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6.4 Geostrophic Flow 123Sect. 3.17) compares the inertial period with the time scale of a process. The Coriolisforce influence or even controls processes that have time scales of or exceeding theinertial period. On the other hand, the ratio between the nonlinear terms and theCoriolis force is called the Rossby number and is define by:Ro = U fL(6.7)where U is a typical speed, f is the inertial period, and L is the lengthscale of aprocess. Again, this is a comparison of time scales, whereby L/U is the time ittakes for a f ow of speed U to travel a distance of L. For small Rossby numbers(Ro