PDS: The Planetary Atmospheres NodeRossby Number
A small Rossby number indicates that the geostrophic approximation is valid ( i.e., du/dt = dv/dt = 0) and that the Coriolis acceleration, v · f, is greater than the horizontal fluid acceleration, du/dt.
Ro = (du/dt)/(v f) 
(v2/r)/(v f)
= (v/r)/f = U/(f0 · L)
For the GRS, hurricanes, etc.
Ro = (Vt a/b2/
3)/f
2 = [ cos2(
) + (a4/b4) · sin2() ] /
[ cos2() + (a2/b2) · sin2() ]
where Vt is the tangential velocity, a is the semimajor axis,
b is the semiminor axis, and is the angle between the semi-major
axis and the direction of motion.
Mitchell et al. 1981 JGR 86 p.8751.
| terrestrial | Red Spot | FA/BC/DE | small spots | |
| Vt | 110 m/s | 120 m/s | ||
| a | 1.1E7 m | 4.9E6 m | ||
| Ro(0) | .36 | .36 | ||
| Ro(90) | .04 | .08 | ||
| g.lat | 22 S | 33 S | 40.5 S |
