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Estimate the Rossby number for \( U = 10 \) m/s, \( L = 10^{6} \) m and \( f = 10^{-4} \) per second.

  • A.1
  • B.0.001, which would place the flow well inside the regime of exact geostrophic balance always
  • C.0.1
  • D.10, which would indicate that rotation plays almost no part in the dynamics of this flow at all

Answer: C

Substituting gives \( Ro = 10/(10^{-4} \times 10^{6}) = 0.1 \), a value small enough that the geostrophic approximation is good to about ten per cent, which is the basis of quasi-geostrophic theory. A value near one would indicate that inertia and rotation compete, as in a tropical cyclone core, and a much larger value would describe a tornado or a laboratory flow where rotation is irrelevant. Making this estimate before choosing an approximation is a habit worth acquiring early.

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