Difference in Supergradient/subgradient wind and Supergeostrophic/subgeostrophic wind

[Andrew Clope]

Reading through Tim Vasquez' book chapter one (Physics) and the way it seems to be described they sound very much like the same thing. Both dependent on PGF and the Coriolis Effect?

It seems almost as if Gradient is naming friction as a factor, but geostrophic is implying it?

 

[Andrew Clope]

So essentially could I assume that gradient winds are surface level sort of and geostrophic are upper atmosphere

 

[Jeff Snyder]

The geostrophic approximation assumes that the only two forces affecting the horizontal motion of an air parcel are the pressure gradient force and the Coriolis force. This is a problem, since it doesn't account for curvature of the flow -- geostrophy requires straight flow. When the flow aloft is curved (e.g., is characterized by troughs and ridges), air parcels also experience a centrifugal/centripetal force. The gradient wind is that which results from the effects of the pressure gradient force, the Coriolis force, and the centrifugal force. Gradient wind "balance" is typically more valid in the mid- and upper-troposphere.

You'll notice that viscous effects (i.e., friction though interaction with the ground, turbulent mixing in the planetary boundary layer, etc.) are absent from both of these "winds". As such, they aren't really an adequate to describe the forces affecting air near the ground. In addition, much of the "interesting" weather occurs as a result of ageostrophic accelerations (that is, processes that affect and are affected by the ageostrophic wind, or the difference between the "real" wind and the geostrophic wind). Down near the ground, where friction is more important, air parcels are not in geostrophic balance.

If the wind is "supergeostrophic", then the wind speed is higher than the geostrophic wind speed; if the wind is subgeostrophic, it is weaker than the geostrophic wind. Supergradient and subgradient winds, although less commonly used, would be analogous to super- and subgeostrophic winds although pertaining to gradient winds. You don't need to think of these as different "winds" but rather as estimated "winds" if only certain forces are included (or as components of the total wind).