1. Class #14: Monday, February 9
Gradient wind balance
Geostrophic adjustment and friction
Thermal circulations
Monday, Feb. 9, Class #14

2. Gradient wind balance
Includes the horizontal pressure gradient force, the Coriolis force, and the centrifugal force.
Applies when the wind blows along a curved path.
The centrifugal force is oriented outward.
Is stronger than the geostrophic wind (super-geostrophic) around high pressure centers.
Is weaker than the geostrophic wind (sub-geostrophic) around low pressure centers.
Monday, Feb. 9, Class #14

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4. Adjustment to geostrophic balance
Starts with the horizontal pressure gradient force.
Continues when the moving air responds to the Coriolis force by curving.
The moving air continues to respond to the horizontal pressure gradient force.
Rightward curvature (NH) continues.
Eventually the wind becomes geostrophic.
Monday, Feb. 9, Class #14

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6. Including friction: Guldberg—Mohn balance
Friction slows the wind.
Friction reduces the strength of the Coriolis Force because the wind is weaker.
The wind cannot turn to the right enough to completely balance the horizontal pressure gradient force.
The wind blows across the contours toward lower pressure (or away from high pressure).
There is inflow to lows and outflow from highs near and at the surface, where friction matters.
Monday, Feb. 9, Class #14

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11. The thermal circulation
The sea breeze is a thermal circulation.
A thermal circulation has both horizontal and vertical air motions.
The horizontal pressure gradient force is most important in a thermal circulation.
Upward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column.
Monday, Feb. 9, Class #14

12. More on the thermal circulation
The thermal circulation begins aloft.
In diagrams of the thermal circulation, “H” and “L” refer to the horizontal pressure gradient, not to the vertical pressure gradient.
The thermal circulation comes about because hydrostatic balance requires that the warmer air column expands compared to the cooler air column.
Monday, Feb. 9, Class #14

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14. The sea breeze Is a daytime circulation.
Depends on differential heating at the surface between land and water.
Has the warmer air column over the land, which absorbs more incoming solar radiation.
Has the cooler air column over the water, which absorbs less radiation.
Monday, Feb. 9, Class #14

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22. The sea breeze and the land breeze
As solar heating diminishes in the late afternoon, the sea breeze weakens.
At night, differential cooling occurs.
The cooler air column is over land, where radiational cooling is more rapid than over the land.
The warmer air column is over the water.
The land breeze develops at night.
Monday, Feb. 9, Class #14

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