (gravity, pressure gradient force) • Imaginary forces (Coriolis, centrifugal) • Force balance and resulting horiz. wind • Geostrophic wind • Gradient wind • Adjustment to balance • The thermal “wind” (change in geostrophic wind in the vertical)
Wind direction is the direction from which the wind is blowing. Wind direction is expressed in degrees Units: m/s or knots 1 knot = 0.5 m/s 1knot ~ 1 mile/hr Wind speed is expressed in terms of the flagpole
Newton’s law of motion • A body at rest tends to stay at rest; a body in motion tends to stay in motion, traveling at a constant speed in a straight line. • A force exerted on a body of mass m causes the body to accelerate in the direction of the applied force. Force = mass x acceleration
A force has direction and magnitude (it is a vector). Adding two forces is vector addition.
Forces that move the air • Gravitational force (g=9.8 m/s2) • Pressure gradient force (-1/rho x dp/dx or --1/rho x dp/dy). It points toward lower p. The pressure gradients causing the wind are horizontal. • Coriolis force • Centrifugal force • Frictional force
Pressure gradient force pushes from higher to lower pressures PGF Magnitude depends on value of pressure gradient
Vertical structure in the atmosphere • What about pressure? • Hydrostatic equation: balance between pressure gradient force and gravity. • dp/dz = - rho g • Ideal gas law: • p = rho R T Let’s go to the board! z = - H ln (p/p0), where H is scale height and is only constant if T is constant. In other words, p = p0 exp(- z/H)
Next: Coriolis forceEarth’s rotational speed is greatest at the equator and exactly zero at the poles
Coriolis deflection Coriolis force deflects moving air to the right in the Northern Hemisp. Coriolis force deflects moving air to the left in the S. Hemisphere
The magnitude of the Coriolis force (CF) is proportional to the wind speed and sine of latitude CF= f x V, Where f is 2xEarth’s rotation rate xsin(latitude)
Centrifugal force: arises because the trajectory is curved. CENTF= V^2/R, where R (radius of curvature) is positive for cyclones, negative for anticyclones
Frictional force is proportional to the wind- speed and directed opposite to the wind dir. FF = -k V (where k describes the roughness)
Summary: The Forces • Gravity – the strong silent type • PGF arises from pressure gradients generated by differential solar heating –leads to wind. Only then do the other forces start acting. • CF, CENTF, FF all depend of V, wind speed.
Atmospheric force balancesSum of forces = mass x accelerationBalance when the forces add up to zero • Hydrostatic balance in the vertical (gravity does not cause wind). • Strong horizontal PGF means strong wind • CF changes wind direction not speed • CENTF only acts on curved flow • FF slows down the wind (regardless of direction).
Wind blows counterclockwise around lows (cyclonic wind in cyclones), clockwise around highs (anticyclones) Force balance not quite right since we have curved flows Add centrifugal force
Gradient balance results in gradient wind • Represents balance of three forces • It is an excellent approximation to free atmospheric flow. • Around highs it is supergeostrophic • Around lows it is subgeostrophic
Adjustment to balance • The atmosphere tries hard to stay in balance, but it is constantly being pushed away from it. • The atmosphere adjusts very quickly (in a matter of minutes) to imbalance.