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 < Vg, 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 > Vg, 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…
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