Note that the wind speed for gradient flow differs from the wind speed of geostrophic flow. Let’s see why. Start with the geostrophic balance (Equation [10.36]) and rearrange the equation to get an expression for the geostrophic wind speed:
Replacing the pressure gradient force ( ) with –fvg in the gradient balance equation results in an equation that relates these gradient velocities to the geostrophic velocity:
In a regular low (middle, figure below), R > 0 so that vg > V. The velocity in a curve around a low-pressure area is subgeostrophic.
In a regular high (right, figure below), R < 0 so that vg < V. The velocity in a curve around a high-pressure area is supergeostrophic.
Think of it this way. The pressure gradient force is independent of velocity and so is always there for a given geopotential gradient. In a regular low, the centrifugal and Coriolis forces, both dependent on velocity, sum together to equal the pressure gradient force, whereas for geostrophic flow, only the Coriolis force does. Thus, the velocity in the gradient balance case must be less than the geostrophic velocity for the same geopotential gradient.
So how do subgeostrophic and supergeostrophic flow affect weather?
Supergeostrophic flow around ridges and subgeostrophic flow around troughs helps to explain the convergence and divergence patterns aloft that are linked to vertical motions.
Look at the figure below, starting on the left. Going from geostrophic flow in the straight section to supergeostrophic flow at the ridge’s peak causes divergence aloft. This divergence causes an upward vertical velocity, which causes a low pressure area and convergence at the surface. As the air rounds the ridge’s peak, it slows down to become geostrophic, and then continues to slow down even more as the flow becomes subgeostrophic around the trough, thus causing convergence aloft. This convergence aloft causes a downward velocity, which causes high pressure and divergence at the surface.
So downwind of a trough is the favored location for divergence aloft, upward motion, and a surface low. Downwind of a ridge is the favored location for convergence aloft, downward motion, and a surface high. Since ridges form around high pressure aloft and troughs form around low pressure aloft, we see that the high aloft is offset relative to the surface low and the low aloft is offset relative to the surface high.
Thus subgeostrophic flow and supergeostrophic flow aloft are directly related to the formation of weather at the surface. Other factors like vorticity are also very important. The video below (1:09) describes how the gradient wind flow aloft affect surface weather.