Gradient Winds

Gradient Wind (or flow) develops only in the absence of friction, when considering curved flow and flows perpendicular to the contours, for the same reason as in geostrophic flow. However, gradient wind is not truly geostrophic because it is constantly moving, thus undergoing an acceleration. Nonetheless, this time, in order for the air to follow parallel to the contours there must consider the effects of the centrifugal force as well as the pressure gradient force and the Coriolis force.

Subgeostrophic Flow is when V < V_g, air curves cyclonically (counter-clockwise), and the CF needs greater than in the geostrophic case in order to balance the PGF.

Supergeostrophic Flow is when V > V_g, air curves anti-cyclonically (clockwise), and the CF does not need to be as great as in the geostrophic case in order to balance the PGF.

Putting it all together…