Rossby number

The Rossby number, named for Carl-Gustav Arvid Rossby, is a dimensionless number used in describing geophysical phenomena in the oceans and atmosphere. It characterises the ratio of inertial forces in a fluid to the fictitious forces arising from planetary rotation. It is also known as the Kibel number.

It is defined as:

- where U and L are, respectively, characteristic velocity and length scales of the phenomenon, Ω the angular velocity of planetary rotation, and φ the latitude. The term 2 Ω sin φ is the Coriolis acceleration.

When the Rossby number is large, the effects of planetary rotation are unimportant and can be neglected.