The Rossby number (Ro) named for Carl-Gustav Arvid Rossby, is a dimensionless number used in describing fluid flow. The Rossby number is the ratio of inertial to Coriolis force, terms and in the Navier–Stokes equations, respectively. It is commonly used in geophysical phenomena in the oceans and atmosphere, where it characterizes the importance of Coriolis accelerations arising from planetary rotation. It is also known as the Kibel number.
The Rossby number (Ro and not Ro) is defined as:
where U and L are, respectively, characteristic velocity and length scales of the phenomenon and f = 2 Ω sin φ is the Coriolis frequency, where Ω is the angular frequency of planetary rotation and φ the latitude.
A small Rossby number signifies a system which is strongly affected by Coriolis forces, and a large Rossby number signifies a system in which inertial and centrifugal forces dominate. For example, in tornadoes, the Rossby number is large (≈ 103), in low-pressure systems it is low (≈ 0.1 – 1) and in oceanic systems it is of the order of unity, but depending on the phenomena can range over several orders of magnitude (≈ 10−2 – 102). As a result, in tornadoes the Coriolis force is negligible, and balance is between pressure and centrifugal forces (called cyclostrophic balance). Cyclostrophic balance also commonly occurs in the inner core of a tropical cyclone. In low-pressure systems, centrifugal force is negligible and balance is between Coriolis and pressure forces (called geostrophic balance). In the oceans all three forces are comparable (called cyclogeostrophic balance). For a figure showing spatial and temporal scales of motions in the atmosphere and oceans, see Kantha and Clayson 
When the Rossby number is large (either because f is small, such as in the tropics and at lower latitudes; or because L is small, that is, for small-scale motions such as flow in a bathtub; or for large speeds), the effects of planetary rotation are unimportant and can be neglected. When the Rossby number is small, then the effects of planetary rotation are large and the net acceleration is comparably small allowing the use of the geostrophic approximation.
References and notes
- M. B. Abbott & W. Alan Price (1994). Coastal, Estuarial, and Harbour Engineers' Reference Book. Taylor & Francis. p. 16. ISBN 0-419-15430-2.
- Pronab K Banerjee (2004). Oceanography for beginners. Mumbai, India: Allied Publishers Pvt. Ltd. p. 98. ISBN 81-7764-653-2.
- B. M. Boubnov, G. S. Golitsyn (1995). Convection in Rotating Fluids. Springer. p. 8. ISBN 0-7923-3371-3.
- Lakshmi H. Kantha & Carol Anne Clayson (2000). Numerical Models of Oceans and Oceanic Processes. Academic Press. Table 1.5.1, p. 56. ISBN 0-12-434068-7.
- James R. Holton (2004). An Introduction to Dynamic Meteorology. Academic Press. p. 64. ISBN 0-12-354015-1.
- Lakshmi H. Kantha & Carol Anne Clayson (2000). p. 103. ISBN 0-12-434068-7.
- John A. Adam (2003). Mathematics in Nature: Modeling Patterns in the Natural World. Princeton University Press. p. 135. ISBN 0-691-11429-3.
- Lakshmi H. Kantha & Carol Anne Clayson (2000). Figure 1.5.1 p. 55. ISBN 0-12-434068-7.
- Roger Graham Barry & Richard J. Chorley (2003). Atmosphere, Weather and Climate. Routledge. p. 115. ISBN 0-415-27171-1.
For more on numerical analysis and the role of the Rossby number, see:
- Dale B. Haidvogel & Aike Beckmann (1998). Numerical Ocean Circulation Modeling. Imperial College Press. p. 27. ISBN 1-86094-114-1.
- Zygmunt Kowalik & T. S. Murty (1993). Numerical Modeling of Ocean Dynamics: Ocean Models. World Scientific. p. 326. ISBN 981-02-1334-4.
For an historical account of Rossby's reception in the United States, see
- Jeffery Rosenfeld (2003). Eye of the Storm: Inside the World's Deadliest Hurricanes, Tornadoes, and Blizzards. Basic Books. p. 108. ISBN 0-7382-0891-4.