GFD Lab X - Ekman layers: frictionally-induced cross-isobaric flow
We bring the cylindrical tank, filled to a depth of 10 cm or so with water at a uniform
temperature, up to solid-body rotation at a speed of 5 rpm, say. Sprinkle a few small
crystals of Potassium Permanganate in to the tank. Note the Taylor columns.
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The figure below shows schematics of a cyclone (low pressure system) and an anticyclone (high pressure system). In the free atmosphere, where the flow is geostrophic, the flow just rotates around the system; cyclonically around the low, anticyclonically around the high. Near the surface, however, the wind deviates toward low pressure, inward in the low, outward from the high. The boundary layer within which this happens is known as the Ekman layer. Because the horizontal flow is convergent into the low, mass continuity demands a compensating vertical outflow. This Ekman pumping produces ascent---and, in consequence, clouds and rain ---in low pressure systems. In the high, the divergence of the Ekman layer flow demands subsidence (Ekman suction) and are dry, which is why high pressure systems tend to be characterized by clear skies.